Re: Regulating High Voltage
From: CFoley1064 (cfoley1064_at_aol.com)
Date: 11/15/04
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Date: 15 Nov 2004 20:05:43 GMT
>Subject: Regulating High Voltage
>From: "Mike" mike@notemail.com
>Date: 11/15/2004 5:55 AM Central Standard Time
>Message-id: <cna5g4$c7o$1@news6.svr.pol.co.uk>
>
>Hi,
>
>I'm trying to design a high voltage DC power supply, which will be used for
>charging up a capacitor. To start with I only want an output of 400V DC.
>The supply voltage to the supply will be around 10V AC.
>
>My current design consists of a simple step-up transformer (10V AC to 400V
>AC), a bridge rectifier (AC to DC), and a smoothing capacitor (reduce the
>ripple). With this design (I think) I should be able to get a high voltage
>of something around 400V DC. As it's used to charge a capacitor I don't
>want the voltage to be any higher than 400V. But I don't want the voltage
>to be much lower than 400V. So I guess I need someway of regulating the
>output. It seems a zener diode is the way to go, but I can't find any that
>will regulate 400V (only 200V).
>
>So my question is, does anyone know a way to regulate the output voltage
>without using zener diodes? I could add / remove turns from the transformer
>until I get the correct voltage, but this is far from ideal (not to mention
>tedious).
>
>If it makes any difference, the current requirements of the supply are 10mA
>at 400V.
>
>Thanks for any help,
Hi, Mike. Hmmn. Quick 'n' dirty cap charger. 400VDC, 10mA max, 10VAC from a
transformer available.
Before you start, remember that you're dealing with potentially lethal voltage
here, and you should observe appropriate safety precautions.
Also, you should be aware that there are good isolated, current-limited high
voltage power supplies available at hamfests, ebay and such that will give you
exactly what you want with no hassle and at good prices. Just provide the 40K,
10 watt series resistor (use 2 ea. 20K, 5 watt in series) and you're good to
go.
Also, if you happen to have a constant voltage transformer handy, you might not
need a regulator at all. Just set up your transformers and rectifier, and
tweak a voltage divider in using a "Dividohm" power resistor and another diode
to make it so your peak no load voltage (cap fully charged) is 400V.
Having said that, the prior post mentioning zeners is also a good idea,
although I'd probably use a string of 10V and 12V 1 watt zeners (1N4740,
1N4742) to do the job. Of course, I happen to have a small drawer full of
these. Use any 1W zeners below 40V. That would allow me to tweak in final
voltage to within a couple of volts of where I wanted to be, and these zeners
are commonly available. If you've got access to It would look something like
this (view in fixed font or M$ Notepad):
____ T1 ___ ___ ___ +
o-|_--_|---o--. ,---. .--|___|-o-|___|-o-|___|-o-->|--o
FU1 | )|( | | R1 | R2 | R3 | D1
| )|( | | | | |
.----' '-. | .-------. | | | | 400V
| | | | | | | | | |
| | T2 | '-o ~ + o-' | VZ1/-/ |
| o--. ,-' | | +| ^ .-.
120VAC | | )|( | BR1 | --- | | |
| | )|( | | --- | R4| |
o----' '-. | | C1 | VZ2/-/ '-'
| | | .-o ~ - o-. | ^ |
| | T3 | | | | | | | |
| '--. ,-' | '-------' | | | |
| )|( | | | | |
| )|( | | | | | -
o--------o----' '---' '--------o-------o-------o------o
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
T1 - T3 120VAC pri, 10VAC sec, 20 VA plus transformers
R1 100 ohm, 10 watt
R2 5000 ohm, 5 watt
R3 40,000 ohm, 10 watt (or 2 ea. 20K 5W)
R4 330K 1 watt (bleeder resistor)
C1 10uF 600V or greater electrolytic
BR1 -- 800V bridge rectifier, 6A or more
D1 1N4007
VZ1 - VZ2 String of 1N4740 (10V) and 1N4742 (12V) 1 watt zeners on a perfboard
(you'll need 33 to 37 total -- stack up the '42s to get over 400V, then replace
'42s with '40s to get down to 400V+/-2V)
Out of the above components, you'll probably have the most trouble finding a
cap. You might want to scrounge one out of some older toob stuff (check the
cap first to avoid misery).
The idea is that you get the 10VAC from your first transformer, and backfeed
three more of the same type of transformer, applying the 10VAC to the
secondary. You'll get around 120VAC out at the primary of each (make sure to
drastically oversize the transformers for this job), and putting them in series
will give you 360VAC, which should rectify to a little more than 500V. R1
limits the surge current at turn-on, saving your bridge rectifier and possibly
the transformers. C1 accepts the charge, and then R2 gives you a voltage drop
for the shunt zeners. The 5K value sets around 20mA steady state, out of which
10mA will be stolen from the shunt by the load when cap is initiating charge.
20mA (no load) can be handled easily by the 10V and 12V zeners. The 40K R3
limits the current charging the cap, and R4 is the bleeder resistor necessary
to discharge C1 when you turn off.
One thing I used to do was have a microswitch interlock, so that if somebody
opened the door to remove the cap or otherwise placed themselves in peril, the
power would automatically be turned off, and the bleeder resistor (which could
be made much lower to discharge more quickly) was switched into the circuit.
Think carefully about your application, and how to minimize the danger, before
you start. After somebody gets hurt is too late. Don't forget the fuses, but
remember that they're not personal protection devices.
Good luck
Chris
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