Re: Modifying Electrophoresis Power Supply for High Voltage Microamperage

On Dec 30 2007, 8:52 pm, SMH <s...@xxxxxxxxxxxxxxxxxx> wrote:
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?

I suggest you start with a general-purpose high-voltage
power supply on eBay. For example, a Spellman RHR 15
15kV supply capable of delivering up to 10mA (that's
150 watts max), without any awkward minimum-load stuff.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=120126870617
You may be able to get one of these for less, if you're
willing to wait 6 - 18 months with email search notice.

You can make a high-voltage precision current limit
using 1.2kV MOSFET transistors in series. This is a
risky thing to do at higher currents, where they make
high-power oscillators at RF frequencies (solve with
ferrite beads on the gate and source leads), but it's
not so hard at low currents like 50uA. Can you tell
us your minimum and maximum setpoint currents, and
your minimum and maximum voltage-compliance range?
(There needs to be a way to power a resistor stack).
.

Relevant Pages