Halogen Electronic Transformer Short-circuit protection confusion - Page 1
Forum section: electronics / projects

author: Buriedcode
author_rank: Super Contributor
timestamp: February 10, 2025, 09:18:31 pm
content: Hi,I have a few electnic transformers for halogen bulbs that are dead. Now, they're so cheap I just replaced them, but out of curiosity about how these work, I traced one out and simulated it.You know the kind, very efficient with a high power factor, but output ridiculously noisy pseudo-AC at 12V. Pretty impressive for such a simple circuit.However, I can't work out how - what I presume is a short-circuit protection/over current circuit - works.I have tripple checked the layout and my schem, and it simulates just fine in LTspice, but I don't see how D5 can ever conduct through R8 to trigger a fault/over current condition. From the schematics on the web, the circuit is pretty much like all the others, except they have the current sense resistor on the bottom transistors emitter, which makes sense, but this one doesn't.Am I just missing the obvious or being dense? The only curious thing that my simulation doesn't account for is the fact the main output transformer is wound on a split bobbin, with quite a large gap between windings, for what I presume to be safety purposes, so it must have high leakage inductance.

author: moffy
author_rank: Super Contributor
timestamp: February 10, 2025, 10:18:13 pm
content: Do you want to share the LTSpice file as well so others can simulate it, also the DIAC...sub file?P.S. Here is a nice article, which you've probably seen, but provides a design example:https://www.st.com/resource/en/application_note/cd00003902-electronic-transformer-for-a-12v-halogen-lamp-stmicroelectronics.pdf

author: Buriedcode
author_rank: Super Contributor
timestamp: February 11, 2025, 01:53:41 am
content: Quote from: moffy on February 10, 2025, 10:18:13 pmSure. The DIAC model file is a copy/paste of ST's own for the DB3. I originally found a model off LTspice groups.io which *seemed* ok, but the simulation wouldn't work - turns out that model had a slow rise time of about 250us - where-as the real DB3 is way less, quoted as 2us. Both had a reasonable threshold, but the ST one kick started the oscillator reliably in LTspice due to is faster response.Note: remove the txt extension as the forum doesn't allow *.sub extension. Also, the UF4007 model is built-in the the DIO file I use. Any fast rectifier with >600V should work. The original circuit used FR05.

author: moffy
author_rank: Super Contributor
timestamp: February 11, 2025, 03:18:35 am
content: I must admit that the current detection/clamping seems weird, the typical circuit is simpler.

author: Buriedcode
author_rank: Super Contributor
timestamp: February 11, 2025, 05:05:51 am
content: Yeah, I'm baffled. I kept thinking that I must have traced it out wrong, but no, that diode definately connects to the capacitive divider. The other end of the main trasnformer conects to a thermal cutout switch, which then goes to the transistor drive transformer. I even desoldered most of the parts to get a good look at the silkscreen on the component side to see the diode polarity marked there.I only posted this because I was so confused and wanted a sanity check. That and I have trouble saying "well, who knows?"

author: moffy
author_rank: Super Contributor
timestamp: February 12, 2025, 05:53:39 am
content: After a few adjustments, didn't have UF4007 so used VS-E7MH0112 instead, didn't have MJE13005, so used something similar, and the DIAC sub had an issue with a parameter, fixed that and it ran. No matter what I changed the load value to, down to 0.2R D5 wouldn't conduct in a meaningfull way, above a few 100s of uV, something is not quite right, perhaps that's why they failed in the first place?

