Transformer Issues - Page 1
Forum section: electronics / repair

author: vidarr
author_rank: Frequent Contributor
timestamp: August 14, 2018, 01:40:37 pm
content: I have a 120/240v to 12vdc transformer (from a cheap, car battery charger) that I am having trouble getting to work. It *was* working but the wires were corroded and a couple fell off. I replaced the wiring, but now, nothing is happening.There is no 240vac where I am am, so I need to wire it for 120vac. Does anyone know what wires I hook to the mains power? Yes, I looked this up online. There was no similar transformer examples. I even watched videos of how to build a transformer. For some reason I cannot figure this out. Maybe there is a short in one of the coils?Thank you very much!V

author: madires
author_rank: Super Contributor
timestamp: August 14, 2018, 02:07:51 pm
content: Such transformers have two 120V primary windings usually. They are wired in series for 240V mains or in parallel for 120V mains. Use a DMM (continuity check/resistance measurement) to figure out the windings. The 12V winding should have a quite low resistance (short thick wire) while the primary windings have a larger resistance (long thin wire). For verifying the windings take another transformer (more safe than playing with mains directly) and feed its secondary into the primary of the charger's transformer and measure the output voltage. The voltage is reduced by the transformation factor (120V / 12V = 10). If the test transformer outputs 12V then the charger's transformer should output 12V / 10 = 1.2V.

author: ArthurDent
author_rank: Super Contributor
timestamp: August 15, 2018, 01:17:01 am
content: The little circuit board the 4 wires on the bottom go to is the back part of a slide switch on the primary side to connect the 2 primaries in series or parallel. One winding goes from the left center pin to the right top pin and the other winding goes from the right center pin to the left top pin. If I'm right the line would be connected to the two top pins. Check it with an ohmmeter to see if that is how it is wired. If it is, then connecting the top and center left together and to one side of the 120 VAC line, and the top and center on the right together going to the other side of the line should allow the transformer to work. If you try this wire a 25W bulb in series with the line to see if it is wired correctly. if the bulb is bright, there is an error.

author: vidarr
author_rank: Frequent Contributor
timestamp: August 15, 2018, 11:14:47 pm
content: I tested with a multimeter and I am not understanding what I am getting. It reads about 700 from black to brown and like 1600 from black to to blue and black to green (multimeter set to 2000k Ohms). But, the numbers are moving up and down a bit. I do not have another transformer to test with and I do not feel safe plugging this thing in to the mains.What kind of readings should I be getting?edit: Thank You!

author: james_s
author_rank: Super Contributor
timestamp: August 16, 2018, 01:33:50 am
content: Are you touching the probe tips with your fingers? That sounds about like what I'd expect to measure across sweaty fingertips. Did you strip the enamel from the ends of the wires before connecting them?

author: ArthurDent
author_rank: Super Contributor
timestamp: August 16, 2018, 02:43:58 am
content: Reread my post #2. You should get the same resistance reading from black to brown as you get from blue to green, and open circuit from black/brown to blue/green.

author: vidarr
author_rank: Frequent Contributor
timestamp: August 16, 2018, 03:32:53 pm
content: OK, I cleaned everything off and fixed a couple kinks in the wires to make sure everything is connected 100%. Now, nothing is open. I get a value "1" for all your conditions you said above. My meter is not broken and I am using clips, not my fingers. I guess this transformer is toast unless you have another idea. That really sucks if it's dead. It will take about six months to get a new one.Thank You.