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INTRINSICALLY SAFE DESIGN

ENGENUICSENGENUICSENGENUICSENGENUICS TRAININGTRAININGTRAININGTRAINING

Intrinsically Safe Design

Module 6


IS Design Course Notes Module 6.docx of 20

Document Revision History

Version Author Release Date Comments

1.0 Jason Long 2015-NOV-01

1.1 Jason Long 2015-APR-18 Update for Module 6 online



Contents Elemental Fundamentals: Components That Improve Intrinsic Safety ........................................................ 4

Infallible Resistors ....................................................................................................................... 4

Fuses ........................................................................................................................................... 6

Series Blocking Diodes .............................................................................................................. 10

Voltage Clamping with Zener Diodes ........................................................................................ 14

Current Leakage .................................................................................................................... 18

Bias Current ........................................................................................................................... 18

Power sharing ....................................................................................................................... 19

Summary ..................................................................................................................................................... 20



Implementing protection using the tools from our “tool box” is the subject of this module. As in

the previous module, each part we discuss has a summary at the start which covers the main

points to keep in mind. The subsequent descriptions fill in the details on how to properly use

the parts.

Elemental Fundamentals: Components That Improve Intrinsic Safety

There are many ways to make an IS product safe. Remember that a source of ignition can be a

spark or a surface temperature hot enough to ignite the surrounding atmosphere. Higher

voltages lead to higher spark potential, so it is in the best interest of the IS product to keep

voltages as low as possible. If high levels of current are flowing in a circuit and the circuit is

interrupted, an arc can occur causing ignition, so less current is better which of course implies

more resistance is better. The product of voltage and current is power and the less power that

can be delivered through traces and to components, the less heat those parts will dissipate.

60079-11 introduced COWS and infallible components that along with spacings give all the

available options to implementing intrinsic safety. A product can be made IS by using these

techniques or strategic combinations of them to successfully limit energy while still ending up

with a circuit that actually does what it needs to do. It is often a very thin line between the

two.

Infallible Resistors

IS Summary

- The only device that may limit transient currents

- Assessed at rated value +/- tolerance (whatever makes the calculation worst)

- Infallible if maximum power dissipated is less than 2/3 rated power and part is physically

large enough to meet spacing requirements

- Must not be bypassed by other components or traces in normal operation or under fault

- Can still be faulted with countable fault, but only to completely open state

- Can power share in series within tolerance bounds

- Use four in parallel to power share under two countable faults

- Low resistance values often protected by fuses

Infallible resistors are essential in any IS product. Their linear, non-time dependant properties

make designing them in relatively simple. A resistor can be used to limit current and/or energy

to safe levels. It is the only device that can always acts instantaneously by Ohm’s Law, and thus

is the only device that can mitigate transients. Resistors can dissipate power and provide a

degree of isolation from one circuit to another.

Since you likely want to stick with surface mount devices for your product, there is a practical

physical limitation to resistors that set the maximum allowable power to 2W. These resistors

are in 2512 packages which are fairly large. 2010 1W packages are often used, and sometimes



you can get away with 1206 packages that are usually rated 250mW but some are 500mW.

Sometimes very high valued resistors are used in which case 0603 parts are acceptable if under

potting due to the size of the device. In this case, power is really negligible since the resistance

is so high. We have never come across an SMT resistor that was not accepted as a CLR as long

as the specifications were met. Datasheet values can be taken as proof of compliance.

When calculating the required power rating for CLRs, be aware of when the system voltage or

current caps the power calculation. This tends to define to ranges of resistors that can ever be

considered. Values from 0 to 2 Ohms will usually be current-limited by a fuse so P = I²R is used

for power calculations where I is the fuse current. As the resistance gets higher the peak

nominal voltage starts to dominate, so P = V²/R should be used.

In their role as current-limiting for short-circuit current, low values of resistance are typically

implemented since the application is most likely in series with the main power source. Values

under 2 Ohms are common and thus almost always need to have a series fuse to “defy Ohm’s

law” by clamping maximum current. The fuse can be placed before or after the CLR as long as

the CLR cannot fault to ground before the fuse (so we show the space between R and F as an IS

region indicated by orange outline). The maximum current is calculated based on nominal

power supply voltage and usually assuming that the CLR shorts to ground somewhere

downstream.

For a typical lithium-ion powered system, there is a “no man’s land” of resistor values from

about 2 – 15 Ohms that are not realizable because of the size of the voltage drop, or because of

power requirements. If you are having trouble with resistor power ratings, splitting the parts

up in series is an excellent way to reduce the per-resistor power.

