Power supply 300Vdc / 400 mA

[Paolo Cravero]

Hello.

I got the idea on this group and now it is time to revamp it.

Electrophoresis power supplies generate DC well above Nixie needs with a current rating I would not dare to design myself.

While they are usually listed on the popular auction site, you never get a low starting price like this (UK seller):

https://www.ebay.it/itm/Bio-Rad-PowerPac-Basic-Adjustable-Laboratory-Power-Supply-150VA/402239857773?hash=item5da75d246d:g:n3MAAOSwHc9eXuqc

(Not my auction, etc etc)

I recently bought the previous release and it is up to its promises with these quirks:

1. It has a fan, so it is not silent.

2. It requires you draw at least 4 mA regardless the voltage set.

3. The output is a bit noisy (see the picture, 170V to 22kohm load), not an issue for a Nixie.

4. V/I setting is in 1V/1mA steps.

I suggest to download and read the manual before purchase. The newer unit from the link still has a fan and minimum current. 

If I connected all my Nixies I would still not hit the 400 mA limit.

Paolo

[jb-electronics]

Thanks for sharing! Does it have built-in current limiting? 400mA is quite deadly at these voltages...

Jens

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[Paolo Cravero]

Il Mar 21 Apr 2020, 21:30 jb-electronics <webm...@jb-electronics.de> ha scritto:

Thanks for sharing! Does it have built-in current limiting? 400mA is quite deadly at these voltages...

According to the manual it shuts down if it senses a change in load resistance. I haven't tried it on my unit and haven't found documented how much delta it needs. That is a good experiment for these lockdown days (looking for suitable resistors in the junk boards box).

Paolo

[gregebert]

The 4mA minimum load is livable; even at 300V it's only 1.2 watts. It's probably required for safety reasons so that you dont have an open unregulated output. In fact, a lot of switchmode supplies require a minimum load, otherwise the output goes to zero volts.

Please be VERY careful with this supply; one mistake and you could kill yourself. If you put a 5K resistor in series with the output, it will limit the max current to less than 60mA which shouldn't be enough to kill you, but will be a nasty shock. Fatal current is 100-200mA.

[Paolo Cravero]

On Tue, Apr 21, 2020 at 10:49 PM gregebert wrote:

Please be VERY careful with this supply; one mistake and you could kill yourself. If you put a 5K resistor in series with the output, it will limit the max current to less than 60mA which shouldn't be enough to kill you, but will be a nasty shock. Fatal current is 100-200mA.

Thanks for the heads up. Would you suggest a more complex current limiter that could be used in case of larger currents, with a lower voltage drop? I need to build and adapter box with the dummy load and easier accessible banana sockets, so the limiter circuit can go in there.

Thank you,

Paolo

[gregebert]

I use a simple NMOS current-limiter in my HV supply. Basically, there is a resistor on the source lead, and the voltage-drop across that resistor provides negative feedback. To set the current-limit, I have a 10-turn potentiometer to set the gate voltage of the MOSFET.

In the posted schematic, the current-limiter on the left (Q1) uses a 100 ohm sensing resistor, and limits current from 0  to 100mA, up to 180V.

The limiter on the right (Q2) is for 0 to 20mA, up to 500V and uses a 1K sense resistor. Both transistors need hefty heat sinks in worst-case operating conditions (voltage set to max, current set to max, outputs shorted).

I wouldn't risk my life with this circuit, but I did test it on my finger at 500V, around 150uA, and barely felt the current. It's perfectly safe for nixies, and I've used it to evaluate all of my valuable tubes (b7971 and R|Z568M) with no damage.