an electronic ballast

8/2/2019 An Electronic Ballast

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An Electronic Ballast (Inverter) to Power HID Lamps from 12 Volts DC!All content Copyright 1999, Jonathan B. Smick. All rights reserved. Reproduction of the contents of this page fornon-commercial and non-profit purposes only is permitted if the following conditions are met: (1). This document is

reproduced in it's entirety, including this notice. (2). There is no charge except to cover costs of copying. (3). Credit

is given to Jonathan B. Smick as the source of this information and as original creator/inventor of any circuits, circuit

concepts and circuit usage concepts included in this site (except as otherwise specified).

WARNING AND DISCLAIMER

DO NOT attempt to build this circuit unless you have prior knowledge of and experience in electronics safety,

construction practices and techniques. This circuit produces possibly LETHAL voltages and currents. High

frequency arcs can cause severe burns on contact. HID lamps can explode with great force if abused (or even in

normal operation), resulting in the scattering of hot particles of glass/quartz of temps. of 1000 Degrees Centigrade or

more. HID lamps produce substantial heat; do not operate in combustible environments or near combustiblematerials. Do not stare at the light directly, eye damage can result. Do not operate HID lamps whose outer bulb has

been damaged, or broken; lamp explosion or exposure to dangerous shortwave UV radiation can result. The author of

this article cannot be held liable for any injury, loss of life or limb, or property damage whether resulting from the

proper use or misapplication of the information contained herein, nor for any damages, incidental or consequential,

whether caused by accidental or intentional use, misuse, or ignorance of the safety concepts and design principles

contained herein.

IntroductionHere is a circuit that can be used to power a variety of small (50 w or less) HID lamps, as well as (with

modifications) fluorescent lamps up to about 40w, or HeNe lasers, flashes/strobes, and neon tubes, etc., from a 12 V

car or marine battery or medium-size gel cell.

Perhaps the best thing about this circuit is that it can be built with almost all Radio Shack parts. This circuit does

require a homemade transformer. This transformer is not hard to make, just a little tedious for hand winding. Because

this circuit gives the lamp high frequency AC (except during starting), none of the limitations associated with DC-

output supplies apply (i.e., not using certain types of HID's, which cannot tolerate DC operation).


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How The Circuit Works...COMPLETELY REVISED AND UPDATED 11/21/99

The circuit as shown in the above schematic was originally designed for use with the Philips 50w metal halide lamp,but will also operate 40/50w mercury lamps as well, without modifications (although the starting circuit, D1-D6, C1-

C6, and R3, is not needed and can be omitted for use with the mercury lamp). C7 is also not absolutely needed for

the mercury lamp and can be left out, although it is recommended to use it (in series with the lamp) anyway (it helps

limit the current, along with the airgap in the core of T1).

HPS (High Pressure Sodium) lamps of 35-50w will also work with this circuit, although the turns ratio on T1 may

need to be adjusted to provide the proper operating voltage to the lamp (these generally need lower arc voltages than

other HID's) and consequently, the starting circuit (diode voltage multiplier) would then need more sections to

provide sufficient voltage to ionize the lamp with the lower transformer output voltage. This is easy to do, as all it

entails is adding more "sections" (2 diodes and 2 caps per section) as needed to the multiplier stack, so that there willbe from 750 Volts to 1kV DC output from it (open circuit) to start the lamp. (If it is desired to go higher than this on

the starting voltage, use higher PIV rated diodes, [Not available from Radio Shack] or 2 or more RS 1kV units in

series in each "leg" of the multiplier). I have started HPS lamps from as little as 480 Volts, so excessively high

starting voltages shouldn't be necessary.

LPS (Low Pressure Sodium) lamps will work without the starting circuit either (as in the mercury lamp case) as the

open-circuit from T1 is sufficient to start these. The wattage will need to be adjusted downward, however, as

outlined below if using 18 or 35 watt lamps.

