Post by thread:[OT] Bleeder resistor.

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_OUTrubenTakeThisOuTsbbs.se ------------------------------------

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 *****

On Fri, 7 Nov 1997 12:23:14 EST Mcorio <.....McorioKILLspam@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.

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}> From: Ruben Jonsson <.....rubenKILLspam.....sbbs.se> > To: EraseMEPICLISTspam_OUTTakeThisOuTMITVMA.MIT.EDU > Subject: [OT] Bleeder resistor. > Date: Friday, November 07, 1997 2:55 AM > > 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

> << 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}>Lee McLaren wrote >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 >The Jacky's wrote >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 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: rubenspam_OUTsbbs.se ------------------------------------