IRF840 Overheating

1 year 4 months ago - 1 year 4 months ago #9618 by ChristineNZ
I'm new to building the Nixie Clock Rev 4 kit.

I built the kit as per the Construction Manual and initially the clock kit powered six IN-8-2 Nixie tubes correctly with
no problems. I added the duel Neon Lamp option, powered from the HV OUT pin, instead of the LEDs. This change appeared to work. I left the clock running for about two hours, when I returned all Nixie tubes where dark. Turning the power off and on again I found that no power was going to the Nixie tubes and within 30 seconds the IRF840 became too hot to touch. Testing the test point on the board, originally correctly showing 170V, it now showed the input voltage of 8V.

I purchased a second Rev 4 kit, and replaced the original MEGA8-P chip with the new one, no difference. I have checked all solder connections, tested all resistors, and visually confirmed that components are in their correct locations and for the capacitors, and diodes correct orientation. None of the capacitors/resistors show any signs of distress.

I have attached a photo of the board. The low voltage side of the circuit appears to work correctly, I can hold down the reset button at power on and the LED will flash and then start cycling around its colours. If I remove the MEGA8-P chip the IRF840 does not heat up, however of course there is no 170V at the test point. I have removed all Nixie tubes and Neon Lamps, again this made no difference.

I believe the IRF840 is not being switched on/off correctly.

Please help, any help would be greatly appreciated.

Last edit: 1 year 4 months ago by Ian.

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1 year 4 months ago #9621 by Ian
Replied by Ian on topic IRF840 Overheating
Sorry to hear that it failed so quickly: This is a bit of a thing with components, either they die quickly or they last a long time.

Just before we start, was the IRF840 running very hot while it was running?

So, if you hold down the button while powering on, this should set the clock back into test mode. It will flash the back light colours wildly for a couple of seconds, and then run the HV generator for 60 seconds before shutting it down.

I read that you did this, but I couldn't see if the result was overheating or not. Is it overheating in test mode? Is there HV in test mode?

I would simplify things for a moment while we figure out what is going on.

Please remove the K155 and the opto isolators. Please also temporarily unhook the neons. This will have the effect of isolating the error either onto the board (which I expect) or onto the external components (less likely).

Please try again with test mode. Do things get hot? Is there HV?

If we have HV now, we will have to figure out where on the external wiring the issue is.

If there is no HV and things still get hot, I suspect either the 400V capacitor, the UF4007 or (as an outside shot) the controller (unlikely, given that you have a replacement).

Is the 400V cap bulged on the top in any way? Does the UF4007 look suspicious in any way?

Let's see how we get on with those couple of steps and then we are close to the solution.

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