The starting circuit concept is thus; when power is applied, there is an essentially infinite impedance at the lamp,since it has not yet ionized; this causes a DC voltage to build at the output of the multiplier, until the lamp's firing

voltage is reached. The gas breaks down, and immediately provides a lower-impedance path for the transformer T1's

HF AC output through the capacitor C7 and the lamp, than is presented by the multiplier string (whose caps charge

up again and due to resistor R7, remain in a relatively charged and high-impedance state). Therefore, the multiplier is

effectively bypassed during the lamp's operation by C7, and the lamp receives high frequency AC. As the lamp


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warms up the impedance of the path through it (and C7) decreases until it stabilizes at some low value at full

brightness. The continued DC contribution of the multiplier string during lamp operation is negligible, due to it's

high impedance relative to the path through C7 and the lamp. R3 limits the available output current from the

multiplier string, as only voltage, not current, is needed for initiation of the lamp arc. This also ensures that only a

minute amount of power is wasted in the multiplier string during lamp operation.The remainder of this ballast is a conventional high-frequency Royer-type push-pull oscillator inverter with a ferrite

core transformer. It is essentially similar to the inverters used to power small fluorescent lamps in pocket andcamping lanterns, etc., except of course beefier. The oscillation frequency, with a 1mm airgap in the core of T1, is

approximately 20 kHz under a heavy or full load, and quite stable, but somewhat variable and of lower frequency

(~15-16 kHz) under light or no load (as in an open circuit condition or during lamp warmup). This characteristic is

typical of simple inverter circuits, and doesn't impair operation in any way (more on this later). This frequency range

can be adjusted to suit individual lamp/wattage requirements, to optimize efficiency, etc. by adjustment of T1's core

airgap.

Building The Circuit

This ballast can be built by any acceptable method, on perfboard, a PC board or point-to-point. Layout is not critical,but high-frequency leads should be kept short, if at all possible, to minimize losses. Also, remember to provide

sufficient separation between high- and low-voltage leads, and to case and/or ground. (At least a few millimeters).

T1 is a high-frequency ferrite-core transformer, which you will have to wind yourself. The cores are from the Radio

Shack (273-104)rectangular "toroid snap-on chokes", which come two to a pack. You must make two "E"-core

halves out of them. To do this, disassemble the ferrite from the plastic housings (this is easy- they just slide out). Do

this for both chokes. You'll now have four "C" cores. Put a pair of these together, end-to-end, and you'll see that,voila, you have an "E" core half! Take "Super Glue" or similar cyanoacrylate cement and bond these two "C" cores

together end-to-end, making sure not to use too much glue, but making a tight bond with minimal gap (maybe clampthem together somehow for the 30 sec. or 1 min. or so it takes for the glue to cure fully) to form one "E" core half.

Make sure all edges and surfaces are as perfectly level and lined up as well as is possible. This can be tricky since the

glue "grabs" instantly and repositioning if you make a mistake is, well, nearly impossible! Follow the same

procedure with the other pair of "C" cores. You'll then have your two "E" core halves.

For a nice nylon bobbin to wind on, get a Radio Shack "Noise Eliminator Kit", no. 270-030, (this is used to remove

alternator whine from the power lead of car radios). You need to disassemble the choke to get its bobbin. To do this,

first determine which side of the core has "E" laminations and which has the "I's". Clamp the "E" side of the core in a

vise, give the "I" core top piece a sharp rap from the end or side with a hammer (it is held on with epoxy) and itshould come right off. (Be careful if anyone/thing is in the vicinity in case it flies off!) Now just pull out the bobbin

from the "E" side of the core. If it is a tad stubborn, *carefully* (to avoid damaging the bobbin) place it on a flat

surface, maybe between the parted jaws of your vise, for instance, supported by the bobbin flanges on one end andtap out the core half with a mallet and some sort of rod or bolt (so you're actually tapping the center post of the core

only). Now that you have your bobbin, remove the tape and unwind the wire from it completely. The only other

thing you have to do (and this is rather tedious, but necessary) is to carefully file out the sides of the inside center

