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Want info on old bench power supplies

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Cobalt60

New Member
I would like some info on old power supplies. I recently purchased a Power Designs 3650-S, which is rated for 36V at 5A. First of all, knowing what year this may have been made would be nice. Also, I noticed on the back it claims an input of 440W. But doing the math, its max output would be 180W. So does this thing actually waste that much energy?

I am really intrigued by a lot of the old electronic equipment, especially when it is functionally equivalent or better than the newer stuff. Its why I bought this one for less money and higher output specs as compared to a newer "less classy" one. But what are some of the design differences between an older power supply and a newer one?

And when I received this, I had to open it up because there was some loose metal inside (brackets for the little rubber feet). The component that by far caught my attention most was what appeared to be an enormous capacitor. I mean, this thing was bigger than a soda can! And I think it had screw terminals for it. Is this really a cap? How do I discharge this? Is it as deadly as it looks?

Also, the current limiting knob seems to be non functional. The power supply does limit current on a short, but when I turn the current knob all the way down and apply a load it does not max like it should.

Well, any information on some of the history of the design of such power supplies will be greatly enjoyed.

Thanks

-Chris P
 

crutschow

Well-Known Member
Most Helpful Member
Those are tough supplies. I used one when I started work more than 40 years ago (gasp) and they still use them in the lab, so yours could be pretty old.

I believe that as the DC output voltage is changed, they use a variac to also control the ac voltage to the rectifiers. (That's why there's such a big control knob which has significant turn resistance.) This maintains a more or less constant drop across the series pass transistor, independent of the output voltage, minimizing the transistor power dissipation (and improving the supply efficiency). 440W maximum input power does seem a little high.

The large can is likely a big electrolytic cap. You need a big one to filter 5A of rectified AC. It shouldn't be particularly dangerous since there's probably never more than 50V on it.

The current limit should allow you to adjust the current to near zero, if it's working properly.

The main difference between that supply and present supplies is probably just newer semiconductor part types and more ICs in the new ones. Basic linear power supply design hasn't changed much.
 

Cobalt60

New Member
Oh wow I think that is so cool that the knob turns a variac. I watched it in action when I had it open. Thanks much for all the info. Im still trying to learn here, so if you dont mind continuing to talk about power supply design Id certainly appreciate it. A few questions I still have would be, do any lab/bench power supplies use a switching regulator in anyway, or is this generally a bad idea due to noise? Also, I am sort of under the impression that variacs have very good efficiency, close to that of using switching, is this true?

I was thinking about designing a power supply, and it would go something like variac -> rectifier, -> smoothing cap -> adj linear reg, with the linear reg ideally being constantly set only slightly below or at the bottom of the ripple. Which, I think you have just informed me, is exactly how my 3650 is designed.

And lastly, Id again have to bring up the difference of input and output wattage on this supply. 440W -> 180W seems really high. With this design, shouldnt theyre be high efficiency, with the output wattage close to the the input wattage?

Thanks again
 

crutschow

Well-Known Member
Most Helpful Member
Yes, switching supplies are usually not used for bench supplies because of the switching noise. This is particularly important when working with analog circuits, but generally not a problem with digital circuits.

The trick with using the variac is, how do you maintain a constant difference between the raw dc and the dc output? I believe the Power Designs has a pot ganged to the variac shaft to control the output voltage at the same time the variac output changes.

440W input power does seem like a lot for 180W output if the variac is controlling the raw dc. The variac itself has quite high efficiency (probably >90%). Don't understand myself why the input power is that high.
 

tcmtech

Banned
Most Helpful Member
I like how some of the old lab quality designs are also. I have scrapped a few that were baddly abused or damaged beyond repair from misuse. And have rebuilt a few and sold them also.

The biggest and most interesting one I ever had was a 0-3000 VDC 500 ma unit. It used the same variac to step up transformer design as the smaller step down ones, But instead of diodes it used a pair of vacuum tube rectifiers and a pentode vacuum tube regulator about 4 inches in diameter and 9 inches tall!

I had no use for it so I put it on eBay and sold it. The person who purchased it was so dumb he bid it up to $550 to win it and planned to use it to run a low voltage ham radio transmitter with it. :p

He even asked lots of questions about it too before he even bid on it. He persistantly reasoned that being adjustable With a maximum output of 1500 watts it shoud be able run his stuff. :confused:
It came down to him having no Idea of the differences between amps, volts, and watts! :eek:
He got it and tried running his whatever it was and fried everything!
Then reported me to eBay for being a bad seller plus he wanted me or eBay to replace all of his stuff or he was going to get his lawyer involved!
I contacted eBay and told them what went on and they reviewed the sale and said he got what he deserved! My discription was very acurate and my contacts with him specificaly said it would not work for what he needed. :D

Wish I would have kept it now! :( If I come across another one I will next time!
 

Sceadwian

Banned
0-3000V DC 500ma? TCM you're breaking my heart. I could get into so much trouble with something like that =)
 

tcmtech

Banned
Most Helpful Member
Yea thats why I sort of regret selling it. But sometimes you do got to eat too!
It came out of some sort of millitary equipment. It was part of a transmitter of some type. The power supply was the only thing left that I found intact. Everything else was smashed or had a torch taken after it. :(

Having an air base near here I see alot of good stuff go to the salvage yard.
And some of its still brand new in the box and never been opened. But still tagged defective.
Our tax dollars hard at work! :mad:
 

Sceadwian

Banned
For something like that I'd skip a meal =)
 
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