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Old 9th April 2012, 09:56 AM   #1
Giaime is offline Giaime  Italy
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Default Help me mod an electrophoresis power supply

Hello all,

I bought an electrophoresis power supply, very cheap, to be used as a bench HV supply.

It is an Amersham EPS601:

Click the image to open in full size.

Here is the user manual:
http://www.blossombio.com/pdf/products/UG_18113019.pdf

Already asked for the service manual / schematics, but they won't send me.

The problem is that this supply has the middle of the two output terminal "referenced" (1kOhm) to earth, so it's a 300V supply really. The supply in fact can detect current leakage from the terminals to earth, and it automatically disables the output voltage in this case.

How to mod it to disable this feature and make it fully floating, so I can connect to earth whatever output connector I need?

Here is the PSU internals:

Click the image to open in full size.

As you can see, it's basically a 2 output isolated flyback converter. Each output has a diode bridge after the transformer and the middle reference of the two outputs is used also as a ground plane to the digital part of the supply. Then, this ground plane is connected to earth (and chassis) with those components:

Click the image to open in full size.

which are a 1k resistor, a capacitor and some sort of diode (?).

I thought about removing them but if someone has experience with this power supply and can suggest me a possible solution...

Thanks in advance
Giaime
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Old 9th April 2012, 10:39 AM   #2
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If you can identify the SMPS controller IC you might be able to gleam the information from its datasheet.

A bit of a long-shot I know.
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Old 9th April 2012, 12:36 PM   #3
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The component labeled V20 is a transil or a transorb, some device capable of shorting itself when the voltage between terminals exceed the design one, protecting the rest of the circuit.

The ic appears to be an LM338, but it is a voltage regulator of 5 amperes, so I believe that it must be LM339, a voltage comarator.

As a first test, try to short the 1K with a lower value, say, 10R, and do some check. If the overcurrent becomes "harder", then shunt it definitively with a 0R.
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Old 9th April 2012, 12:53 PM   #4
Giaime is offline Giaime  Italy
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It's a 339, I added the main IC names in this image:

Click the image to open in full size.

I think the PWM controller is the two TL074, with the LT1244 as a PWM modulator but after a couple of opto-isolator to deal with the mains insulation. The LM339 probably is to sense the earth leakage and probably for overcurrent too (or this last one is covered with primary sensing of the power mosfet current?)

Quote:
Originally Posted by Osvaldo de Banfield View Post
As a first test, try to short the 1K with a lower value, say, 10R, and do some check. If the overcurrent becomes "harder", then shunt it definitively with a 0R.
But the idea should be to open it, not to short it, to let the output voltage completely float over earth

Last edited by Giaime; 9th April 2012 at 01:19 PM.
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Old 9th April 2012, 01:46 PM   #5
Giaime is offline Giaime  Italy
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So, the main opto-isolation is between primary-referenced PWM comparator (LT1244) + power mosfet, and the rest of the circuit.

In this way, I cannot obtain a full isolation of the two outputs without disconnecting the circuit from chassis/safety ground (would not do that).

Anyway the "secondary" circuit has to sense the output voltage in some way to regulate it, and given the mid-referencing probably there's only one voltage sensing, on one 300V supply, the most positive one. I have yet to find the involved components.

If I could stack the other 300V power supply on top of the voltage regulated one, I'll have a regulated 600V supply.
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Old 10th April 2012, 03:10 PM   #6
Giaime is offline Giaime  Italy
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Hello,

so I've analyzed this further and I traced some schematics of the output zone:

Click the image to open in full size.

the two output voltages (one positive and one negative) are obtained with two independent half-wave rectifiers (3x diodes in series). They are stacked on top of one another, and the mid voltage (in the pic as "mid-reference") is currently shorted to the control/digital ground plane.

The voltage sensing for regulation is made with a 1Mohm resistor from the out+ supply voltage to the control circuit. I have not found any current sensing component for the output voltages, so I guess the current sensing is made at the primary side, sensing the current in the power mosfet: no need to worry about that, so.

So, the paths between the output zone and the control zone are:

- the 1Mohm resistor to sense the out+ voltage
- the mid-reference is shorted to the control ground
- safety ground is isolated from all that, only the 1k sensing resistor connects it to the control ground
- there are some missing components (probably for another version of the PSU: see in the pic "missing components") which have 2 lines connecting with the control circuit (both have a small SMD capacitor from them to control ground, one of them has 100k resistor to the control circuit and the other one has a component missing, so no link to the control circuit).

About the last item, those lines appear to be shorted to the mid-reference voltage (thus control ground now).

If I arrange the connections as following:

- double the 1Mohm resistor to get the same voltage ratio
- open the connection between the mid-reference and control ground
- short control ground to out-
- short control ground to safety ground
- cut the missing component traces (since the mid-reference after this operation would appear to be 300V above ground, I would isolate that from the aforementioned small SMD capacitors)

I should be able to get a ground referenced 600V supply.

What do you think about it?
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Old 15th April 2012, 02:58 PM   #7
Giaime is offline Giaime  Italy
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Hello all,

I tried the mods but it didn't work. The unit has "sudden load change" diagnostics which conflicts with the bench PSU use (and that's software, so inaccessible).

Also, it now does measure a significant current drawn (50mA) even without load connected at the terminals, so I guess it has something wrong. I will try to bring it back to original specifications to see if it still works.
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Old 22nd April 2012, 04:49 PM   #8
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Hello Giaime
Congratulations on Your Graduation

LT1244 - High Speed Current Mode Pulse Width Modulators - Linear Technology
"They are improved plug compatible versions of the industry standard UC1842 serie"
classic UC3844 for little switching power supply
that drive mosfet with heatsink and supply H.T. output.
http://www.ee.bgu.ac.il/~dcdc/Lab/uc3843.pdf
(Blue transformer's probably used to power +12 +5 CPU. )
the best way would be to change program-rom
but LT1244 can also operate without cpu control. (see on datasheet)
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Old 7th June 2014, 08:50 PM   #9
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Hello Giaime

Did you get anywhere defeating some of the safety features of this power supply? I have bought a similar modle - but good for +/- 1750V (3.5KV total) It looks to be the same PCB as yours, but fully populated.

I had some success simply shorting out the 1K resistor between chasis ground and the centre point of the outputs.

You are right - it only voltage senses the positve rail. I am considering lifting a couple of diodes on the negative rail to disable it, then changing the high value voltage feedback resistor to give the right reading when only the + side is running, and try loading the supply to chasis ground.

If you never hear from em again, I got it badly wrong...
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