Modifying 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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Ask John L. about his high voltage regulator using a diode in a feedback loop.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Does this mean that the diode is used in the avalanche mode in an attempt to get HV regulation with one part? Noisy as all heck and temperature sensitive as all heck and may be unreliable. If the diode was used as a "leaky resistor" in the reverse direction, that makes the situation a lot worse. If someone needs high voltage regulators in the 60uA region, check out the following:

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Reply to
Robert Baer

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.

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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).

Reply to
Winfield

He controls it via the filament voltage.

More tubes..

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
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Reply to
Spehro Pefhany

Winfield wrote in sci.electronics.design:

I have attempted to explain my predicament in a highly illustrated web page in which I put in a few hours.

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As to minimum current, I have no minimum current: the procedure is an isoelectric focusing technique in which 1 mm thick (when rehydrated) strips of polyacrylamide are loaded with a protein mixture and the high voltage is used both to establish a pH gradient along the strip, and the proteins have a net charge at a pH and move to a pH zone where that net charge is zero (the "isoelectric point" or pI...the pI = pH at which the net charge on the molecule is zero) and the molecules stop movement...no current. Theoretically all proteins move to that point in the generated pH gradient where they have zero movement and thus zero electric current...no more charged particles move to create a current. In practice, there is always a net slight current of about 10 microamperes when the voltage is about 10,000 volts...so the currrent reduction follows an asymptote...and current reduction to the asymptote can be used to monitor completion of the procedure.

As to maximum current, each gel strip on the system can carry 50 microamperes maximum. There is an electrophoretic power supply sold by Bio-Rad in which you enter the number of strips and it automatically allows the current maximum of the entire system to be the number of strips times 50 microamperes...of course, this assumes all gel strips will have approximately equal resistance which is a generally safe assumption; of course if one strip has little resistance and all the others have a lot, that gel might take far more than the 50 microampere current it is rated for and burn up, which is why they tell you not to run strips where the sample type varies too much...for example, where you have rehydrated/infused the gel strip with a protein solution having a high salt content and the other strips do not have it...that is sure to create problems since conductivity and therefore resistance varies.

The mimimum load feature is even present on these thousands-of-volts- metering-in-the-microampere-range scientific power supplies, although they trip in a way I am not aware..perhaps if the current goes down to below 5 uA. The idea is to sense that resistance is so high that there is no effective focusing (electrophoresis) going on, and you are more likely to have a very high resistance condition that would cause the strip to burn up due to Joule heating, and you would lose your probably already focused sample. The voltage shuts down in that event. It is less of a condition in which you might have a fire, and more about preventing one's gel strip from turning to a black or brown ash, which we have seen on occasion.

Thanks for your interest.

Reply to
SMH

Spehro Pefhany wrote in sci.electronics.design:

I have defined my problem with a detailed web page:

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I will attempt to implement your recommendation. Thanks.

Reply to
SMH

Your problem intrigues me. I am split between modifying one of your existing supplies or building a compensator. Just where the heck are you?

JosephKK

Reply to
Joseph2k

Just stay with it, this sounds like a relatively simple project. You need to get your hands on one of these OC100HG or similar:

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The actual control is fairly easy, sensing low side currents and servoing the low voltage LED drives to maintain your minimum loading for the power supply as well as limiting the IEP strip current.

Reply to
Fred Bloggs

Its nice to have an electronics shop at your institution to do stuff like this. I would suggest getting a HV supply and using resistive current limiting as a simple method. I would also try defeating the sense circuits on the electrophoresis supplies. Unfortunately, the supplies are likely to have noise problems and voltage control problems with small output currents. Some voltage sinking will probably be needed, and current control may not exist.

greg

Reply to
GregS

I see your EC3000P only goes to 3kV, is that sufficient?

I suggested a constant-current sink to keep the supply on, as a convenience, because, while using a resistor would work, you'd have to keep changing the value if you change the voltage very much. If not, a resistor should be fine. But I take it simple current monitoring is not sufficient, because you also need a shutoff. A comparator + flipflop on each strip could drive a 5kV relay to provide shutoff. These items should cost less than $200, depending on how many strips you want to run at once. Safety interlocks, protective covers, etc., should of course be included. A maximum-current shutoff as well, although perhaps that's already in the EC3000P supply?

If you have a schematic for your model 4000, it shouldn't be too hard to get it repaired.

Reply to
Winfield

On a sunny day (Thu, 03 Jan 2008 14:23:22 GMT) it happened snipped-for-privacy@zekfrivolous.com (GregS) wrote in :

problems

will

I am not exactly sure what you want, but if I had to make one I would use some old BW TV chassis, rob the horizontal out / HV transformer, run it from some small 1:4 pulse, from a primary regulated 3-12 V. That way you cam make from a few hundered volts to > 10kV. Sensor resistor, comparator, flipflop, start button. A PIC with LCD display (no meters) for currents and voltages. Measure voltage exactly with some high voltage high value resistor divider. Dunno about output capacitance, but some 100 pF is in HV triplers. Some control loops.

power | 3-12V DC regulator | | | TV ouput stage with transformer - single recifier of tripler

-----------------0 + | | | object | PWM generator R | | | | -0 - | ---------------)-----------------------------------------------------| --| | | R Volt | R current | | | sense | | sense | | /// | /// | | | | -------------------- flipflop --------- comparator

----------| | | | | |U start | | PIC -- DISPLAY button current trip | | setpoint | |i |

------------------------------------------------------------------------------

Or something like that, so parts perhaps 60$, and labour 5000$ only.

Reply to
Jan Panteltje

There is one for sale on ebay at the moment, see:

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Price given as $179 + shipping.

Hope this is of some interest.

Regards,

Sarah.

Reply to
Sarah E. Bailey

Good find, Sarah. Here's a shorter URL for.

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Reply to
Winfield Hill

Don't forget the 40 lbs.

greg

Reply to
GregS

problems

will

-----------------0 +

object

| -0 -

--|

Volt | R current

sense | | sense

| ///

|

----------|

|
|
|
|
|

------------------------------------------------------------------------------

variac + neon transformer + rectifier & filter - the last two aint trivial at 15kV, but parts are obtainable.

Cheers Terry

Reply to
Terry Given

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