hole of the bobbin to allow your ferrite cores to fit (they won't unless you remove a little plastic from the inside of

the bobbin, maybe .020" total (just a guess). Get a small, fine flat file (like the type used by modelers/hobbyists that

comes in a "needle file" kit) and begin working on the side walls only of the bobbin's interior, periodically checking

for fit with your cores. You want a tight, sliding fit, not a loose, sloppy one so don't remove too much plastic. Makecertain the cores fit all the way in when you're done, there should be no taper in the interior hole of the bobbin, it

must be flat and parallel (hint: file alternately from both sides so you don't get a taper). Once this slightly tedious task

is complete, you are ready to wind your transformer! Using this method to get a "real" bobbin is vastly superior to

the previous method I had detailed and have since removed from this site, as it made for a lot more work than

necessary and was far less convenient and versatile. (With a bobbin you can now experiment much more easily with

different windings and core airgaps, and can also rewind/replace the assembly much more easily should you need towithout scrapping the whole transformer!)

Begin winding the secondary, or output (O)winding, using no. 30 AWG enameled wire (from, you guessed it!) RadioShack's magnet wire 3-spool pack (278-1345). Recommendation: if you can get the "Heavy Nysol" insulated type of

magnet wire from Mouser Electronics, it will have both better heat resistance and dielectric qualities than Radio

Shack's wire and is far preferable. Mouser does not carry no. 30 AWG but they do have no. 28 AWG which is just

fine. Try to wind your secondary fairly neatly, closewound (wires side-by-side, in contact). This is easy if you keep

the wire feeding off the spool in tension with one hand and use your thumb on the other hand to hold down the

already-wound turns. Avoid kinks at all costs, but a few overlapped or otherwise scramble-wound turns are ok (just

don't get *too* sloppy). I have found it unnecessary to use an insulating tape to divide the layers of the secondary


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(e.g. between each layer of winding), as was outlined here prior to this update, although it is imperative you do

insulate the three windings from each other. I have found that Teflon pipe "dope" tape is excellent for the purpose- it

is thin and stretches to conform to irregular surfaces yet resists heat and has high dielectric strength and low loss at

high frequency. At any rate, for the secondary or output winding you simply need to wind enough layers (generally 7

or so) to give a total of 400 or so turns (not particularly critical, although optimization for your particular applicationis advised). Each layer can be directly on top of the previous one. Overwrap the whole output winding with several

*tight* layers of Teflon, secured with Scotch tape.Then comes the primaries, or drive and feedback windings! These are few turns, each in one layer, so not bad at all.

Wind the drive (D) winding first, of no. 18 or 20 AWG enameled, (again, Nysol wire is preferred) 15 turns, then loop

over or otherwise leave a long center-tap lead, then another 15 turns, closewound and placed over the center of the

output winding, to give a 30-turn center-tapped winding. Twist the leads from the center taps together. Secure the

winding with tape, and put a layer of Teflon over it. Now do the feedback (F) winding, in the same manner, only this

is no. 28 or 30 like the output was and is 10 and 10 turns, for a 20-turn center-tapped winding. Each 10 turn winding

of this should be centered on the Drive winding's 15-turn halves. Secure with tape and cover with several layers of

Teflon. Then overwrap the whole winding assembly with a tight layer of Scotch tape. Assemble both "E" core halveson the bobbin, with an airgap between the halves of the core. This airgap serves two functions: it limits the current

delivered to the lamp thus providing the "ballasting" function, and it varies the resonant frequency of the transformer,

depending on the gap spacing (larger gap = higher frequency due to less inductance of the core - simple!) A spacer of

the appropriate thickness, made of some kind of cardboard or plastic and inserted between the two core halves in the

center of the bobbin's hole may be needed. A "ballpark" gap to start with would be 1 mm. Your transformer is now

complete although experimenting with the gap setting is necessary and will be covered later.The details on the rest of the parts required for this project are as follows:

