# Power Supply Assistance Requested

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#### EvilGenius

##### Member
Hello
I have opened up the case to a large transformer that converts Input: 120V, 60 HZ, 6A to Output:12V AC, 25A, 300W
I need to convert it to a 20A, 12V DC with 12V minimum requirement.
I measured the output of the transformer with a Multi-Meter and it gave me 12.2VAC with no load
My logic says that it wont work for the following reasons, but I am open to suggestions and clever thinking to make it work.
12.2VAC going thru a bridge rectifier (min Vf of 1.25V) will reduce the voltage to 10.95vDC without a load
With a load (20A x 12v = 240W) I am estimating that my voltage drops to 90% of this value or 10.95 x 0.9 = 9.86v which is not acceptable
The idea is to utilize what I have or scrap it and buy a 12VDC 400W power supply at $28.00 Any suggestions or cheap ideas are very much welcomed.... Last edited: #### MikeMl ##### Well-Known Member Most Helpful Member Here is what Duncan says: Download, put in better values than I did, and re-run. Have fun. #### KeepItSimpleStupid ##### Well-Known Member Most Helpful Member 12.2VAC going thru a bridge rectifier (min Vf of 1.25V) will reduce the voltage to 10.95vDC without a load DC voltage from a bridge = 1.414 * 12.2 Vac- 0.6 * 2 ; DC Current is less than rated AC current. Power out cannot be greater than power in. EDIT: slight wording change #### EvilGenius ##### Member DC voltage from a bridge = 1.414 * 12.2 Vac- 0.6 * 2 ; DC Current is less than rated AC current. Power out cannot be greater than power in. Are you saying the voltage after a high power bridge rectifier is 32.8v? I am flexible on load current as long as I get 12 to 12.2V dc. I need minimum of 15A but prefer 20A plus. P.S.: My loads are LED's and resistors, so there is no inductance load. #### EvilGenius ##### Member I just read a bit more. I believe what I measured with Multi-meter was Vavg. If so Vrms=1.11 x Vavg = 13.54v Is this correct? If so, If I put a 0.8 ohm load on it, I will get 11.7v, 14.5A at 170W. Is this correct? If yes, I will be perfectly happy with that. Nevertheless I will need a very large capacitor bank! Last edited: #### MikeMl ##### Well-Known Member Most Helpful Member A VU meter is for indicating Volume Units, i.e. audio. Are you using a Digital Multimeter in the VoltsAC mode? If so, they come in two types. One measures true RMS. The other actually measures something else, but is calibrated to display Vac RMS, but only if it is being used to measure a sine wave. So what did you measure with nothing connected to the transformer secondary? #### EvilGenius ##### Member A VU meter is for indicating Volume Units, i.e. audio. Are you using a Digital Multimeter in the VoltsAC mode? If so, they come in two types. One measures true RMS. The other actually measures something else, but is calibrated to display Vac RMS, but only if it is being used to measure a sine wave. So what did you measure with nothing connected to the transformer secondary? The multimeter I used is inexpensive so I will no assume it has sophisticated electronics. With that in mind I put it on 200V AC setting and measured the secondary voltage of transformer without a load. #### MikeMl ##### Well-Known Member Most Helpful Member The multimeter I used is inexpensive so I will no assume it has sophisticated electronics. With that in mind I put it on 200V AC setting and measured the secondary voltage of transformer without a load. Can you use a lower AC range like 20Vac? That will result in a more accurate reading. #### EvilGenius ##### Member Can you use a lower AC range like 20Vac? That will result in a more accurate reading. Unfortunately not. I have 200 and 750 setting. What's on your mind? I might just board the darn thing with bridge rec and large cap bank with no load and see what DC output I get. Then we go from there. #### MikeMl ##### Well-Known Member Most Helpful Member I reran the Duncan simulator with some better estimates of Transformer impedance and Filter Capacitor ESR. Here is what is predicted: Peak voltage of ~13.3V; minimum voltage of ~10.8 when the load approaches ~20A. The peak current in the rectifier bridge is >60A, so it will have to be a big bridge. You could reduce the ripple some by going to an even bigger filter capacitor, say 220mF which would keep the minimum voltage at ~12V #### EvilGenius ##### Member Peak voltage of ~13.3V; minimum voltage of ~10.8 when the load approaches ~20A. The peak current in the rectifier bridge is >60A, so it will have to be a big bridge. You could reduce the ripple some by going to an even bigger filter capacitor, say 220mF which would keep the minimum voltage at ~12V I got similar results using another online sim. I did not want to complicate the situation but I have two of these transformers in the big box. So I can divide the load current up with a common ground and get away with 14.5A per transformer. At 0.8 ohms 14.5A I got 11.7v with cap of 0.4F (low ripple). I went on Mouser to price 0.1F cap and they are$15 each x 2 = $30 just for the caps then add cost of BR to it. How do these PSU companies build such large-amp devices so cheap? 400W 12VDC for$28 free shipping!
I think I will give up making what I have work since there are cheaper alternatives. Your thoughts.....