EEVblog #530 - Electrophoresis Power Supply Teardown

What's inside an EC Apparatus brand constant voltage constant current power supply used for Electrophoresis - the separation of particles in a fluid by using an electric field.
You can get these supplies quite cheaply on ebay if you look around.
A quick reverse engineering of the circuit provides some insight into what's happening.
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[jamieostrowski]

These work great for reforming high voltage capacitors

[tra]

You probably don't want to hear that term while talking to your doctor. I used to work in a clinical reference lab and sat near the electrophoresis bench. I don't recall all the different assays they used but I wouldn't want my doctor ordering any of them on me! Excellent tear down Dave!

[whysguy]

One of the best teardown vids on this channel! :) I wish Dave would take the time to explain the circuit in all of his tear-downs like he did in this video. Usually he will point to some component or sub-circuit, and say "nothing interesting there", and then my hart sinks. Love the channel! i would love to see more explanations like this.

[ivanv]

Thank you very much for not mentioning the model or brand in the title. That should keep eBay prices low if the demand keeps relatively low.

[HambertHM]

My mother does biochemistry and this is soooo familiar to me, lol. Her power supply is custom made from a friend of her who does electronics, it's even older than me and still running! Recently the platinum electrodes failed and needed a new tray.

[KX]

The load on this is a block of polyacrylamide gel. As long as the voltage is vaguely constant the particles will move at a fixed speed through it depending on their charge. Allow them to move part way through the gel and remove the voltage and you can work out their charge from their position. Noise on the voltage doesn't make much difference and the gel isn't exactly going to breakdown from the spikes.

[mikeselectricstuff]

I took apart something similar a while ago made by LKB - initially spent ages tracing why it wouldn't work - eventually found a mechanical interlock on the output sockets that didn't quite kick in with standard 4mm plugs. It used a more conventional flyback transformer topology.

[tubical]

All my DIY linear HV PS got these fluctuation problems without my online USV.
Nowadays the mains voltage is rather reduced than setpped up. The days of the old Tube based radios and television sets are gone forever! So we have to deal with it, or spend much more money to get rid of it.
For low output current (10-100mA) wind your SM PS tranny some more sec. windings to keep aprox. 100V between input and output. This helps the regulator, as well as a good ground! But causes more heat to dissipate

[Afrotechmods]

Electrophoresis is cool stuff - you can read out DNA sequences with very simple equipment! And Dave is right - you don't need a high degree of precision on the output to do it. If anyone wants to learn more about it sign up for the MITx course 7.00x.

[Mikkel]

I suspect the fluctuations on the output are caused by fluctuations in the mains input voltage. I think the output voltage is set purely based on the timing of the IGBT switching in relation to the zero crossing, and not regulated based on any sensing of the actual mains voltage. This means that the output voltage is a fixed fraction of the mains voltage, controlled by the voltage setting, and any fluctuations in the mains voltage will appear on the output.

[jrevillug]

Interesting stuff - that overshoot was a bit unexpected...

One thing: as all this stuff is running from the mains frequency, it'd be worth using the line trigger. Much cleaner waveforms, and less fiddling with the trigger settings...

[otherphoenix]

The circuit behaviour looks very much like a full wave thyristor rectifier, but using the diode bridge and IGBT instead. I.e. The zero crossing pulse starts a timer (firing angle) in the uC and the IGBT turns on just before the zero crossing of the AC waveform based on the timer. Easy test - the higher the voltage the earlier the IGBT should turn on with respect to the optocoupler pulse.

I also found a manual but can't link the address! I put it on the forum.

[tubical]

an additional "benefit" would be overshooting and/or very high peaks with spike errors and may some low frequency ringing may occur as well.
Adding more low freq-gain to the error correction is not the right way.
Stabilisation must be prior to the SMPS input.

[maeanderdev]

i guess there is going to be an IGBT fundamentals friday soon...
Dear Dave, I'd appreciate that

greetings from Germany,
Mäander

[mausball]

Traces on the breakaway tabs match up with the lower edge of the UI board. They were built and tested together, then snapped apart I think.

[mausball]

electro-for-eee-sis ;-)

Dad got his PhD in biophysics/biochemistry. Used often for DNA and protein analysis.

[chuckvanderbildt]

Great stuff, thanks dave. I've been wanting to take one of these apart, but would have gotten in trouble.

[EEVblog]

Simple matter of the Rigol being on the teardown bench, and the Agilent being on my main working bench up on the instrument rack. I have two separate benches were I shoot videos, albeit right next to each other.

[EEVblog]

The overshoot was the collector voltage in constant current mode. In voltage mode it does not overshoot on the output as I showed. I didn't test for current overshoot

[Nate_dog]

Electrophoresis moves DNA/ Protein through a gel matrix, this matrix depending on the concentration can have different pore sizes. So depending on the size of the DNA/Protein you can separate by size/charge..with smaller moving faster vs larger particles. The Voltage pretty much just dictate how fast it takes to run your gel. Higher voltage results in faster movement.