wemos 4 digit clock - Not getting 170v on High voltage

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6 months 4 days ago #10308 by Farsider
Hi Ian
Thanks for the quick postage of my 4 digit wemos clock.
I’ve looked in the forum but can’t find any answers.
I have built the low voltage stage and tested fine and have also built the hv stage.
When I test this I’m only getting about 74v at the 170v test point and the mosfet seems to be getting hot.
The most I can get is between 68-74v when adjusting the pot on Rv1.
I’ve checked component installations and they all seem ok.
Power supply is a 9vdc 1.66A switchmode regulated powerpack.
Unloaded, the power pack provides a 9.05v output when tested.
I don’t have access to a power supply to limit current so I’m wondering if the amperage of the power pack is causing the mosfet to heat up.
I have also tried a 9V DC power pack unregulated (1A) and have the same results.

I can’t see any issues with solder joints or bridging either.
Hope you can help me.

ps: I noticed that I don't have capacitor c9 included in the kit. I can't see when its supposed to be installed in the construction manual but its included in the BOM at the end of it. Do I need this part?
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6 months 4 days ago #10309 by Ian
The construction looks very good from what I am able to see, so I don't think it is anything to do with poor soldering.

The MOSFET getting hot is a sign that it is spending too much time in the linear region, whereas it should be switching cleanly and not producing any heat. Normally it should run barely warm.

A couple of things to (double) check:

Please check that the T0-92 device Q1 is a 2N3906, not an MSPA42. The 2N3906 is responsible for pulling the inductor down cleanly and decisively, thereby causing a voltage spike that can be rectified into something useful. If you don't have this, we'd be getting the symptoms we are seeing: (hot MOSFET, HV not up to spec. Just a note here: the 34063 is an open-emitter device, and it can turn an external device on sharply, by doesn't have a path to earth in order to discharge the gate capacitance if the 2M3906 is not there. This is my best guess at the moment.

Make sure that the 1N5819 and the UF4007 are in the right locations. D2 is the 1N5819, the others (D1, D3) are the UF4007. I can't quite make it out from the picture.

Please remove and re-seat the MC34063: I have had some problems with tarnished pins, and although I don't think this is a problem here, let's remove the doubt.

If you happen to have an MC34063 laying around, could you try swapping it out?

If this doesn't help, would you be prepared to return the board (at my cost) for investigation. It's a very new kit, and I'd be interested to see what happened.

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6 months 4 days ago #10310 by Ian
C9 is totally my bad: The parts list was wrong.

Do you have anything you could throw in there? It must be able to withstand the input voltage comfortably, so in your case 9V. More capacity is better and a low ESR is even better than that. If you don't have anything, I will throw a couple in the post for you.

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6 months 3 days ago #10311 by Farsider
I’ll check D1,2&3 and reseat the 34063.
I do have a 1000uF 25v and a 330uF 25v cap in the drawer. Will either of them be suitable , otherwise The cap c9 I’ll just get from the local electronics store.
Will c9 have any effect in this not reaching voltage?
Thx

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6 months 3 days ago #10312 by Ian
Most important on reflection is to check that the 2N3906 is right.

Either of the replacements you have is fine, as long as they fit physically. I'd probably for for the 330uF one, but the exact value is not that important.

The missing C9 might have a minimal effect, but I wouldn't expect it to cause an issue like you are seeing. I suppose it's possible that it might contribute to the stability of the circuit. Please try putting C9 in place first and then we can see if that helps. It might be that we both learn something with that. ;)

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6 months 3 days ago #10313 by Farsider
No luck Ian
C9 hasn’t changed anything
Not sure what to do now.
Did you want to send a kit out and I’ll return this one to you for investigation?
If you think it’s worth it I can go and get another mc34063.
Cheers

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