HV Power Supply failues

[JBro63]

Hi all, looking for insight in to PSU problems. Have had two NCH8200s fail very close together after only a few weeks of use.

In December, I finally got round to building four IN-12 4 tube clocks using a new PCB and some leftover tubes and drivers. They are driven using a K155ID1 for each tube (no multiplexing) and MJE340 for the INS-1 colons. Current limiting resistor is 18K. Each tube anode has a PC851 optocoupler to turn the tube off if no presence is detected.

My previous IN-12, IN-14 & IN-18 clocks use the 8200 or 6300 and are still going strong. They are driven using HV5530s.

Both 8200s have gone full short across the input. Anything about the K155ID1 / PC851 that is harsher on a HV supply? Anything I can check for with a scope?

Thanks

[Max DN]

You must have acceded peak current and the mosfet is gone. If they are both gone, no point repairing or replacing with same HVPS, it’ll happen again.

If interested, I have a much more powerful power supply for sale here:

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[JBro63]

Thanks. They look similar in output to the 6300 but that would need a new PCB :(
Curious what the neon is for? Power indicator?

The 8200 is rated for 30ma - I measured 9ma on the prototype when it's running 4 tubes, 13ma for 6 tubes so it should be within limits.

[Max DN]

Yes, I figured out electronics and windings of the transformer as I wanted to embed it in my nixie clock design. Yes, the neon is just for fun, as an indicator that HV is on.

In my experience 8200 can take 30ma but not for long as the transformer is very small and gets hot (I may be wrong). I think that even if you are only using 13ma, the peak current (and in-rush current especially at start up matters so much). If you have a way to check that, you’ll see. If you don’t have an oscilloscope, try to use a power bench supply and limit the current to, say 50mA. It’s a cheeky way to test peak current at start-up, not accurate but gives you an idea. The power supply will enter into an overcurrent mode, at least for a few seconds until the current stabilises within the transformer. There are ways (hardware and software to minimise in-rush current, but it’s not very simple and requires plenty of testing).

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[Richard Scales]

Hello,

I had a number of NCH8200HV failures though specifically, this was when I was using a 12V power adapter, I have many more boards using a 5V supply and none of these have failed.

I went straight to the designer who took a look at my board and came up with an answer.

l  Root Cause: Catastrophic MOSFET (NCH8200HV) failure observed across all modules.

l  Power Delivery Concerns:

The 12V input bypasses critical bulk capacitance prior to distribution.

Current PCB layout lacks immediate electrolytic buffer post power input

l  Critical Design Recommendations

1. Capacitor Placement Protocol:

12V rail must implement bulk electrolytic capacitor (≥470μF) within 10mm of input connector.

Since then I have added a 470uf electrolytic as suggested and have had zero issues.

What input voltage are you using: 12V, 5V, something else?

 - Richard

[Jeff Walton]

Protect your device by adding this to the input.  Reduces noise, spikes and protects the NCH8200HV.  The HV module is sensitive to spikes.  Wall-wart power supplies are not all created equally.  No failures since adding - had a couple nixie clock failures with the NCH8200HV before doing this. 

Jeff 

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[gregebert]

I admit I'm stubborn, and all of my nixie clocks use linear power supplies, EXCEPT one, which was a nixie watch. Getting that booger to work in a small volume was a lot of work and it never will run as reliably as an old-school linear supply. The HV supply for my NIMO clock is also a switcher, and has a ridiculous amount of control on it (yep, software can specify how many kilovolts it wants to throw at the NIMO tubes, and can check the temperature of the MOSFETs driving the converter).

I've never had any sort of failure with the linear supplies, and I have seen one DCDC converter fail (one of those direct-replacements for the TO-220 LDO regulators). I really like how efficient they are (no heat sinks!!), but they have not yet proven themselves to be as durable. The latest rev of my main board for clock projects has A/D converters so I can have software monitor the low-voltage power supplies (most of which are derived from some sort of DCDC converter), and I'm planning to add something so that the nixie supply can also be monitored.

[JBro63]

Many thanks all.

I have the caps available and will source the TVS and try that.

@Richard - yes 12v 1amp cheapo power supplies from Amazon. Ironically I went with 12v as my simple logic assumed the 8200 would have to work less hard. On a bench supply the clock draws about 250mA.

Can I ask which drivers you're using? I'm curious if the K155ID1 is a factor. My first ever clock using these had a different PSU and ran without issue. This is the first time I have combined the driver and 8200.

I saw reference to a bleeder resistor (150K 1W) and 33uF 250v cap across the HV output on a DC-DC boost circuit. Could this afford additional protection to the PSU?

Thanks

[newxito]

Having to add a 470uF electrolytic somewhat defeats the purpose of such a small power supply. I only used this PSU once for a flat clock base and it's working perfectly so far. I used 4 x 47uF MLCCs in parallel to keep the hole thing flat.

[Richard Scales]

I would tend to agree with the thought that using 12V means it has to work less though it seems the various spikes that can come from a 12V wall wart can sometimes be all too much - hence the cap (as well as optional TVS etc as Jeff suggested).

I use the Microchip HV5XXX drivers like HV5522 etc which demand a 12V supply and logic levels (Absolutely NOT my intention to start that particular discussion regarding using them at lower logic levels!).

I cant really comment on the protection of the output - my suggestions are based purely on the recommendation of the original designer. It is a fact that I have never experienced any issue when using them with a 5V supply and I guess I could pick up the 5V rail from the 5V buck converter that I already have installed but to go from 12V to 5V just to get back to 170V seems somewhat counter intuitive.

- RIchard

[newxito]

By the way, my attempt to make a similarly flat nixie power supply failed miserably. I tried using the CJ5143-ALC flyback transformer which is 4.6mm high. It worked somehow but I was never happy with my design.

[David Pye]

I'm moving to try to power mine from usb-c as there are a range of autonegotiation boards that can request supply voltages from 5-24volt from a suitable PD PSU.

David

On Mon, 16 Feb 2026, 14:08 newxito, <axt...@gmail.com> wrote:

By the way, my attempt to make a similarly flat nixie power supply failed miserably. I tried using the CJ5143-ALC flyback transformer which is 4.6mm high. It worked somehow but I was never happy with my design.

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[Richard Scales]

I've just tried one of those and for sure it works though I don't think that USB-C with PD capability is as ubiquitous as some suggest. 

For myself I prefer to distribute 12v from  beefy 10A supplies to individual clocks but that's only really because of the number I like to display. 

For regular folk I can see that  USB-C-PD could work well. 

- Richard

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