Q1,Q2 are the common TO-3 case style 2N3055's (don't use TO-220 types). Any power transistors of similar or(preferably) better ratings (Pdiss, Ft, Hfe, Vceo, etc.) should work, make sure they are in TO-3 cases or similar for

low thermal resistance. PNP types simply require reversal of power supply polarity. These transistors must be heat

sunk. **Update** Unless you are really lucky and your transformer is reasonably symmetrical and transistors are a

matched pair (good luck trying to find a matched pair of '3055's at the Shack, or anywhere else for that matter!)

you'll most likely find that one of the transistors will become hotter than the other. This is due to unequal sharing of

the load by the transistors, thus, I recommend getting a matched pair (or sorting through random batches until you

find two that are reasonably well matched as evidenced by both of them becoming equally warm in operation).

Q1 and Q2 should preferably be something more substantial than '3055's. Don't take too literally the published specsfor the 2N3055 as concerns Pdiss (115 w) and Ice (15 A), these are in my experience fanciful ratings that might be

achieved if you used cryogenic cooling! It seems that at 2A or so collector current per unit they will run ok if

adequately heatsunk; much more than this and you are asking for a meltdown in short order no matter how good yourheatsink is.

Nevertheless, any transistors used still need adequate thermal conducting mass in their heatsink for proper longevity

(keep in mind that heat from close proximity to the lamp is a factor, after all, heat is heat, regardless of source!) This

means don't use those pressed, aluminum sheet metal "miniboxes" or the like as a chassis/heat sink alone (without

supplementary heatsinks), it is far better to use a diecast aluminum box, which is thicker and has adequate thermal

mass- this way you don't necessarily need separate heat sinks (use the box as enclosure/heat sink), which keeps the

size nice and compact. In using the box as a heatsink, remember, the box itself (rather than the lid) has more mass, somount your transistors there. Don't forget mica washers and thermal conducting grease (RS # 276-1372)under each

unit, and the proper insulated hardware (the cases of most TO-3 transistors are common to the collectors). Radio

Shack has complete TO-3 mounting kits with mica washers, cat. no. 276-1371. With reasonable heatsinking, Q1 and

Q2 should be comfortably warm, NOT too hot to touch, after several minutes of operation.

R1 and R2 are 1w metal-oxide resistors, which come two to a card. Put the two 100 ohm units in series to get the 200

ohm value and the 10 ohm ones likewise to get 20 ohms. Wirewounds might work but I'd be concerned about theirinductive properties. *Update* Wirewounds ok for certain portions of this circuit, as for the emitter ballasting

resistors if used as will be covered later.C1-C6 are ceramic disc units, rated .01uF at 2kV.

D1-D6 are 1kV PIV, 2.5 amp rectifiers. Higher voltage (if you have) is ok, and may be preferred for some

applications (see above under "How The Circuit Works, starting circuit concept"). The current rating is way overkill,

but we're sticking with RS components, remember?

R3 may be a little small (technically it's rated for 250 Volts max.) so maybe four 2.2 or 2.5 meg. units in series

would be better (recommended).


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C7 is the one "clinker" (every project has one of these!)- you can't get it from Radio Shack. I'm not sure but probably

Digi-Key or TechAmerica might have something similar. (Mine was a "junk-box special"). Get preferably above

1kV, up to 2kV rating, or so, and NO electrolytics!! If you build the version of this circuit for mercury or fluorescent

lamps only, you don't need a 1 or 2kV unit, 600 Volts is fine, you can get away with 400-500 Volts rating, *maybe*

as low as 250 Volts (or even omit C7 altogether). Alternatively, if you desperately need C7, you *might* be able toparallel ten RS 2kV .01uF units. If you do, keep the leads short! *Update* the capacitor doesn't necessarily need to

be .1 uF, in fact a single .01 uF HV ceramic disc might do here, depending on your wattage requirements, and onT1's core airgap.

