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'[OT] Bleeder resistor.'
1997\11\07@020852 by ruben

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Hello everybody.

I am currently designing a high voltage DC power supply
that will generate up to about 50 000 Volts at 1-2 mA.
Since there is a voltage trippler at the output it has
a capacitance which doesnt automaticaly get discharged
when the power is turned off. Because of this I will put
a bleeder resistor in the range of 500 - 1000 M ohm over
the output.

My problem is that these resistors are not very common and
rather expensive (I need 2 because each unit will have
both a positive and a negative high voltage). So I'm
asking if there is anybody on this list that know of
any supplier for these kind of components or perhaps
another way to solve my problem.

I know that this isn't exactly the right place for this type
of question, but there seems to be many people here that
know alot about much. And by the way, the unit is controled
by a PIC.

TIA
------------------------------------
Ruben Jonsson
AB Liros Elektronik
Box 9124
200 39 Malmo
Sweden
Tel +46 40 14 20 80
Mail: spam_OUTrubenTakeThisOuTspamsbbs.se
------------------------------------

1997\11\07@123057 by Mcorio

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In a message dated 97-11-07 02:09:37 EST, you write:

<< My problem is that these resistors are not very common and
rather expensive (I need 2 because each unit will have
both a positive and a negative high voltage). >>

Ruben,
I assume you are going to pot this circuit as surface leakage will be a very
large parasitic when you are using 500-1000M resistors. In either case, you
could cut your cost in half by using a single resistor (2x the resistance)
between the plus and minus voltage. I am interested to know how you make
out...good-luck.

Mark A. Corio
Rochester MicroSystems, Inc.
200 Buell Road, Suite 9
Rochester, NY  14624
Tel: 716-328-5850
Fax: 716-328-1144
http://www.frontiernet.net/~rmi/
***** Designing Electronics for Research and Industry *****

1997\11\07@153302 by Mike Keitz

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On Fri, 7 Nov 1997 12:23:14 EST Mcorio <.....McorioKILLspamspam@spam@AOL.COM> writes:
>In a message dated 97-11-07 02:09:37 EST, you write:
>
><< My problem is that these resistors are not very common and
> rather expensive (I need 2 because each unit will have
> both a positive and a negative high voltage). >>
>
...
you
>could cut your cost in half by using a single resistor (2x the
>resistance)
>between the plus and minus voltage.

Then the resistor has to be rated for 100KV, and twice the power of  each
single resistor.  Also, it's usually useful to sense the current flowing
through the resistor in order to regulate the output voltage.  This means
having one end of the resistor close to ground (connected to ground
through a much smaller resistor, forming a voltage divider).

20 years ago such resistors were common in color TV sets.  They were
often part of a voltage divider to set the focus voltage.  The resistance
was on the order of 300M, and the operating voltage 30KV, so even if you
could still find one (ECG used to sell them for about $25) it may not be
suitable for this project.

1997\11\07@203008 by Paul Britton

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You should really use 4 resistors ie 2 in each place for redundancy in case
one goes open circuit, wouldn't want to kill anyone would we?
What's the PSU for? Photmultiplier tubes, laser? or something else?
Sounds interesting anyway.....later.........................Paul

                             <PBrit00001spamKILLspamaol.com>

1997\11\07@235901 by Lee McLaren

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Normally in high voltage circuits you need physical seperation as well
and electrical, what they do in tv high voltage circuits is use a large
number of say 10 Meg ohms resistors in series and in a Z physical
pattern so you get the physical seperation as well.
Most resistors have a voltage rating as well as a power rating so you
may have to use a larger number of resistors so that the individual
ratings are not exceded. This will also give you some fail safe ability.

regards

Lee McLaren

1997\11\08@113743 by The Jacky's

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Ruben  I've seen this solved by using a lot of cheap, comparetively low
ohm, resistors in series.  This is also how we make test loads for HV power
supplies...gj

----------
{Quote hidden}

1997\11\10@061929 by ruben

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> << My problem is that these resistors are not very common and
>  rather expensive (I need 2 because each unit will have
>  both a positive and a negative high voltage). >>
>
> Ruben,
> I assume you are going to pot this circuit as surface leakage will be a very
> large parasitic when you are using 500-1000M resistors. In either case, you
> could cut your cost in half by using a single resistor (2x the resistance)
> between the plus and minus voltage. I am interested to know how you make
> out...good-luck.
>
> Mark A. Corio

Yes it will be poted. The problem with one resistor is that it has to
withstand the double voltage. Also I think there will be a relatively
low ohm resistor in series with the 'bleeder resistor' to ground so I
can do some measuring of the output voltage.

>Paul Britton wrote

>You should really use 4 resistors ie 2 in each place for redundancy
>in case one goes open circuit, wouldn't want to kill anyone would
>we? What's the PSU for? Photmultiplier tubes, laser? or something
>else? Sounds interesting
>anyway.....later.........................Paul>

Im thinking of supervising the outputvoltage with a voltage divider,
which also means that I can check if the bleeder resistor is open
circuit.

The PSU is going to be used for discharging static electricity by
ionizing the air. It will be designed so it can't supply more than
3 mA which is considered safe for this kind of equipment.

{Quote hidden}

Most 'normal' resistors have a voltage rating of a 250-500 Volts.
This would require 100 resistors to get a voltage rating of 50 Kv,
which is far too many for this design.

I asume that the stated voltage rating for a resistor is for open
air mounting. Since I am going to pot the bleeder resistor together
with the voltage trippler the voltage rating for each resistor
could be increased so that instead of 100 resistors I could perhaps
use 20 22Mohm resistors. Is this assumption correct?

Another idea I have is to put bleeder resistors across every
capacitor in the voltage trippler, which decreases the voltage over
the resistors. I havn't tried this yet though.







------------------------------------
Ruben Jonsson
AB Liros Elektronik
Box 9124
200 39 Malmo
Sweden
Tel +46 40 14 20 80
Mail: rubenspamspam_OUTsbbs.se
------------------------------------

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