I need some help with my 4 digit wemos clock.

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5 months 1 week ago #11182 by Galesloot
Hello to all.
This clock has run for 11 days according to the wemos stats without problems.
When I started the clock from another power socket, the light disappeared from the clock after maybe 1 or 2 minutes.
I immediately disconnected the power but I could immediately smell that it was really wrong.
The IRF640 had turned blue and the leg 3 was molded.
I then ordered 2 times IRF640 and 1 time MC3406. Replace this next day: HV output 5 volts. No luck.
All other parts still looked undamaged. What's next?
I also replaced Q1 with a BC557 from my own stock. HV output still the same 5V.
I found an HV generator diagram on this forum. So it was time to dust off my scope.
All tests are now done with a 9V (6lr61) block battery.
On pin 2 of the 34063 is a square wave of 71 kHz with a duty cycle of 12 usec high and 2 usec low! The same on the mosfet gate.
On pin 2 or the mosfet 5v with some very short spikes.
I would expect that the mosfet would also switch off for 1 usec. but that doesn't happen.
So I replaced irf640 one time more with no other results. Can it be the L1?
I have removed the cover from the L1 to be able to look at the wires. Looks undamaged. I would like to replace it but this part is unfortunately not available at my suppliers. Bad luck again.

So are there any sugestions I can do? Maybe I can wrap a 100uH coil on a toroidal core for testing?
Any ideas ......?
Could a fuse in the ciruit have prevented this?

Thanks in advance for your help from a retired person (74) from the Netherlands.

Best regards, Wim.
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5 months 1 week ago #11184 by Ty_Eeberfest
Am I correct in thinking that this is a Wemos Nixie clock, not one of Ian's giveaway Wemos VFD clocks?

I am not 100% sure that a 9 volt block battery can source enough current for the clock circuit to work right. For a 4 digit clock you can expect a draw of ~200mA.

Your troubleshooting method seems good. My only question is what is the amplitude of the square wave at the MOSFET's gate? I would expect about 4 - 5 volts, more or less.If it's too low the MOSFET won't trigger. If you find it is low I would suspect the 2N3906 / BC557 is partially conducting when it should not be. But you said you already replaced it once.

The frequency and duty cycle of the gate drive square wave are reasonable. The waveform is very asymmetrical because the MC34063S sees low voltage at its sense pin (pin 5) and it is trying as hard as it can to bring up the voltage by holding the MOSFET on for most of each cycle. But the voltage is not rising so it "maxes out".

I doubt that inductor is the problem. You said you see about 5 volts at the output of the circuit. If the inductor had an open winding you would see 0V.

So... I don't have much of an answer for you. Please let me know the amplitude of the square wave at the MOSFET's gate and we can go from there.

Look into it later when the dust is clearing off the crater.
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5 months 1 week ago #11185 by Ian
The 4-Digit Wemos clock has an HV generator controlled by the more or less traditional MC34063 boost converter. The controller is not involved at all in the generation of the HV, so this simplifies things.

I agree with Ty that the 9V battery will probably not work. At this point I would go back to the old power supply an use it only for a few seconds, you can easily tell if the circuit is in trouble by testing the output and the heat generated by the MOSFET. If there is a problem, it heats up enough for you to feel in about 5 seconds.

I have two additional pointers:

Please check the resistor R3. If this goes open, there is no path to ground for the MOSFET and it can't turn off. You would be able to see this as a lack of square wave at the MOSFET gate, as Ty already mentioned. I don't think that is the case here (you see spikes on the gate), but give it a pull to make sure that it is not intermittent.

The other thing is the divider made of R4, R5 and RV1. Please check that this measures out correctly.

If all else fails I suppose that it has to be the inductor L1. If it got hot enough to melt the MOSFET, it got hot enough to damage the coil.

To be sure, I'll put one in the post to you.
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5 months 1 week ago #11189 by Galesloot
Thank you both for all your help.

Yes Ty it is the nixie clock and ok no battery but now I use a newly purchased 9v 1 amp power supply.
The first supply I used was 7.3 V and I find that a bit marginal.
I took measurements again and first without the 34063 in. Vin on pin 6 = 8.5V.
Then with 34063 in and Vin on pin 6 = 4.9V now!
The mosfet becomes hot and dissipates the voltage drop.
With the scope on the gate same square wave as before but now I have looked more closely at the levels. High 3.8V and low 0.2 V.
Maybe the mosfet is good, but I can't measure it on the drain. However there are some fast spikes.
It seems as if L1 and the mosfet both act as resistors and that L1 has lost its inductive effect.
R3, R4, R5 and RV1 are ok.

It took me some time to understand what is happening here in this circuit.
It was totally new to me. Learned something again.

I think the clock is a very nice kit and when I had it ready I demonstrated it to my radio amateur friends at a local club meeting.
They thought it was a very nice little thing and bring back memories to old times.

Why the clock failed is still a mystery for me. What about a sort of a fuse somewere?

Regards, Wim.

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5 months 1 week ago #11191 by Ian
A self resetting fuse would be a good idea.

The failure could be due to one thing I am looking into. The inductors have a very poor tolerance, +/- 20% if I remember correctly. They are already at the lower end of the range for safe operation, and if you get one that is -20% (effective 80uH) that's going to cause the circuit to get hot.

I think that an inductor of 150uH would be a safer value, given the poor tolerance. With +20% that would go to 180uH, which would still be a safe value for the boost conversion.

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5 months 1 week ago #11192 by Ty_Eeberfest
I agree with Wim - it seems that the inductor has no reactance, only resistance, and a very low resistance at that. I can only guess what happened but would love to see that inductor taken apart and the failure analyzed. My guess is that it got so hot that the enamel (lacquer) burned or melted, resulting in shorted windings. With no reactance there is no inductive "kick" and so the output voltage does not come up as expected. The chip tries to increase the output voltage by shifting the duty cycle of the gate drive toward longer and longer MOSFET on time. With the MOSFET on almost all the time and the inductor offering pretty much zero impedance the MOSFET heats up because of its small but not zero internal resistance (Rds).

Well, that's my theory. I can't prove it but it's the only theory I have.

Wim, I don't know how far you dug into the operation of that circuit but I recommend getting the data sheet for the MC34063 if you haven't already done so. It gives a pretty good description of how DC/DC cinverters works and the calculations involved in designing one. It's useful knowledge since variants of these "transformerless" power supplies seem to be used in everything these days.

Look into it later when the dust is clearing off the crater.

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