An optional component is an isolation choke in series with the +12 Vdc supply lead. This should be capable of up to

6 amps or so, and can be hand-wound on a ferrite core, of the type of material used in flyback transformers. The

value is not critical, several tens to a few hundred uH is ok. By isolating the inverter, it helps to reduce the losses a

little, and so you'll get a little more "oomph" out of it. Also, it helps to reduce "hash" and interference on a car radio,

etc., caused by the oscillator. In newer cars, it may be possible for this high-frequency "hash" to interfere with the

operation of the car's "brain", so it may be a wise idea to add a bypass capacitor, tantalum or 'lytic of about 10uF at

35 V or so in addition to this choke across the 12V input (on the supply side of the choke). It is by no meansnecessary to have these parts, though, for portable or non-critical mobile operation.

Assuming you've wired the circuit correctly, you should have no problems in powering it up. If it won't oscillate,

disconnect power and reverse the connections to *one* outer set of leads ONLY (don't do both windings, and leave

the center taps as is!) of either the drive or feedback windings.

Don't operate the circuit with no load for extended periods; it may be damaged. Do not operate with starting circuit

connected but no load (lamp). The HV output will be excesive and will probably flash over somewhere and maydamage/blow something. The lamp, if a metal halide or HPS, will start about 0.5 to 1 second after application of

power (this delay caused by charging the multiplier stack to the ignition voltage of the lamp). If a larger airgap (ofthe order of 4-5mm)is used (higher frequency, remember?) this lag is eliminated, but output watts will be less.

Mercury/LPS lamps will start instantly in any case.

The arc in many HID's (especially metal halide) may appear to "snake" or change colors or otherwise appear to be

rather unstable. However, the circuit operates the lamp stably once it has warmed up. This arc instability is normal

during warmup for high frequency operation of high pressure discharge lamps. Warmup is gradual, taking a few

minutes as in 60 Hz operation, and relatively easy on the lamp. Current drain at 50 watts output and 12 volts in is

about 6 amps. (No, this circuit is not "over 100%" efficient, although you may not need a *full* 50 w, due to the high

frequency, to get full rated lumen output from the lamp. This would result in the illusion of "overunity" efficiency,but it just ain't so!)

Modifications

This basic circuit is very versatile, not only for operating HID lamps or fluorescents, but a variety of other uses aswell, with a few basic adaptations, to be outlined here.

For wattage reduction to power different types of lamps as mentioned below and elsewhere, you have a number of

options. The most obvious and simple is to use less than 12 volts to power the circuit. You will have to determine the

right voltage for your application experi- mentally as I have not as of yet had opportunity to work up a chart showing

various outputs versus input voltage. There also may need to be changes to T1's secondary turns count if you do this.

The circuit should oscillate at 6 volts in, maybe somewhat less.

Another possibility is to change the airgap in T1's core; this will also vary the frequency of oscillation as previouslymentioned. A gap of 1mm seems to allow the most wattage and results in the lowest frequency (about 20 kHz under

load, 15-16 kHz at no or light loadings). A gap of 4 or 5mm increases the freq. substantially, (exact value not yet

measured but probably something of the order of 40-50 kHz) and reduces the output current as well (although output

voltage does not seem to be affected as much as one might think, in fact, the circuit seems to "like" to be run at

higher frequencies). The starter ignites the lamp instantly (no "lag" time) when the circuit is run a higher frequency,

although this might be due to faster charging of the diode multiplier stack. In any case, *don't* try to adjust the coregap with the circuit energized; this may put unusual stresses on the components, and could be a shock hazard as well.

Yet another current-limiting method is to vary the value of C7, this can be done alone or in conjunction withvariations in gap setting. Obviously, smaller values will result in lesser current output; make certain the voltage

rating of the capacitor is sufficient.

An additional possibility (though maybe undesirable) is to put resistance in series with either the 12 volt supply lead

or in series with each emitter lead of the transistors. The latter method is preferred, as each resistor has to carry only

1/2 the current. Also, the degenerative feedback produced when such "ballasting" resistance is introduced into the

emitter circuit is desirable on two counts; it helps compensate for a mismatched/unbalanced pair of transistors, and it

also reduces the possibility of "thermal runaway". Try a 1 ohm, 10 w wirewound, or 2 of these in parallel for 0.5


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ohms at 20 w (these are available from Radio Shack). The only unfortunate consequence resulting from the use of

resistors is that they dissipate, thus waste, some power. This may not be a major problem in certain specific

applications, however.