author: Buriedcode
author_rank: Super Contributor
timestamp: February 12, 2025, 03:49:41 pm
content: Quote from: moffy on February 12, 2025, 05:53:39 amSame here! But I think I've worked out whats going on.One of the replacement units I bought were higher quality - Osram Halotronic HTM105. Pretty much exactly the same specs, 105W max, 230-240V in, 11.5V out. Thought it would be a good opportunity to see how this one works.Well, its almost identical, save for pull down resistors on the base of the switching transistors and.... that over current/short circuit protection circuit. This one also has that strange diode from the capacitive divider going to a current sense/low value resistor to "ground". However, this one has 4 1206 2.2R resistors, as two in series, two in parallel, arranged on teh PCB in a square - surrounding a 0805 thermistor.That thermister is connected to the DIAC to ground - so when this heats up, it pulls down the voltage across the DIAC charging cap, preventing it from reaching the 33V breakover voltage, so it never fires, and doesn't start the oscillator.Since both these units use this diode config, I can only assume that it relies on this diode breaking down over a certain voltage, and conducting through those current sense resistors.In the original circuit - the one I provided the LTspice file for - that uses the voltage across that resistor dfor a charge pump to charge a high value cap, that turns on a NPN to prevent the DIAC circuit from charging. In the Osram one, it uses the temperature of those current sense resistors to reduce the resistance of the termistor to achieve the same thing.You can test this in LTspice - decrease the load resistor to something silly like 1R - and check the capacitive divider node voltage (node 11 on my schem). It has oscillations from the switching, but these increase in amplitude as the load current increases. It follows the input voltage/2, but the oscillation rides on top of this, increasing its peak amplitude to >input voltage/2. With a high output current, these oscillations are substantial. On my simulation, I measure the peak voltage at node 11 to be >280V.So, I'll see if I can knock up a high voltage circuit to test the breakdown of that diode to test this...

author: Langdon
author_rank: Frequent Contributor
timestamp: February 12, 2025, 04:39:10 pm
content: What, exactly, does the ground symbol denote?Do you mean like 3rd-prong?Chassis ground?

author: Langdon
author_rank: Frequent Contributor
timestamp: February 12, 2025, 04:42:37 pm
content: Considering it broke, why are we assuming it makes sense?Maybe it's just trash (E-Waste).Perhaps some cheapo current-limiting thing.Where does power leave the circuit?

author: Buriedcode
author_rank: Super Contributor
timestamp: February 12, 2025, 05:56:21 pm
content: Quote from: Langdon on February 12, 2025, 04:39:10 pmLTspice needs a ground. It just denotes the common "ground" for the circuit - nothing to do with the Earth pin on the input, its just the negative side of the rectifier.Quote from: Langdon on February 12, 2025, 04:42:37 pmPower leaves the circuit on the secondary of mains trainsformer - L3 with a load of R9.Also, the fact it isn't working doesn't change the PCB layout or connections...

author: moffy
author_rank: Super Contributor
timestamp: February 12, 2025, 08:46:53 pm
content: That is truly weird, using the 2.2R resistors to heat up a thermistor which effectively controls the conduction angle of the self resonant circuit, by the controlled breakdown of a diode! That is a head scratcher and I have to thank you for posting it, I've never seen anything like it before.

author: Buriedcode
author_rank: Super Contributor
timestamp: February 12, 2025, 11:00:56 pm
content: Quote from: moffy on February 12, 2025, 08:46:53 pmCheers, as I said, I wouldn't normally post about these sorts of things, but its just so odd. Thought at least one other person might find it curious

author: moffy
author_rank: Super Contributor
timestamp: February 12, 2025, 11:23:21 pm
content: Using the resistors to heat up the thermistor is an unusual way to maintain some degree of voltage isolation, again wierd for why that might be necessary.

author: Buriedcode
author_rank: Super Contributor
timestamp: February 13, 2025, 12:53:12 am
content: Just so people dont' think I'm being silly:I haven't marked up everything because it'll just get too cluttered, but you can see the thermistor in parallel with an 0805 cap, which is also shorted by the SOT23 NPN when its active.Honestly, I thought the origginal cheaper unit was weird, but this Osram one is weirder, albeit much better build quality and clearances.Edit: Desoldered the diode connected to these current sense resistors - its a RGP10J. Which apparently has a reverse breakdown of... 600V.So, it looks like it doesn't breakdown, perhaps that capacitivide divider node goes negative in over current conditions? Maybe the lose coupling between the windings on theoutput transformer has something to do with it. I dunno, I give up

author: moffy
author_rank: Super Contributor
timestamp: February 13, 2025, 01:31:26 am
content: It looks like the 2R2's are thermally? bonded to the PCB as is the thermistor, running the thermistor track under one of the resistors means the voltage isolation isn't high. How would you design something like that, what formulas would you use?