Splitting resistors up is an easy way to cut the power requirements for each resistor. If you are

trying to increase resistance and ending up with power requirements that are too high

(governed by P = I²R), then using series configuration you get immediate benefit by using two

parts. The power reduction is almost half – it will be slightly lopsided as one of the resistors will

be considered at R + tolerance and the other will be considered R – tolerance.



If you are trying to decrease resistance and getting power requirements too high (governed by

P = V²/R), then you can use parallel combinations. In this case, however, you need at least four

resistors in parallel to start getting benefit since two of the parts can be opened with countable

faults. From the electrical perspective you design for all of four of them in place, but for IS you

design with two removed by the countable faults.

One life-saving (or at least potentially cost-saving) trick you can use resistors for is to essentially

“add a layer” to your circuit board. In fact you can use this in non-IS designs, too. If you are in a

situation where you have just a few traces that are not routable using the number of layers in

your board, you can jump over traces with 0R resistors. For the non-IS case this likely is not

going to factor in very often, but for IS it might come up more often than you think.

On some IS PCBs, you will not be allowed to via perhaps on the whole board, or perhaps in a

particular location. More commonly, you may only be able to use ground vias but not other

signals that could bring potential voltage through the PCB that would otherwise be isolated by

the PCB material itself. Using a jumper resistor is a far better solution than doing a blind or

buried via.

Fuses

IS Summary

- Assessed at rated value x 1.7

- Must be rated for voltage and breaking capacity (current)

- Must be UL or IEC certified


- Cold resistance is considered infallible and can be used as part of current-limiting

resistance for short-circuit current protection; in most cases, 10 samples of the fuse will



have to be provided to the test lab to determine worst case cold resistance and the

extremes of the target device operating range

- Cannot be faulted

- Must be potted if energized in hazloc

In classic electronic design, fuses limit current. In IS design, the main purpose of a fuse is to

limit power. This is because transient currents that are potentially incendive occur in a much

faster time frame than the fuse can react. Therefore the role of a fuse limits current for the

purpose of preventing too much power being delivered to components downstream.

For assessment, the standard stated that the current through the fuse is assumed to be 1.7x the

rated fuse current. Fuses have some amount of DC resistance which is often specified as “cold”

or “nominal” resistance. This resistance can be considered infallible and used when considering

spark current limiting, but the fuse rating itself does not influence spark assessment. While

some agencies might accept the datasheet cold resistance, most likely 10 samples of the fuse

will have to be provided for measurement over the operating temperature range of the

product.

Remember that fuses are the key component in setting the maximum power available in the

circuit, and we want this power to be under the maximum for your product’s specified ambient.

That being said, there is nothing stopping you from using multiple fused paths to distribute

power. Just make sure the two branches of current cannot be recombined through countable

or non-countable faults.

The example above is indeed ideal, and the two circuit domains will likely need to

communicate. However, this could be done through high valued infallible resistors on high

impedance lines so current would not be able to flow.



Finding a fuse for IS applications is not difficult. We tend to use Littelfuse, Vishay, and Bourns

most often. Aside from ensuring that the fuse you are considering meets the voltage and

current breaking capacity requirements and is UL or IEC listed, take some time to carefully

examine the datasheet for other significant information. For discussion here we will again

reference the Littelfuse series 466. For the 200mA part in the 466 series that has a nominal

cold resistance of 1.16 Ohms per the datasheet, the actual tested resistance at -20°C is around

0.6 Ohms. You can get a rough estimate of what it might be using the derating curve (called a

“rerating” curve in the datasheet to sound more positive, we suppose).

It shows us that at -20°C, the fuse rating is about 113% and the subtle little note below the

table mentions another 25% derating for continuous operation. So our 200mA fuse with 1.16

Ohms resistance can be looked at as 276mA in cold temperatures. Or by extension we could

apply the factor to resistance and assume the cold resistance is 1.16 * 0.75 * 0.88 = 0.77 Ohms

which is still higher than the measured value but at least in the ballpark.

The really tricky part here is that calculations based on the fuse current could be done assuming

-20°C operation and so a much higher fused current would be used and this makes a significant

difference when you also factor in the 1.7x multiplier and start doing I²R power calculations!

However, any power calculation and handling of parts would also scale down due to the

ambient so in most cases the agencies stick to the standard that requires 1.7 x In which is the

room-temperature spec. Most fuses will blow faster at higher temperatures, so perhaps just

make sure you consider the electrical side of the design in this case.



