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Normally I would just go and breadboard the design out, but since this is using mains power, I would rather ask experienced people first.

I want to build a desktop 170V DC power supply to fiddle around with nixie tubes. I came up with this circuit: Original diagram

The Transformers will be(looking at the cheaper end parts, both will be around 7-10 VA):

  • T1 : 230V -> 9V
  • T2 : 230V -> 12V but in reverse.

The capacitors will be:

  • C1 : 1mF 25V
  • C2 : 47uF 450V

So will this circuit work and how should I improve it?

EDIT:

OK, here is an updated circuit taking into account all the replies. Updated circuit diagram
I missed a lead on that regulator, also a resistor on the HV warning led, ignore, please

Some highlights of modifications:

  • 7.5k Bleeder resistor, which should dissipate the power in a second or so after turning the HV off
  • L7805 regulator for keeping the +5V clean
  • HV on warning LED
  • Fuses on all positive terminals
  • It will be in either a plastic or wooden enclosure, to eliminate risk of touching anything inside
  • All work on the circuit will be done after the HV is turned off, the warning LED unlit and voltmeter reads zero on panel
  • T2 will be 18V

  • output terminals will be this kind or similiar:
    Banana plugs

  • all connections inside will be maniacally heat-shrink tube wrapped to counter any chance of a short

  • will probably etch a PCB for that
Limiter
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    I'd add a bleeder resistor across C2, so its charge will dissipate to a safe level in a few seconds. Otherwise you might get a shock long after you switched off the mains. And note that, even with isolation form the mains, this is a potentially deadly circuit! – Wouter van Ooijen Sep 09 '14 at 12:56
  • Thanks, also I was thinking, maybe I could move the high voltage switch before T2, so I could use this as a 9v PSU as well, feasible? – Limiter Sep 09 '14 at 13:31
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    I don't like having the only bleeder resistor be switched. I would feel safer having the HV warning LED were powered from the high voltage supply via resistor, and there was also a bleeder resistor directly across the cap. If the LED lights up when the circuit is on, that will mean that it's should work to discharge the cap when the circuit is switched off. If it doesn't light up, unplug everything and wait long enough for the "direct-across-the-cap" resistor to discharge everything before poking around the circuit. – supercat Sep 09 '14 at 21:39

3 Answers3

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I don't agree with the accepted answer. As to your questions:

  1. No it is not completely safe, since it involves high voltage, but it won't burn up unless you short it, and it does provide galvanic isolation so it's less likely to be in a position to deliver a lethal shock.

  2. You will get about 11-14V at the point marked "9V". Too low to be reliably regulated down to 9V and a bit high for (linear) regulation down to 5V (but okay for 5V if you don't draw much current).

  3. The point marked 170V will actually be at about 240VDC with a light load. That's because the AC voltage is RMS so you get about 1.4 times higher. You're using a 450V cap so it won't hurt the capacitor. You might be better to use an 18V transformer.

  4. 1000uF/25V means you if you draw 100mA you'd get about 1V p-p ripple on the 11V line, which is fine for regulation down to 5V.

  5. Nixie tubes typically take a few mA. If you have (say) 12mA, the ripple on your 240V line with 45uF will only be a few volts, which is quite acceptable.

I agree that (say) 200K 1/2W bleeder resistor across the 45uF would be a good idea, and a suitable fuse on the input (there is no need to make the fuse more than a few hundred mA).

Take appropriate precautions when dealing with high voltage. Typical precautions include, but are not limited to, not probing the circuit when power is on, using appropriately rated test equipment, keeping everything neat, using a GFI/RCD, discharging the capacitor manually before touching things, even if you think there is a bleeder, working with (at least) one hand in your pocket, etc.

Spehro Pefhany
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    I usually work with both hands in my pockets. Accident count is almost zero, since I've started doing that... Can't continue typing, gotta go back to work. – DerManu Sep 09 '14 at 15:31
  • I don't think a GFCI will help him with the output floating. The return should be earthed and the fuse put in the proper place (the line / hot input). – Adam Lawrence Sep 09 '14 at 15:37
  • @AdamLawrence If he touches the fuse, fuse holder, input terminals or the 9V transformer primary it might save his life. – Spehro Pefhany Sep 09 '14 at 15:39
  • I'm not sure he appreciates the risks involved with playing around with a +170V DC output. The GFCI will of course protect inadvertent contact with the mains. If he's putting this in some sort of box, an earthed output return should be part of the safety consideration. – Adam Lawrence Sep 09 '14 at 15:42
  • I might wear rubber gloves when testing. I am planning a wooden enclosure or similiar with only the terminals with no bare contacts on the front. I will edit question with updated design. – Limiter Sep 09 '14 at 20:39
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You may not find a transformer which outputs 170V from either 9V or mains. In this case, use two transformers. Connect primary sides parallel, secondary sides in series. Resulting voltage will be sum of secondary voltages of two transformers. But you have to be careful on the polarity of the secondary sides.

