Debugging a high voltage nixie driver - Page 1
Forum section: electronics / projects

author: roli
author_rank: Regular Contributor
timestamp: March 15, 2015, 01:53:06 am
content: I am yet again turning to you with some issues that I've been having.Basically I am trying to build a nixie clock. And before I do that I need a nixie driver. I've come across two very similar designs:http://www.instructables.com/id/Nixie-Tube-HV-Driver/http://blog.xyzio.com/2011/09/05/a-low-cost-nixie-tube-power-supply/I plan on using the second one for my final design. But I decided to build the first one (since it has less components) to test everything on a breadboard before I design a PCB. But I've been having some issues. Basically - the voltage that this thing outputs is too high. It should output somewhere around 200V. Instead I get about 380V out of it - and it starts falling slowly if I keep the thing on. And I can't figure out why. Adjusting the trimpot seems to make no difference.The first time I tried it, I was using some not properly rated resistors at the output and they started producing smoke. I then replaced them with properly rated SMD ones soldered to a couple of wires. These seem to be fine. Nothing seems to be getting hot (the mosfet gets a bit warm but not much) except the output capacitor which is rated for 250V - probably has something to do with the output voltage being too high.Any ideas on where the problem could lie? I don't really know much about power supplies like this. I know the basic idea behind how they work but not much more. Certainly not enough to diagnose a problem with one.

author: Psi
author_rank: Super Contributor
timestamp: March 15, 2015, 02:04:42 am
content: I would check the BC547, the pot and the 220k are still ok after the smoke issue. They provide feedback and shuts off the 555 when the output voltage matches what the pot is set for.If the transistor failed it might be running with no feedback and producing the highest voltage it can.Or if you accidentally used a 220R resistor things would fail as well.The fact that you say the pot does nothing confirms you have no feedback.Simple switchmode psus tend to either work fine or explode. So any smoke and you have to start again and confirm what has died and what is ok.

author: roli
author_rank: Regular Contributor
timestamp: March 15, 2015, 02:21:20 am
content: I tried replacing the BC547 and something new is happening now. The output is now either 8.5V or 380V depending on how far I turn the trimmer. And when I reach the point where it drops I noticed something else. The voltage drops to about 100-240V, pulsates around for a second or two and then drops to 8.5V and stays there.The resistor is 220k. I checked again. Though you did remind me of something - after the burning resistor issue I replaced the combination of 2.2k resistor and 100pF cap with a combination of 1k resistor and 47pF cap - the values used in this place with the second design. Simply because I didn't have any properly rated 2.2k resistors.EDIT: I've also tested the mosfet with my multimeter. And it seems to be working.

author: Psi
author_rank: Super Contributor
timestamp: March 15, 2015, 03:20:18 am
content: If the pot has been melted inside you might get a weird scale where the output changes wildly.

author: TerminalJack505
author_rank: Super Contributor
timestamp: March 15, 2015, 04:31:52 am
content: You can test to see if there is problem with the pot by removing it and putting a 1k resistor in its spot. Take the feedback to the BC547 from the more-positive terminal of the resistor (the point between it and the 220k resistor.) This will basically configure the circuit to produce the lowest selectable voltage--about 130V to 140V.It looks like there's a bit of a bug in the circuit. This is what may have bit you. There should probably be a resistor between the ground and the pot. That way, when the pot is at its minimum setting, it will keep the output voltage at a sensible level. Or maybe the circuit designer had a minimum load requirement to keep the voltage level down.

author: roli
author_rank: Regular Contributor
timestamp: March 15, 2015, 12:40:25 pm
content: You were right. Thanks! I replaced the pot with a 1k resistor and yes. The output was 141V. Strangely enough I didn't ever see the pot getting hot. I combined a few resistors together and I am now getting the desired 200V.I've noticed that the other schematic has a 470 ohm resistor there. That should probably work?Maybe another question related to this design... Can I use FMMTA42 in place of the BC547 in the final design? I am using them throughout the circuit anyways and it would uncomplicate the part list a bit.- To answer my own question - yes, FMMTA42 works fine.