We did get into an argument with an agency about this one day a few years ago which was

never settled because the result was still acceptable but it was a dangerous precedent to set.

Long story short, be aware that this is out there.

All that being said, you do not HAVE to use the fuse resistance in your IS considerations and can

instead rely fully on additional current limiting resistance to mitigate short-circuit transient

currents for spark. Be really careful, though, because the lower you go for fuse rating, the

higher the resistance become and you can start to have serious impact on the electrical

operation of your product due to the voltage drop on what effectively becomes your total

source impedance. Fuses less than 200mA start to increase resistance rapidly. The lowest

rated fuse in the 466 series is 125mA and it has a cold resistance rating of 4.0 Ohms! If your

product draws 100mA, that is a solid 400mV drop just on the fuse.

In most cases, fuses must be potted to exclude atmosphere since arcing or high temperatures

can occur as the fuse blows. As we mentioned when reviewing the encapsulation section of

60079-11, the encapsulation process must ensure that a minimum of 1mm thickness surrounds

the protected component. To guarantee this and to make the production process reasonable,

you will likely have to build a plastic fence or other mechanical feature around the fuse that can

be filled up with potting compound. This will likely be a custom part that will add time and

money to the design process to produce. Do not forget to add locating holes to ensure the

fence is always in the correct location. As long as the potting compound bonds to the fence,

then the thickness of the fence can be included in the 1mm calculation.

There are fuses that are pre-certified for IS including the encapsulation and thus do not require

additional encapsulation. There is only 1 that we know of that will meet Class I Zone 0

applications and it is also from Littelfuse:



We have not come across any that are surface mount. There are a few of things we do not like

about the 259 series and have thus never used them:

1. They are not surface mount, so even though you save the step of encapsulating, you still

have a thru-hole part to deal with. That is arguably orders of magnitude better than

dealing with encapsulation.

2. They are huge. 13mm long with 8mm diameter?!?

3. There is no 200mA part, although the 125mA part has only 1.7 Ohms of nominal cold

resistance and the datasheet specifies the minimum resistance at both -20°C and -40°C

which should alleviate the requirement to test 10 samples.

So if you can afford the space and since you will probably have a few other thru-hole parts

anyway, the 259 series of fuses are a good alternative.

Series Blocking Diodes

IS Summary

- Most common device that can completely block energy between circuits (unidirectional)

- COWS if maximum current that would flow if diode was shorted with 1.5x factor is less

than rated current

- Do not typically have power spec


- Can be shorted with countable fault (or opened, but that is typically good)

- Must use three in series so one remains after two are faulted

- Often protected by fuses to limit current on power lines

- Can be protected by current limiting resistors on signal line

- Main disadvantage is voltage drop for normal electrical operation

The intent of blocking diodes is to completely prevent energy from coming out of a circuit or

device. We have already seen the application where they are used on a charging path to

ensure that battery energy is not available at the charging terminals when the device is in use.

Properly rated diodes are COWS, not infallible, and 60079-11 states that they can be faulted to

short circuit with a countable fault. While they could be faulted open, that would be entirely

safe. The condition for these COWS is that the diodes must be rated for the steady-state

current that would flow in their place if they were replaced as a short circuit. The standard

does not explicitly require the diodes to be rated for power, though this is essentially

redundant because if they are rated for steady state current, then that implies they are rated

for P = IV where V is the voltage drop of the diode (in the case of signal or Schottky diodes, V is

not very high).

The other use case for blocking diodes was discussed in this module when looking for ways to

“hide” capacitors. Any circuit can be “hidden” with blocking diodes as long as it still functions

electrically which is often the challenge because of the voltage drop even on ultra-low forward



voltage Schottkys. Of course if communication needs to be bi-directional, then you have issues

to deal with there. One of the case studies covers a complete scenario so we will bring all of

the relevant details together. For this section, we will explore a theoretical example.

The circuit above is a modified version of the blocking diode circuit we showed before with the

introduction of a fuse that is perhaps inevitably required in the context of a charging path. In

this case, you will be stuck with fairly large physical parts to allow any appreciable charge

current. Realistically 1A is about the maximum current you can design for and still use surface

mount parts that are relatively common. A 1.0A fuse x 1.7 x 1.5 safety factor requires the

diodes to be rated 2.55A which translates to 3A or higher parts. Voltage drop will factor in here

on the electrical design side of things as you will have to deal as much as 1.5V total drop. This

should explain why we use 6V chargers for lithium ion batteries!