author: Langdon
author_rank: Frequent Contributor
timestamp: February 13, 2025, 02:01:09 am
content: Quote from: Buriedcode on February 13, 2025, 12:53:12 amIt looks like a relatively well-made PCB.A bit like the PCB of the Beomaster 4000 I opened to dust inside.So odd.Likereallyodd.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 13, 2025, 02:06:22 am
content: I think it might be some sort of low-tech inverter perhaps.(where the lights dim according to temperature, which is common)(I'm being sarcastic)

author: Langdon
author_rank: Frequent Contributor
timestamp: February 13, 2025, 02:15:17 am
content: Is U1 a TRIAC?Where's the gate?Remeber that the positive (and negative) output of the full-wave recifier of the input do not provide normal, smooth DC, but pulsating DC!Pulsating DC is quite similar to AC.This is more of an AC circuit with elements of DC weaved in .

author: moffy
author_rank: Super Contributor
timestamp: February 13, 2025, 03:12:49 am
content: Quote from: Langdon on February 13, 2025, 02:15:17 amIt's a DIAC, similar to a TRIAC but with a defined breakdown voltage, when the breakdown voltage is exceeded it fires like a TRIAC and recovers its blocking ability when the current through it approaches zero.Di = 2 terminal, Tri = 3 Terminal

author: Buriedcode
author_rank: Super Contributor
timestamp: February 13, 2025, 07:19:32 pm
content: Quote from: moffy on February 13, 2025, 01:31:26 amI'm guessing that all the SMD parts are glued to the PCB for wave soldering - with many TH parts, and those very large TO-263 I think wave soldering is likely and that would require fixing the 0805/1206 parts to the board.So now I have two designs, slightly different, that I can't fully explain. And this all started because one unit failed - again, they're so cheap I just replace, but I like to knowwhythings fail, and I still can't find fault with any of the parts on that first unit

author: Langdon
author_rank: Frequent Contributor
timestamp: February 13, 2025, 08:00:20 pm
content: Just buy a 12V DC LED driver/PSU from like Delta or Quanzhou Smarts Electronics or Mean Well and then wire up some new LEDs to the 12V.(I'm not one of those people who thinks that LEDs are the answer for all illumination; but for low-voltage, LEDs are pretty great)

author: themadhippy
author_rank: Super Contributor
timestamp: February 13, 2025, 08:23:52 pm
content: QuoteDid you bother reading the very first post? or are you just spouting nonsense for the fun of it? The whole idea of this thread is not how to repair the device,but asking for guidance in understanding how the device actually works.

author: moffy
author_rank: Super Contributor
timestamp: February 13, 2025, 09:17:43 pm
content: Quote from: Buriedcode on February 13, 2025, 07:19:32 pmI just finished repairing a 7 chanel amplifier, AVR360, several chanels were overheating and shutting down. I spent ages looking for failed components with a multimeter, I even had good chanels to compare against but couldn't find anything. Finally I put a scope on a bad chanel and saw multi MHz oscillations, hence the heating. The fix was adding an extra compensating cap across the feedback resistor, only 3.3p. The lesson, always use a scope, and nothing had failed, component characteristics had just shfted enough to produce instability. In your case maybe some component has shifted enough to stop the oscillation without actually failing, those transformer feedback oscillators can be fussy.

