Using a salvaged transformer in a diy bench power supply? - Page 1
Forum section: electronics / beginners

author: PotatoBox
author_rank: Regular Contributor
timestamp: July 26, 2017, 04:49:56 pm
content: So, I have a salvaged transformer that will output 22 volts with no load. I have no idea what it was I salvaged it from. It was some sort of TV channel selector with a remote. But anyways, I have no ratings for how much current it can output. Is there a way to figure this out? Would it be a good idea to use this transformer or should I just go out and buy a transformer so that I know the ratings of it?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 26, 2017, 04:57:28 pm
content: A channel selector does not sound like high power. One can get a first approximation from the weight and the ohmic resistance of the wires.Another important point for a lab power supply is the safety: the transformer should be good for a class II device, so it is safe even without a PE connected to the output side. Most transformers good in that respect, but not all. Anyway I would be careful with transformers reused from some cheap units.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 26, 2017, 05:37:16 pm
content: Small transformers usually have a load regulation between 10 to 15%.22 volts open circuit would mean between 19 and 20 volts at rated load.If you have power resistors, you may use them increasing the load until you obtain that voltage.Measure the load current.

author: PotatoBox
author_rank: Regular Contributor
timestamp: July 26, 2017, 11:39:30 pm
content: Quote from: schmitt trigger on July 26, 2017, 05:37:16 pmSo I would just do this through trial and error? Like slowly ramping up the resistance until I get 20 volts, then measure the current?

author: rstofer
author_rank: Super Contributor
timestamp: July 26, 2017, 11:51:28 pm
content: Quote from: PotatoBox on July 26, 2017, 11:39:30 pmTechnically, ramping down the resistance. But, yes, you have the right idea. The problem is, unless you have a variety of power resistors, this is going to cost more than buying the right transformer in the first place.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 27, 2017, 01:30:48 pm
content: With small transformers most of the voltage drop is usually due to the ohmic resistance. So one can just calculate the voltage drop from the resistance. It is only with the very small (e.g. < 3 VA) transformers where it does not work that well, as the magnetizing current gets really important in these. Increasing the load until the voltage drops by something like 10 % does not help very much: the larger the transformer, the lower is usually the drop in voltage at nominal load. To a good approximation one can get the same from measuring the ohmic resistance and doing the math. The main part is to estimate the power loss the transformer can dissipate without getting to hot. One unknown here is the quality of the insulation and thus the maximum rated temperature.

author: PotatoBox
author_rank: Regular Contributor
timestamp: July 27, 2017, 04:32:07 pm
content: Quote from: Kleinstein on July 27, 2017, 01:30:48 pmHow do I measure the ohmic resistance?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 27, 2017, 04:39:35 pm
content: Most DMMs have a resistance range. So measuring resistance should not be so difficult - just make sure the transformer is not powered.A few auto-ranging DMMS don't like the high inductance of a transformer - so it might help to shorten the secondary side when measuring resistance on the primary.

author: tablatronix
author_rank: Regular Contributor
timestamp: July 27, 2017, 04:44:18 pm
content: Does it have ANY markings on it?Like xx-xx-xx or xx xx

author: PotatoBox
author_rank: Regular Contributor
timestamp: July 27, 2017, 04:47:23 pm
content: Quote from: tablatronix on July 27, 2017, 04:44:18 pmYes it has a variety of marks on it.On the top of the transformer there is:M-013T 6C24Hand on the side of the transformer there is:24E019HDTAMRADIO

author: PotatoBox
author_rank: Regular Contributor
timestamp: July 30, 2017, 10:38:52 pm
content: Alright I did a little more digging and apprently TAMRADIO is a brand of Tamura. I looked on Tamura's site for legacy datasheets but no luck.Any ideas?

author: Zero999
author_rank: Super Contributor
timestamp: July 31, 2017, 10:08:17 am
content: Weigh the transformer and compare it to a data sheet for a similar type of transformer of approximately the same dimensions, core type (usually either toroidal or laminated for small units), frequency rating (50Hz units will be slightly heavier than 60?Hz units) and mass.