One of our favorites in this application is a B340A (SMA packages). The datasheet is included in

the course notes – open it up to take a look. You might notice that there is no power rating for

the diode in the datasheet, but remember that the 60079-11 only specifies a current rating.

The B340A is rated for 3A. However, we still must consider temperature rise of the part to

ensure that at its peak operating value it will not be too hot for the temperature rating we want

for the product. So we introduce now the typical method of determining that.

Most semiconductors will have a datasheet spec for their “junction to ambient” thermal

resistance. The parameter is called “R-theta-J-A” (junction to ambient) and usually written RϴJA.

This is the parameter used to make a paper-based assessment of the max temperature a part

will reach. We are not experts on thermal resistance and cannot comment on the validity of

this approach, though if you Google around you can find some explanations that seem to make

sense and justify it. Note that the Junction to Terminal spec that is also (more) commonly

found on a datasheet is NOT accepted for this calculation.



The table below is copied from the B340A datasheet and you can see that the RϴJA spec is

50°C/W.

For the case of each diode we can assume that its power dissipate is P = IV with I at the fault

current multiplied by the rated forward voltage drop at that current. The 1.5 safety factor is

included in the fault current. The Vf spec of the part is shown here:



So at 25°C and 2.55A fault current, we will be safe and take 0.5V as Vf. We can expect the

diode to be dissipating 2.55A * 0.5V = 1.275W. Therefore the temperature rise is P x RϴJA =

1.275W x 50°C/W = 63.75°C. So at room temperature, our diode will be 25°C + 63.75°C = about

89°C which is no problem at all. If this was the only temperature we had to worry about, we

could rate our device operating peak temperature for T4 at (135°C – 89°C) + 25°C = 71°C just

based on this, and we still have the small component thermal considerations to consider which

allows higher temperatures for small parts. In other words, you have nothing to worry about

here.

If it started to get close to the temperature limit needed for the product, you could be more

careful in reading Vf from the graph to get a lower number or use a different ambient curve

because the forward voltage is less at higher ambient. In other words, the temperature rise is

non-linear so the default assumption the standard makes about linear temperature rise works

against you here. Since we have so much margin, we would not care UNLESS we had other

parts in our design that were very close to their limits. In that case, we might choose to show

all design calculations using the same (more precise) method to be consistent in our design

documentation.

So that is a lot of detail about specifying a blocking diode! But now that this process

information is out of the way, we can proceed through the next sections more quickly. Before

moving on to the next helpful part, we can talk about the other likely application of blocking

diodes.

If the diodes are downstream of the main product fuse, then they are likely okay without any

additional current-limiting. SOD-123 parts are what we commonly use if the current rating

required is under 1A. It is not too difficult to find a part rated high enough, but often the

cumulative voltage drop becomes an issue. There are some “ultra low forward drop” diodes

like the LSM115J from Microsemi (datasheet is included in the course package) that can help to

minimize this.

Even with ultra low forward drop diodes, over temperature and battery discharge you might

still have a problem. One of the ways around this in a battery powered application is to run a

separate power path from just after the main fuse and resistor but before the voltage regulator

to the rest of the product.



This way, the current draw from section “A” does not add to the voltage drop (and noise)

through the blocking diodes. The diode drops occur before any voltage regulation so the

maximum voltage in the product is available. Now the diode voltage drop may be as small as

0.1V per diode so section “B” will see a consistent voltage level while still being isolated.

Obviously (hopefully) section “B” must be physically isolated from the rest of the circuit so the

blocking diodes cannot be bypassed with faults. This is where the tricks come in since most

likely section “A” will have to talk to section “B”. It could be as simple as using very high

resistors on otherwise high impedance communication lines, or lower value resistors that still

provide enough isolation as possible. If you need low impedance input and output to section

“B” then this is not going to work.

That is as generic as we can be to cover the majority of factors involved in using blocking diodes

for protection in practical circuits. The process of designing them in will be the same for any

application, but the details will be different in every case. Sometimes it is very difficult to find

the balance between the electrical and IS requirements and we have done designs where it was

impossible to implement the protection without giving up some overall battery life or requiring

a lower voltage part in what would be equivalent to section “B” in the example to make the

solution work with the diode drop.

Voltage Clamping with Zener Diodes

IS Summary

- Most common device for clamping voltage between circuits is Zener diodes

- COWS if maximum current that would flow if diode was shorted is less than rated

current x 1.5 AND power dissipation is less than 2/3 of Vz x I.


