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Topic: DC dimmer circuit (Read 4233 times) previous topic - next topic

Runaway Pancake

I cannot find anything on a "TC2249": DigiKey, Mouser, startpage, google.
Any you're referred to it as such, "TC2249", repeatedly.
I couldn't find a datasheet.
It's a TC4429 - makes a difference.

You'll probably be terribly insulted, that I'm trashing your creation, and that's wholly not my intention, but I take particular issue with your grounding in addition to other aspects of your wiring and construction technique.

I think that you should work out this "DC Dimmer" disassociated from the other circuitry: just work out the dimmer aspect then you can bring in that other stuff.

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dc42

A few thoughts:

1. I can't see any wires connected to the drain terminal of the mosfet in your photo. Was the load actually connected during your testing? If so, how was it connected (in particular, where was it connected to the ground line), and how much current does the load take?

2. What is the value of the decoupling capacitor connected between the supply and ground pins of the TC4429?

3. What is the nature of the 12V power supply, and what is its off-load voltage? I'm wondering whether it produces a temporary voltage reversal during startup or shutdown, or alternatively that it produces a lot more than 12V when the load is not energised. Assuming you didn't short the output of the TC4429, there must be something nasty happening on the supply pins to cause it to fail, and this would likely cause any other driver chip to fail too. The Arduino is protected to some extent by the diode you put in series with the Vin pin.

4. Why are you using a switching frequency as high as 32KHz? The higher the switching frequency, the more careful you need to be with circuit layout, to keep the inductance of critical paths low.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Pokey

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It's a TC4429 - makes a difference.


Well I got the numbers switched and then kept copying it. :~

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You'll probably be terribly insulted, that I'm trashing your creation, and that's wholly not my intention, but I take particular issue with your grounding in addition to other aspects of your wiring and construction technique.


I'm not insulted. I am still very much a beginner. I know the circuit isn't pretty but I do make sure all the solder connections are very strong. I am grateful for any criticism.

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1. I can't see any wires connected to the drain terminal of the mosfet in your photo. Was the load actually connected during your testing? If so, how was it connected (in particular, where was it connected to the ground line), and how much current does the load take?


There was no load during my most recent test.

In a previous test, with the first TC4429 that I burned, it was connected to the 8 amp lighting. The ground lead on the board was connected to the vehicle ground. I do not know exactly where the ground wires go inside the vehicle, but I do get good continuity between my point of connection and the lighting ground.

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2. What is the value of the decoupling capacitor connected between the supply and ground pins of the TC4429?


1 ?F, 50v.

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3. What is the nature of the 12V power supply, and what is its off-load voltage? I'm wondering whether it produces a temporary voltage reversal during startup or shutdown, or alternatively that it produces a lot more than 12V when the load is not energised. Assuming you didn't short the output of the TC4429, there must be something nasty happening on the supply pins to cause it to fail, and this would likely cause any other driver chip to fail too. The Arduino is protected to some extent by the diode you put in series with the Vin pin.


My testing power supply is a switching transformer. It outputs a constant 11.9v, with or without load (load being the Arduino in this case, it is not powerful enough to power the lights). I do not have any reverse voltage protection between the power in and the TC4429, and after sleeping on it, I am thinking that may be the problem. It was working for a while with the 32KHz PWM. When I came back the next day and powered it up it had failed. So maybe it is something to do with power cycling.

In the vehicle, where the first TC4429 failed, the power is just a 13.8v lead.

Any switch can cause reverse transients can it not? I just didn't think of that before.

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4. Why are you using a switching frequency as high as 32KHz? The higher the switching frequency, the more careful you need to be with circuit layout, to keep the inductance of critical paths low.


To eliminate a disturbing hum which these lights make when pulsed. I had the dimmer working before with an IPS6021 high-side switch (I really like them), but they have a maximum frequency of 1.5KHz. At that frequency the lights sound like a horde of mosquitoes.

I have calmed down since yesterday's bitter disappointment.I will order a couple more TC4429s (maybe I should get a reel!) and put reverse protection diodes on the voltage inputs.

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To avoid such transients, add a decoupling capacitor between the -ve side of the LED and the source of the mosfet.


Between pins 1 and 8 of the TC4429, correct?

Any other suggestions would be very much appreciated. I really do not want to take any more chances, I have learned my lesson. :smiley-red:

Thanks very much.

dc42


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To avoid such transients, add a decoupling capacitor between the -ve side of the LED and the source of the mosfet.

Between pins 1 and 8 of the TC4429, correct?


No, it should be much closer to the mosfet source and the ground side of the LED than that, so that the switching transients don't propagate to the rest of the circuit. I would use 1uF ceramic in parallel with at least 100uF electrolytic.

I can see how the TC4429 might have been damaged when you had the load connected. Without that decoupling capacitor, the inductance formed by the long wires to the power supply, together with the existing 1uF capacitor between pins 1 and 8 of the TC4429, form a resonant circuit. This is excited when the mosfet turns on or off. The resulting oscillation may cause the supply voltage on the TC4429 to exceed its 20V rating, or to go negative.

However, this doesn't explain how you managed to damage a TC4429 when there was no load connected - that puzzles me.

Btw when you disconnect the power supply, the presence of the supply protection diode means that the power supply capacitors in Arduino will briefly supply a burst of current through the output pin and thence through the TC4429 to whatever else is connected across the supply. To avoid this, I suggest connecting a resistor of about 220 or 470 ohms between the Arduino output pin and the TC4429 input.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Pokey

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No, it should be much closer to the mosfet source and the ground side of the LED than that, so that the switching transients don't propagate to the rest of the circuit. I would use 1uF ceramic in parallel with at least 100uF electrolytic.


I see, (I assume "the LED" refers to the 8 amps of DC lighting) I will do that.

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However, this doesn't explain how you managed to damage a TC4429 when there was no load connected - that puzzles me.


Does that suggest the power supply disconnection causing transients then? If you are puzzled by this, imagine how I must feel!

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I suggest connecting a resistor of about 220 or 470 ohms between the Arduino output pin and the TC4429 input.


I will certainly do that.

Thank you!

dc42


I see, (I assume "the LED" refers to the 8 amps of DC lighting) I will do that.


Yes, that is what I meant.

It's just possible that the failure of the TC4429 was caused by a parasitic SCR in it being triggered when you removed the power (although I feel I am clutching at straws here). If so, adding the resistor should avoid that (maybe use a slightly higher resistor than I suggested previously, say 1K).
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Pokey

Phew! It is finally working!

Thank you dc42, for patiently putting up with me!

I added:
A secondary mosfet in parallel (unrelated to the problem) to distribute the amperage draw,
two decoupling capacitors (100?F and 1?F) to the source of the mosfets,
a reverse protection diode to the power input for the whole circuit,
a 1k resistor between the Arduino output and the TC4429 input,
and I reduced the pwm to 25kHz just in case.

It works beautifully!

THANK YOU once again!

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