SCR "switching" PSUs

[eccerr0r]

I got an old bench power supply, it's 18V 15A DC regulated adjustable voltage/current. Looks like it's almost a half century old, probably early 70s technology and uses SCR as the "switching" element - and hence this thing probably is fairly efficient compared to a series pass transistor design. Even after several amperes of low voltage use, this thing doesn't get that hot - its heat sink stays fairly cool.

However it seems to have severe overshoot when turning on from 0V. Anyone know what's normal for overshoot for SCR based PSUs and what to look for to deal with overshoot? Maybe this is normal though? Since SCRs can't be turned off until the cycle ends, it will overshoot, but by how much is normal/expected?

I need to test the capacitors in this system to see if any have rotted. The 70mF large capacitors unfortunately I don't think I'll replace so I won't check, but the small caps may have rotted over the years...

[ben7]

Re: SCR "switching" PSUs

I got an old bench power supply, it's 18V 15A DC regulated adjustable voltage/current. Looks like it's almost a half century old, probably early 70s technology and uses SCR as the "switching" element - and hence this thing probably is fairly efficient compared to a series pass transistor design. Even after several amperes of low voltage use, this thing doesn't get that hot - its heat sink stays fairly cool.

However it seems to have severe overshoot when turning on from 0V. Anyone know what's normal for overshoot for SCR based PSUs and what to look for to deal with overshoot? Maybe this is normal though? Since SCRs can't be turned off until the cycle ends, it will overshoot, but by how much is normal/expected?

I need to test the capacitors in this system to see if any have rotted. The 70mF large capacitors unfortunately I don't think I'll replace so I won't check, but the small caps may have rotted over the years...

Hm, you really should get us a schematic of it.

The issue is probably not the SCR but the way the PSU was designed.
If it was from the SCR, then there could be a lot of ripple on the output, which wouldn't make sense.

What type of SMPS is this? Buck? Or an isolated transformer design?

Also, you should check them 70mF caps. Big caps like that go with a big bang when they are bad.

But, yes, I say first try testing and replacing capacitors.

[PeteS in CA]

Re: SCR "switching" PSUs

SCR regulated P/Ss are fairly efficient, but they operate at the line frequency - 50Hz or 60Hz. Thus, magnetics and capacitors are usually large and heavy.

SCR regulated P/Ss control how much energy from each half cycle is blocked or delivered to the load. Because of the low operating frequency the control loop is very slow, and subject to significant droops and overshoot. A large load transient (large in terms of the capability of the SCR-regulated P/S) may do really nasty things in terms of alternating droops and overshoots as the loop tries to get the O/P back in regulation.

I learned this the hard way. I was using a large Sorensen SCR regulated P/S to supply bulk power to a 1600W or 2000W switcher that used Vicor DC-DC modules. The regulation test involved switching from 10% load to 100% load and noting the difference in the O/P voltage. Normally that test was performed by switching the electronic load quickly between the two load levels. With this particular UUT and power source, that quick large load switch caused the Sorensen to "hunt" - V(Out) bounced back and forth between very large droops and overshoots. The voltage at overshoot was high enough to destroy the Vicor DC-DC modules. We performed that load regulation test more carefully after that.

So, yeah, I could believe you might see significant overshoot on turn-on, especially if you had very light or no load.

[budm]

Re: SCR "switching" PSUs

I second that with PeteS, have not work on the Sorensen power supply since the 80's. They are slow.

[cheapie]

Re: SCR "switching" PSUs

Are these the same as the "SCR drives" that are used to control DC motors?

[eccerr0r]

Re: SCR "switching" PSUs

Yes I believe it is, pretty much using SCRs for a power supply instead of controlling DC motors from AC.

I suppose this is normal for the very large overshoot. Yes, this thing is very slow (120Hz switching) but I would have thought it could respond to transients faster, though 120Hz is slow, it's a lot faster than what the human eye can see, and I can definitely see the large overshoot on the slow voltage meters... I just don't quite understand.
Plus it does have those 70mF capacitors that should filter away a lot of the transients, how could it overshoot so much - the control loop should notice the voltage is getting too high too quickly and should shut off the SCRs which should stop the upward rise... but don't know, yes it would be neat to get the schematics of the control loop...

Even with a fairly sized load, a 12V 60W headlamp, the powerup overshoot is immense and visible - I can see the bulb burning very bright for a split second then drop down to normal brightness. It's that slow... or should it be this slow?

It's an old Harrison Labs/HP PSU...

[eccerr0r]

Re: SCR "switching" PSUs

Necroing my own post.

For the heck of it I measured:

No load: measured 4.7W (for control electronics)
7V 0.6A load: measured 12.2W => 7.5W input - 4.2W output => 56% efficiency; counting control, it's 34% efficient. PF=0.2

12V 4A load: measured 82W => 62% efficiency; counting control = 58% efficiency PF=0.36

Pretty miserable compared to a modern switching PSU and the EU probably would want to kill it with fire due to its miserable PF.

With modern electronics available I wonder if it's worth it to design a circuit to quash its overshoot...

[CapLeaker]

Re: SCR "switching" PSUs

I second that with PeteS, have not work on the Sorensen power supply since the 80's. They are slow.

I third that. Worked on a few of them before.

[PeteS in CA]

Re: SCR "switching" PSUs

For early 1970s technology, 60% efficiency was pretty good. At that point switching power supplies were going into space and flying around in F-14 Tomcats.