- Can be opened with countable fault (or shorted, but that is typically good)

- In most cases, standard allows just two to be used and only one countable fault can be

applied to the pair.

- Often protected by current limiting resistors on signal line

- Main disadvantage is current leakage

- Be careful about bias current



There are quite a few cases where voltages in a circuit need to be clamped because an elevated

voltage is possible somewhere in the circuit. In almost all circumstances this is done with Zener

diode. The main purpose of clamping voltages is to obtain as high an allowed capacitance as

possible to allow the electrical design to work. This might be for the whole design, or for just a

section of the circuit. It is fairly rare to clamp your main power source (e.g. your battery),

because the operating voltage of the product and the Zener voltage of the diodes tends not to

work out to any benefit. But clamping circuits downstream is quite normal.

The application is best illustrated with an example. As usual we will start out simple so the

circuit we will look at is just part of the complete real circuit that would be likely used. In this

case, assume the product is lithium ion powered where the battery runs into an LDO with an

output that sets the product supply rail at 3.3V. That of course has nothing to do with the IS

considerations, but it does have a lot to do with Zener selection. A typical example is

connections to an LCD that has some sort of charge pump or step-up converter to provide

elevated backplane voltages. For a starting example here consider just a single connection into

the LCD (in practice, there would likely be 2-6 signal lines plus power). Assume the LCD has a

boost circuit to raise the voltage to 5.5V for the backplane, but otherwise the signaling is 3.3V.

Notice that the positive and negative traces around the Zeners are indicated as infallible: this is

essential. If the GND side can be faulted with a non-countable fault, then the Zeners are

floating and useless. The GND connection must be infallible all the way back to the battery

connection. If you have a ground plane layer, this is easy as long as you use the correct vias. If

you have to run a trace, make sure it meets the 2mm requirement. Below is almost as compact

as you can get for this structure (SOD-123 packages).



The positive trace does not really need to be infallible, although if you route it in a certain way

such that one of the diodes can be disconnected with a non-countable fault then you are in

trouble – it is much easier to just require infallibility for this part of the trace, and you can route

this without taking up any extra space as most of the trace can be underneath the parts. As

long as this cannot be bypassed between the two domains, only the traces connecting the

diode terminals need to be infallible – the rest of the traces to and from each domain do not

require infallibility.

Using our very familiar lithium ion battery example, we know our spark voltage is 4.2V. This is

substantially lower than 5.5V and we do not want the rest of the product to be assessed at that

level. Due to leakage current through Zeners that we will discuss more later, the highest Vz as

possible should be chosen. In this case, 3.9V + 5% = 4.1V which is less than 4.2V from the

battery, so clamping at this level does not affect the rest of our assessment. If you selected

4.1V Zeners + 5% tolerance = 4.31V, and thus the rest of the product would be assessed at 4.4V

due to rounding. Sometimes that amount of difference can be significant.

Once you have decided on the clamping voltage, verify the IS requirements of the parts you

chose. This circuit above is fine but the Zener power requirements are quite high which means

physically large and more expensive parts.

P = [3.9V + 5%] x [0.200A x 1.7] x 1.5 = 2.1W

There are lots of inexpensive and reasonably small SOD-123 parts but they typically max out at

0.5W of power handling. Working backwards, that means current must be:

0.5W = (Vz + 5%) x I x 1.5

I = 82mA



There are 62mA fuses available but they are fairly expensive and every one would have to be

potted. So instead consider a current limiting resistor.

To set 82mA, consider the 3.8V nominal voltage from lithium ion and assume a short circuit

somewhere in the LCD circuit.

R = 3.8V / 0.082A = 47R (including 1% tolerance).

This is a standard value and likely no problem for a communication signal on a high impedance

line. The power consumption required for this CLR is set by the CLR itself since it holds current

to less than the main 200mA product fuse. The power required with safety factor is:

P = 82mA² x 47R x 1.5 = 0.474W

So a 0.5W 1206 resistor would be fine. However, hold that thought for a moment.

Now we introduce a significant consideration that many people new to IS will overlook. We

calculated the resistance necessary to reduce current and thus reduce power dissipated by the

Zeners. And we just calculated the power handling of that resistor. If we have two paths, the

input resistances have to be considered in parallel because there are now multiple fault paths

that can contribute to the power dissipated by a single diode.



