Estimating transformer VA for DIY lab power supply - Page 1
Forum section: electronics / projects

author: tango17
author_rank: Regular Contributor
timestamp: January 22, 2016, 12:11:01 am
content: I have a power supply for an old telephone system that has the control circuitry burned out. Only the regulator chip is bad, everything else is fine. I would like to convert it into a laboratory power supply with adjustable voltage and current limiting.The original power supply is marked as supplying 22 - 26 volts DC regulated at 5 amps and also supplied two outputs (isolated from each other and the other winding) at 11 volts AC. All three outputs were marked as being capable of supplying 5 amps, one at DC and the other two AC. I measured the output voltage of all three secondary windings and two of them are 11 volts AC as expected and the DC winding is 30 volts RMS AC. I also measured the DC voltage on the main filter capacitor which was 41 volts DC. That makes sense as it is the peak voltage minus 2 diode voltage drops in the bridge rectifier.Is it reasonable for me to assume that the higher voltage winding was rated at 212 VA (peak voltage times 5A) and the two other windings should be rated at 55VA each (RMS 11VAC times 5A)?FYI, the regulated supply used three 2N3055s, which I will scavenge.I'd like to build the adjustable supply to provide 5A regulated 0-30V DC from the higher voltage winding and to put the two 11VAC windings in series, then provide a second regulated DC supply at 0-20 VDC and 3A.Does that seem reasonable, or do I need to derate it more? Any problem with putting two transformer windings in series, as long as I get the polarity correct?Thanks for comments.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 22, 2016, 08:14:40 am
content: The higher voltage winding has a slightly higher power rating. Depending on the capacitor used, it takes something like 7.5-10A AC current to get 5 DC out. So the rating is more like 230-280 VA. Anyway you can expect 5 A DC if the capacitors are not enlarged.It is not clear if the VA ratings of the windings simply add. Especially if both secondary windings are loaded with a recitifier, one might have to reduce the rating a little compared to one part with rectifier and one with ohmic load.Similar for the two lower voltage windings you might not get 3 A DC at rated power to the transformer. So maybe 2.5 A DC out would be safer.For an adjustable supply (lower than 20 V) the 3 pieces 2N3055 do not have much reserve - so you might need 1 or 2 extra to cope with the higher maximum loss.There is no problem putting the two windings in series - it might be a problem with high voltages, but not at less than 40 V.

author: tango17
author_rank: Regular Contributor
timestamp: January 27, 2016, 01:53:22 am
content: Thank you for the helpful comments and suggestions. I realized I have little reserve with only 3 2n3055s, and I may add another. I may also limit the 20v supply to 2.5A.

author: uncle_bob
author_rank: Unknown
timestamp: January 30, 2016, 11:15:00 pm
content: HiThe real question here is:Can I wire the spare secondaries up like I want to and get 5A DC out of the supply?The VA stuff is only a way to get at that answer.There are a few other odd things that get done with transformers past their VA ratings. Things like mutual impedance. That said, what you propose sounds reasonable. The voltage ratings on everything should be fine, the worst case is that the transformer will get a bit warm. It's probably rated for operation at a higher temperature than your lab anyway.One thing to do sooner rather than later:Rig up a load on the transformer. A light bulb or three is fine. Get it pulling 5A in the configuration you intend to use it in. Check both the voltage and current with a meter (do not rely on the light bulb guy to tell you the truth). Let the beast run for a few days. Watch for smoke. If it makes it through that, start building the new supply. You never know when a transformer that *looks* ok really isn't. Check it for leakage between the primary and secondaries after it has been running. You don't want to electrocute yourself.Bob

author: tango17
author_rank: Regular Contributor
timestamp: February 10, 2016, 11:08:43 pm
content: Thanks for the load test suggestion. Good idea.