You are using only a capacitor for filtering. Notice that there will be to much ripple in your DC voltage. No problem though, if it is OK with your project.

You may add a fuse to the DC sides as well.

hkBattousai
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  • Thanks for reply. Should I use a regulator then as the capacitors will not smooth things enough? Sorry for beginner questions, I have the passion, but not the experience here. I will add fuses there too :) I haven't bought the parts so I wil look for a step up to put there. The PSU will be used mainly for experimenting with nixies, but in case I decide to use it for other things one day then I will definitely improve it to be versatile. – Limiter Sep 09 '14 at 12:22
  • If you need an accurately regulated voltage, always use a proper regulation circuit of some sort. Transformer voltage will vary (sometimes quite a lot) with load, so if your "9v" connected circuit can't handle, say, 4.5v to 18v input, you should put a regulator in. – John U Sep 09 '14 at 12:30
  • I'll be sure to add a regulation circuit then. I will probably make it my first project on a PCB, seems like a great project to advance from veroboard. How thick should the traces be? – Limiter Sep 09 '14 at 12:34
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    I've never used a nixie tube before. But I don't think that any regulation is needed for it, or for the LED. For regulating the 9V, you can use [7809](https://www.fairchildsemi.com/pf/LM/LM7809.html). Regulating 170V is a lot harder. Don't bother it for now. By the way, if you need the 9V only for the LED, you should know that you can light the LED from the AC 220V just with a capacitor and a diode; [Ex1](http://www.marcspages.co.uk/tech/6103.htm), [Ex2](http://www.instructables.com/id/LED-on-AC-Mains/) (See rhughes19's 7-month-old comment with pandyaketan's corrections under it). – hkBattousai Sep 09 '14 at 12:47
  • Thanks, I will regulate the 9V because I use it also for driving the nixie logic e.g arduino or similiar. But great to know about driving leds on mains – Limiter Sep 09 '14 at 12:52
  • One more thing: do I need to add a dummy safety load to the transistor as well? – Limiter Sep 09 '14 at 12:53
  • Transistor? You mean 7809? It is an IC. You don't need any dummy load for it. But you can connect a dummy load resistor (bleeding resistor) between values 100k\$\Omega\$ and 1M\$\Omega\$ if you want to. – hkBattousai Sep 09 '14 at 12:56
  • Sorry my mistake, I meant transformer – Limiter Sep 09 '14 at 13:05
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    Your first paragraph is bogus. There's no such thing as a "transformer designed for step-up" that's distinct from a one that's "designed for step-down". For example, if you take a 120V:12V transformer and drive the low-voltage side with 12VAC, the magnetizing current will be 10x the current used when driving the high-voltage side with 120VAC, just as you'd expect. – Dave Tweed Sep 09 '14 at 13:10
  • @Limiter No, you don't need a dummy load for the transformer. Dave: Step down transformers have lower self inductance on their secondary winding. If you want to use a 120:12 transformer reversely with the same magnetizing current, you need \$\sqrt{10}\$ times more turns on the both sides; and the transformer core may need to be redesigned as well. I meant this in my answer. – hkBattousai Sep 09 '14 at 13:20
  • Another question if I may, since I decided to regulate the 9VDC with a variable regulator, would a potentiometer voltage divider on the 170V output be safe? – Limiter Sep 09 '14 at 13:42
  • Why do you feel that the current needs to be the same at both voltages? The magnetization of the core is the product of the current and the number of turns. 1/10 the turns requires 10x the current. Why do you consider this to be a problem? – Dave Tweed Sep 09 '14 at 14:24
  • @Limiter No. Using a voltage divider is a very bad idea; and yet it is impractical. Voltage divider is not something to be used for creating power supplies. Dave: I think you are right at this discussion. I don't want to insist on something I'm not completely sure of. I will edit my answer. – hkBattousai Sep 09 '14 at 14:29
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The 170VDC output is no joke. It is lethal and not to be toyed with. In fact, you need to take measures to ensure that it cannot be touched.

Consider a mechanical enclosure around your nixie tubes, something like a hinged plexiglass box with a mechanical 'dead man' switch. When the cover is down, power is applied and the nixies can do their thing. When the cover is up, the power is disconnected and some bleeder circuit is connected to discharge that big 450V capacitor in the secondary.

We do this sort of thing in production enviromnents - our initial power-up fixtures are exactly as I describe (a clear box with a contact switch that cuts the power when opened).

Adam Lawrence
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  • Thanks, I was going to put it in a box and only touch the circuit when the PSU is off, hence the 170V switch, will think about the enclosure – Limiter Sep 09 '14 at 18:59