With countable and non-countable faults, we can deliver power to D3 from both resistors. So if

the resistors needed to be 47R before, they now need to be 2 x 47R = 94R so their parallel

combination is still 47R. The good news is that this reduces the power requirement of each

individual resistor itself. The bad news is that the signal on each line now has to accommodate

more resistance and still work.

As more lines are added, the resistance of each line must increase. You might think you can

space infallibly space them out but you cannot, because they all originate in your

microcontroller and all terminate in the LCD controller IC. Both ICs can be totally shorted.

There is no requirement that every resistor must be the same, but the parallel combination of

them all must meet the minimum resistance requirement that you calculated. If you are

juggling values around, make sure the minimum value of resistance still meets the power

requirements. It is really best to use a V²/R calculation for power here so you do not have to

keep calculating the current value. If the calculated power is too high, double check that the

resistance has not gone low enough so that the fuse current will start to limit.

We will extend this example in the case study to see the full solution which is a combination of

Zener-clamped lines plus the power line through series blocking diodes.

Current Leakage

Real Zener diodes operate a long way away from the ideal model that you see the first time you

learn about them, namely the way that the devices actually clamp voltage and the current they

leak when biased at less than their rated Vz. This can create a lot of problems for battery

powered devices. The higher the Zener voltage is away from the nominal system voltage, the

better, but inevitably there will be some amount of leakage and some loading of any digital

signal on the line. Different diodes will exhibit remarkably different behavior even though they

may share the same Vz spec. It is important to have a few options and test these on your bench

to characterize them.

Bias Current

All Zener diodes require a certain amount of current before they are fully active and able to

clamp. Therefore one must be careful when choosing a diode to ensure it does the job it is

supposed to do. Interestingly, every agency we have worked with to date does not recognize

this – they essentially assume the diode behaves ideally. This is not safe and we ensure that

the diodes operate properly.

This is mostly a function of the input resistance to the Zener. If you only have a fuse, then you

do not have to worry about it. However, when it comes to clamping voltages within a design

between two voltage domains as we showed in the example where power was limited by

resistors, then you do need to be careful.

First we have to make an important distinction. For the power-handling assessment of the

Zeners, power is supplied from the battery, through the CLRs and dissipated into the Zeners.



However, the Zeners are there to prevent the high voltage on the LCD side from coming back to

the VCC domain. The higher voltage cannot source power, or in the very worst case can only

source as much power as what can be input to the circuit. So the power available the diodes is

NOT 5.5V x 200mA x 1.7. What we do have to be concerned about is the ability of the Zeners to

be biased on correctly.

If we add a path through blocking diodes, we have introduced another power path to the

Zeners. An easy fix is to put a CLR on both sides.

This is where a problem might be introduced. If the resistance on the LCD side of the Zeners is

too high, then the Zener will not be active and the 5.5V will be available through the Zener

clamp. You might not believe this, because we did not believe it the first time we came across

it while trying to solve a particularly tricky IS problem where quite high IS resistors were be

used. But it is true, and you must make sure you design for it. The good news is that the

resistor on the LCD side can be as low in value as the single resistor case as long as it is spaced

such that it cannot be bypassed. That is easy to do.

Power sharing

If you only need two diodes to form an infallible assembly you could argue that adding

additional diodes would enable them to share power. This is correct to a degree, but is not

something that is clearly defined anywhere and will require agency approval. If the agency

allows it, you will definitely be required to test. All diodes have a variation in forward voltage



that has substantial impact on how they conduct. If you do not believe that, connect two LEDs

in parallel to a single resistor and see the difference in light intensity.

All types of diodes have this behavior which means that there is no simple way to determine

how much current and therefore how much power would be dissipated in each part. Testing

would have to be done to find the worst case scenario which could be taken as normative for

that particular part, though it may be subject to routine testing in production at least with every

new reel of parts you buy. We have only seen this used once over a decade ago and the power

sharing ratio was something like 20:80, so there was not that much benefit after all of the

effort. Therefore it is highly recommended to even consider this option. This is completely

different than power sharing in serial connections which is absolutely a viable calculation to rely

on.

Summary

This module has captured the four main components or groups of components that can be

applied to circuits to ensure intrinsic safety. The calculations and implications shown here are

directly applicable to all IS designs, though the specific details and end applications will of

course be different. Even with these generic examples, we have seen how certain steps can

lead to difficulties in both the IS and regular electrical design domains. In some cases, a

successful product will only be possible with a very delicate balance of some combination of all

of these options.

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