As mentioned above, 50w MH and 40/50w mercury lamps work interchangeably on this circuit. Starter not needed

for mercury.Output voltage measured on this circuit (raw T1 output,without starter) was 245 V when rectified with a diode (a

typical DMM won't give accurate AC readings at 20 kHz). However, it charged a 2 uF 400 Volt strobe capacitor(through a rectifier) to 520 Volts almost instantly- amazing that the cap didn't explode! Also, these tests were done

on a weak battery, a full 12 V might give more. So we'll guesstimate the average output at about 275-300 VAC with

12 V in.

35 and 50w HPS lamps may need about a lttle more than half the number of output winding turns on T1, to give

about half the open circuit voltage but possibly twice as many multiplier sections in the starter if this is done (a

sextupler rather than a tripler). Most HPS lamps need about 50-55 Volts for the arc (when operating), therefore since

the ballast open-circuit voltage should be around twice the lamp operating voltage (disregarding the needed HV

starting pulse), you might assume that you would need about half the open circuit as mentioned above. In all casesexpect to need about a kilovolt give-or-take to start any HPS reliably. At some point in the near future, I will be

posting complete spec./data sheets on this site for all types of HID's, including HPS, so stay tuned!

18 watt LPS lamps, like with mercury, do not need the starter circuit, the peak open-circuit is enough to start these

lamps. 35 watt LPS *might* need the starter, (their starting volts are a bit more than for 18-watters, of the order of

500 V); try and see. Of course, you'll need to reduce output wattage to the appropriate level for use with any

HID/fluorescent lamp type rated less than 50 watts, by one or more of the methods outlined above.Fluorescent lamps like the ordinary 40w Rapid Start (RS) and preheat 4-foot tubes, IF genuine 40 watters, (not

energy-saving 25 or 34 watt lamps so common today) will start and operate on this circuit with no mods. needed.(Although they might be slightly overdriven, this won't really hurt them). Smaller lamps like these newfangled

energy-saving things, and standard 30, 20 and lesser wattage lamps of all types will need to have the current output

to them limited by the methods specified above. This circuit is capable, with 12V input and the starting voltage

circuit, of starting ANY fluorescent lamp, even slimline! Without the starting circuit, it will probably start up to a 20

watt preheat, possibly a bit more, but I have not tried it this way on a fluorescent.

The new 35 watt automotive headlamp type metal halides (Types D2R and D2S)will work on this circuit but require

higher starting voltage due to a Xenon starting gas fill rather than the Argon used in general lighting service type

metal halides. Xenon has a higher ionization potential than Argon, thus these lamps need something like 6-20 kV toreliably be started. Their operating voltage, however, is similar to that of the general-service lamps; between about

75 to 100 volts. Either a multiplier having substantially more sections, or another method entirely may be needed to

start these lamps. That is left up to the experimenter.For powering xenon flash lamps/strobes, omit the starting circuit, as well as R3 and C7, and substitute the usual

rectifier/energy storage capacitor/trigger circuit.

To power HeNe or other gas lasers, use the starter circuit as a DC multiplier (no C7 or R3) and use enough sections

in it to give the required voltage (diodes may have to be higher PIV than the Radio Shack ones). Also, some sort of

trigger (with greater output than from the multiplier, something like 5 kV or more) may be needed to start the laser.

Don't forget the proper ballast resistor!

For certain types of neon tubing, or other miscellaneous gas discharge lamps there may need to be more sections onthe multiplier, and/or more turns on T1's secondary. Some neons can use DC, so you could then use the multiplier

without C7 or R3. In some cases a ballast resistor may be needed.

Feedback is appreciated, send comments, corrections, suggestions to [email protected]

an electronic ballast

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