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Trying To Adapt Electrophoresis Power Supply for High Voltage Microamperage

I have an urgent problem. The extent of my knowledge of electrical engineering is a trimester course in university physics in electricity and magnetism; I am molecular biologist/biochemist.

We have some old electrophoresis power supplies we are using for a method called "isoelectric focusing." These power supplies have a load detection system in which the current must be in the MILLIampere range (say 5-10 mA). Our electrophoresis strips (used in proteomics) must have a current NO GREATER than 50 microamperes (50 uA). We have been using one power supply (a BRL 4000 with 4kV dc output) in which its load detection system was FORCIBLY overridden by trying to lock the "DC ON" switch using a wooden stick wedged into it. The power supply is on the fritz now; we're not sure if it was because it was forcibly set on.

We had considered setting up some sort of dummy circuit in which we set up a variable resistor, say from 50,000 to 800,000 ohms, to produce a constant 5 mA current as the voltage output was varied from 250 to 4000 V.

We would than tap into that circuit somehow to get the 4000 V with a limiting 50 uA per strip. We use as many as 10 strips, so the ultimate microamperage limit might be 500 uA.

I need to get something up and working right away, and budget is a factor. We know that it will cost us $8000 to buy a system that is supposed to work with the strips (it puts out 10,000 V and is quite programmable and detects microampere currents...it's called the Bio-Rad Protean IEF).

What sort of schematic should I be drawing that fills the bill? Also, if it's not too much trouble to get higher voltages, can I use a step-up transformer? These strips are able to take on 10,000 V, and the max d.c. input from our old power supplies could range from as a low as 500 V to

3000 V.

What sort of box/container will I use to hold the soldered components...after all, this is very high voltage? Also, I don't really know how to solder them together...any guides? And what about heat buildup: will that be a problem for a resistor on a dummy circuit, which might be putting out heat in the tens of watts range?

Reply to
SMH
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You can't use a transformer on dc. You would have to break into the BRL power supply to get at the ac stage, before rectification. You have to find out what frequency ac you're getting from the BRL so you can choose the right transformer. Then you have to figure out what to do with the output of the boost transformer. How do you rectify, filter and regulate it? Might as well build a whole new power supply from scratch.

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For the curious: BRL 4000 power supply ebay item #

300185064074 wikipedia entry on proteomics:
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/IsoElectric_Focusing

Seems you're asking for a power supply with voltage regulation, and current limiting. The current limiting might not be so important. I mean, you disabled the shutdown function of the old power supply by jamming the switch, so you've been running without a current limit anyway if I interpreted your post correctly.

Here's a cheap adjustable 15 kV power supply ebay item 200187181104 weighs 69 pounds and has way more power than you need

First thing to try to get up and running would be to troubleshoot your existing power supply. Maybe you're in a university where you could find some friendly EE's?

Reply to
gearhead

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The more I read your post the less clarity I have. You said the power supply was "on the fritz," which sounds like you can't get it to work at all. Or have you determined that it works, as long as you put enough load on it, and the problem is that the strips don't draw enough current to get the power supply to work?

Reply to
gearhead

gearhead wrote in sci.electronics.basics:

I very much appreciate the thought you are giving to this.

And so I have created an illustrated web page which I hope provides more clarity. Yes, I agree the original post is not as coherent as it should be, and indeed I was trying to word it for as much brevity as the Usenet demands.

Anyway, please have a look at the web page and I hope it explains my problem.

Preview tinyurl:

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And in addition, I am taking your advice and checking out EBay and other places to see if they have an affordable solution. We are trying to do science out of our own pocket here because we don't have the grant support for this (or else, we would probably spend $15,000 on a solution that would pretty much take away all the difficulty).

Reply to
SMH

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