author: Buriedcode
author_rank: Super Contributor
timestamp: February 14, 2025, 12:04:32 am
content: Quote from: moffy on February 13, 2025, 09:17:43 pmYeah as all the semiconductors appear "OK" (diodes/transistors tend to fail short, but sometimes open) and they test fine outside of the circuit, I agree - need to see whats happening when powered. As it doesn't output anything, or blow fuses, I'm just assuming the oscillator doesn't start up.Also what complicates things is, its all at mains level apart from the output, so I'm not going to scope it when plugged in. But I can check volages with a multimeter. Its a faff for something thats <$15 but I'm invested now

author: moffy
author_rank: Super Contributor
timestamp: February 14, 2025, 12:33:46 am
content: Pity you don't have an isolation transformer, mains with scope ground is nasty. Since its low power, if you had say two transformers say 240:15V, connect the two 15V windings together and use one to power the other, but dependent on having a couple lying around.

author: Buriedcode
author_rank: Super Contributor
timestamp: February 14, 2025, 06:59:06 am
content: Turns out I'm an idiot.When I originally got to fix this, the main filter inductor had sheered off - nice obvious fault - so I replaced it, and tested, but wouldn't work.Seems there was a crack in the solder joint of the output terminal block. When I set it on my bench to test it, I always wired a load (6R8 power resistor) and kept it flat whilst testing. Check it had continuity, but must have put pressure on the joint to make contact. When powered up, it didn't make contact properly = no load on output = no startup. So after all this I had fixed it, but didn't properly inspect the joints.Now, that doesn't make this thread pointeless, because I sitll ahve no idea how that over current (or is it over voltage?) circuit works, on either design. But at least one confusing issue is solved.

author: moffy
author_rank: Super Contributor
timestamp: February 14, 2025, 07:15:11 am
content: Really pleased to hear that you've fixed it, bad solder joints can catch anyone.

author: Wolfram
author_rank: Frequent Contributor
timestamp: February 14, 2025, 10:11:48 am
content: Referencing the original schematic, the AC voltage drop across C1/C2 is proportional to the transformer primary current (and inversely proportional to frequency). Ignoring the transformer magnetizing current, the primay current is proportional to the secondary current in the transformer. The DC voltage on the capacitor junction is given by the duty cycle of Q1/Q2, which should be close to 50 %, so this should be half the DC bus voltage. Once the peak capacitor AC voltage exceeds half the supply voltage, the C1/C2 junction goes below the negative rail and D5 is forward-biased. This drops some AC voltage across R8, which gets doubled by D3/D4/C5/C6, turning on Q4. This pulls down the voltage on C4, disabling the diac statup circuit, preventing the circuit from starting up on the next mains half cycle, and for as long as C5 keeps Q4 conducting.

author: Buriedcode
author_rank: Super Contributor
timestamp: February 14, 2025, 04:22:14 pm
content: Quote from: Wolfram on February 14, 2025, 10:11:48 amAh! Well that would explain both schematics then, its jsut the second uses resistor heating. Is there any way to simulate this in LTspice? I admit the transformer model I have has a coupling factor of 1 and just inductance, with no other parameters.No matter what load I put on there, I couldn't get the C1/C2 junction to go below the negative rail.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 15, 2025, 06:47:37 pm
content: Quote from: themadhippy on February 13, 2025, 08:23:52 pmI just meant don't buy a new halogen light system.Of course finding out how it works is cool!

author: moffy
author_rank: Super Contributor
timestamp: February 15, 2025, 09:11:16 pm
content: Quote from: Buriedcode on February 14, 2025, 04:22:14 pmI think you might need real world measurements to see what is happening, but that is tricky and a little dangerous.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 16, 2025, 12:37:13 am
content: Quote from: Buriedcode on February 14, 2025, 06:59:06 amHappens to everyone from time to time.(After hours of troubleshooting): "Oh! Turns out the fuse was actually blown, that explains why it stopped working!!"

author: Buriedcode
author_rank: Super Contributor
timestamp: February 16, 2025, 01:08:49 am
content: Quote from: moffy on February 15, 2025, 09:11:16 pmYeah I'm not prepared to scope it whilst active. It's a curiosity thats not worth screwing around with a live circuit without a HV differential probe. Perhaps if work slows down and I get bored, I'll try to fully parameterize the main transformer rather than just L and R. For now, I'll call it a day