Modding a Sorensen Supply

[cubemike99]

I recently became the owner of an old (as in, analog voltage and current displays) 150V 3A Sorensen power supply. 150 volts is cool, I suppose, but its not all that useful or fun to play around with. High current, on the other hand, seems a lot more interesting. So, how similar is the construction of my supply to a lower voltage, higher current model? Would it be possible to mod the supply to supply a higher current? Or is the better alternative to sell this supply and buy some new equipment with the money? Thanks for the advice.

[c4757p]

So, how similar is the construction of my supply to a lower voltage, higher current model?

In theory, reasonably similar. In practice, not at all. Provisions for high voltage and for high current are completely different.

Would it be possible to mod the supply to supply a higher current?

By the time you're done it will be a whole new supply.

Or is the better alternative to sell this supply and buy some new equipment with the money? Thanks for the advice.

150V 3A is fairly impressive. I bet you could pretty easily sell it for enough to get the kind of supply you want.

[skipjackrc4]

Old 500W Sorensens go from $200-$400 on eBay, depending on the model, condition, and current market (ie, luck).  I've been meaning to pick one up myself.  I plan on going for a 40V 13A, or possibly a 60V 17A.  I use them all the time at work and love them.  They have a bit of ripple on them at heavy loading as they're not regulated.  I believe the specs call for something around 100mV at full load.  Not something for delicate circuits.  Still, I consider them to be the best value supply for high current (relative to the normal lab supply).  Modern Sorensens are great (we have them at school) but they're not worth the x10 increase in cost over the old ones.

You can find service manuals with schematics for around $10 on eBay.

[cubemike99]

$200 - $400 is not bad at all. Maybe I'll post up an ad here.

In case anyone is curious though, preliminary tinkering shows its relatively trivial to increase the current output of this thing. The current is set via pots on the front panel where a higher resistance is a higher current. On my unit, 680 ohms is 3.5A. Adding 220 ohm resistors in series with the pot does increase the current output. Now, whether you could keep doing this indefinitely or whether something would go pop eventually, I'm not keen on finding out.

[skipjackrc4]

I was just looking on eBay, and $400 may be a bit much at the moment.  A few years ago they were that high.

The transformer won't be able to handle much higher current, so I would be EXTREMELY careful doing that. 

Out of curiosity, where are you located, if you don't mind?

[cubemike99]

Don't mind at all: I'm in the Hartford area of Connecticut in the US.

[skipjackrc4]

I'm in the US also.  I was almost tempted to make you an offer, but I really can't afford a half-useful power supply right now.  Hopefully you will have an easy time selling it to fund something else.

[c4757p]

Yeah, a 150V power supply falls under the category of "Stuff I want just for that warm fuzzy feeling you get from having a well-equipped bench" and would probably get used once. It's been a while since I've been able to justify that kind of purchase... Shame it can't go up to 170V (peak rectified mains).

[skipjackrc4]

Shame it can't go up to 170V (peak rectified mains).

Yep.  I've got plenty of (hypothetical) uses for a 170V supply, or a 340V. 

[cubemike99]

But skipjack, I can hear it calling your name in the transformer hum!

Seriously, though, about it not going to 170V: unmodified, it goes up to 158V. Maybe its possible to tease it up 12 extra volts by way of the pot mod I mentioned. In my very inexperienced opinion extra volts shouldn't be nearly as big of a problem as extra amps. I have no idea how comfortable any of you guys would be doing that, but I'll report back anyway with results.

[cubemike99]

And the results are in. The coarse voltage pot is a 12k. Adding a 4k7 ohm resistor in series with the pot offset the output by 50 volts. I took it up to 170V and it hummed along happily from all I could tell. Then I took it up to 180, where the OVP promptly kicked in. Didn't bother adjusting that to see what the absolute limit is.

[skipjackrc4]

That's interesting.  I have a feeling that, at full output current, that voltage will drop.  They probably leave that much headroom in their voltage control so that constant rated voltage can be supplied at full load current, though 3A isn't that much.  Hm.