AC + DC Bench supply on a budget

[Skilldibop]

Hi Folks,

I'm a long suffering noob tinkering in the man cave for years and it's all gone and gotten a bit more serious and I'm starting to need proper toys. I've upgraded to a proper (well hung lo copy) soldering/rework station, and was also looking to treat myself to a bench supply.  I have a couple projects in mind that need low voltage AC and also like the idea for testing PSU builds on "safe" supplies before hooking them up to a full blown mains transformer. Another thing I like to do is mess around with valve state stuff like guitar amplifiers and i figure even if I had a 30V AC source I could bump that up with a transformer to higher voltages if required for testing tube stuff.

I did manage to find one that I think was 0-32V, 5A with both AC and DC output for around £250. But it was on my phone on the train and I lost the page and haven't been able to find it again. Does anyone know of anything similar in the sub £300 budget zone?  0-30V DC bench supplies are everywhere, but ones with both AC and DC seem hard to find, am I looking for a golden unicorn here? 

Also I am very much a home gamer with limited space so not really in the market for any old 2nd hand 3U rackmount gear off Ebay. I was tempted but my work bench would not accommodate! I was also tempted by some nice old CRT scopes too but again, couldn't sacrifice 1/3 of the entire bench just for the scope.

Any help graciously received!

[dicky96]

For AC couldn't you just use a 240V (or 110V) isolation transformer and a variac connected to the secondary?  That would give you any AC voltage you like from 0V to mains Voltage.  In fact many variacs go from 0V to mains+10%.  Stick a suitable wattage light bulb in series to act as a current limiter if you like to prevent things from going bang due to a short in the load.

As you are in the UK if you don't need more than 110V AC get one of those yellow isolation tranformers they use on building sites, they are easier and cheaper to find than 240V:240V transformers.  You should pick up a used 240V:110V transformer for less than £25 and a variac for less than £50 on ebay.  A light bulb (incandescent type) are getting a bit harder to find but still only a quid or so.

I use a 2000VA 240V:240V isolation transformer and 2000VA Variac in my repair bench.  The isolation transformer also keeps me (relatively) safe.  And the light bulb stops me crapping myself if i connect a short circuit DUT!  I have a switch so I can short the light bulb if i want full power to the load (once i know it is safe)

Otherwise an old transformer out of a scrap Power amplifier would give you something like 70V-0-70V and use a variac on the primary to set whatever output you like. . That would work well for circuits requiring plus and minus rails too - or you could get up to 140V across the whole secondary winding.

[dicky96]

Oh.... just to be certain you know the dangers as you are new to this - the Variac will give you variable AC from 0V to 240V or 240V + 10% but will NOT GIVE YOU ANY ISOLATION FROM THE MAINS so you must use an isolation transformer between the variac and the mains supply, or connect a transformer to the output of the variac.  Don't think you can get away with using just the Variac.

Rich

[Skilldibop]

Thanks for the replies. I had thought about a variac setup but was hoping for something with an adjustable current limit in a relatively out of the box form. Though continuing to look and finding something is proving difficult and it is looking like it's cheaper to get an isolation transformer and variac then mod it with some sort of panel readout and rig something for a display.

I've worked on my own amps for years, most of which are valve powered so I'm well versed in the dangers of HT lines that are referenced to mains earth. While I don't (yet) own an isolation transformer I do always work with shoes on, stood on a insulating surface and work with one hand in my pocket

Also well aware of shorted DUT's for some time. Ever since I was 12 and I tried to test a faulty starter motor off a bike by placing it across the terminals of a car battery    That thing blew up in my face very literally and spectacularly.

[dicky96]

That is what I did - bought a bare chassis variac and built it into a case with AC Voltmeter and AC Ammeter.  On the output i have the lightbulbs, I have a 60W bulb and a 100W bulb.  I can switch those so I have either 60W, 100W or 160W in series with the load (or direct supply from the variac with no limiter).  I personally never needed anything more sophisticated than that as a current limiter.

That works perfectly with the sort of things I work on - TVs, Amps, disco lighting and switch modes PSUs mostly at the moment.

Whole thing cost me about £150 (I used second hand kit from ebay - 240V:240V 2kW isolation transformer cost me about £50, 2kW Variac about the same, volt and ammeter less than £10, new metal enclosure £40, and bulbs/holder/switches/sockets about £10.   I used analog volt and ammeters (matching round ones - very 1930's style) because with the big wheel that rotates the Variac I thought it looks quite like something off a Hammer House Frankenstein horror film.  I just need one of those big spark gap things on top where you get the lightning moving up and down between the angled electrodes - that would really complete the look!!

I then have a  new 0V-30V 0-5A Bench supply from ebay about £40, and a second hand dual variable 0-30V 0-3A, 0-30V 0-3A linear bench supply (I can switch to series 0-60V 3A , parallel 0-30V 6A,  plus/minus 0-30V with 0V 'ground', or two indipendant variable voltage supplies) and that also has a fixed 5V 3A supply with current limit built in as well - got that from the Blackpool Amateur Radio Rally a few years ago for £50.  Actually the rally is 28th April this year http://narsa.org.uk/ and it can be a very good place to pick up cheap second hand electronics test gear and valves etc,  if you live in that area of the UK 

Personally for me that is all I need.  So total for the three cost me £240

Rich

[rdl]

BK Precision 1653A is a 2 amp variable, isolated 0-150 volt AC power supply. It doesn't have any DC output. I don't think I've ever seen a combination of AC and DC in a bench power supply, other than a few that brought the mains transformer secondary out to front panel connections. Don't know how easy it would be to find outside North America though.

[Skilldibop]

I did manage to find one of the ones I saw that put the idea in my head.

It was a PeakTech 6120 and 6125, though on closer examination of the controls it doesn't look like there's a constant current option on that. Also a regulated power supply that requires a DC "smoothing"button doesn't fill me with confidence.  I may just go the DC route and build my own AC supply with a Variac. Seems a better route.

http://peaktechthai.com/images/products/p6125/01-p6125.jpg

http://peaktechthai.com/en/products/power-supplies/ac-dc-power-supplies.html

Looks like you need to get the 5995 to get current control.

[mr.fabe]

I have one of these that provides AC and DC power...