Salvaged power supply repurposing - Page 1
Forum section: electronics / beginners

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 25, 2014, 02:17:33 pm
content: I found this board in an older laser fax machine and want to repurpose it. However, I'm not getting the kinds of outputs I would expect. So, I'm apprehensive about trying to use it. My question is: Does the output fluctuate significantly in response to the load? I'm having trouble understanding the significant disparity between measured output and labeled. Maybe there's a fault on the board? I need 24VAC for a project that will run a typical large cooling fan and thought a power supply pulled from a printer would be ideal.In the first image, the silk screening indicates Gnd, 24v and 5v as one might expect. The problem is, I'm not getting a reading GND pin to anything else. The three fuses are good. I get approximately 50VAC on several of the pins (24V and a couple others) to earth ground at the mains power coming in. The 5V reads about 19VAC. I don't see any voltage regulators, but one of the TO-92 pkgs I cannot read. It's too close to the inductor. One of the TO-92's is a bidirectional thyristor and the other identifiable TO-92 is a regulating FET.Any suggestions on the easiest way to repurpose this board?

author: JoeO
author_rank: Frequent Contributor
timestamp: April 25, 2014, 02:25:01 pm
content: Unless someone has done this already you will have to supply some more info.What make model number fax machine is the supply from?Did you look for a schematic online? Did you find it?You may have to reverse engineer the supply to generate the schematic.

author: miceuz
author_rank: Frequent Contributor
timestamp: April 25, 2014, 02:39:08 pm
content: Try putting some load on the +5 and +24 rails, it might be the supply needs some minimal load to be stable.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 25, 2014, 03:28:46 pm
content: Don't know the make and model of the machine. Already threw away the body and frame. No, I didn't search for a schematic. Couldn't imagine that being a fruitful endeavor. In the past, any board numbers I've searched on produced next to nothing if not nothing. You ever have luck with that?Applying a load sounded like a good idea. The results raise more questions. Keep in mind, the OP says the 50 VAC was to the earth ground and nothing on the GRD pin. After applying the load, I know get 12 VDC on just about half the pins to the GRD pin. So, at least I'm reading something to GRD pin now. But, I'm not finding a 24 VAC or a 5 V (ac or dc) in spite of silk screen. Removing the load seems to have no further effect, other than a couple hundred mV increase in output.What's the chance the board could be dependent upon some external circuitry? A sensor or MCU to make adjustments?

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 25, 2014, 03:37:42 pm
content: Looks like a Brother 2820. Still looking for a schematic

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 25, 2014, 04:31:12 pm
content: Joe, you were right. I searched and found the service manual complete with schematics. They don't show component values, but the connectors are labeled and the general design is there. As a beginner, I don't know how much I can discern from the drawing, but I'll study it a bit and see what I can determine.http://elektrotanya.com/brother_fax2820_2825_2910_2920_mfc7220_7225n.pdf/download.html

author: Phaedrus
author_rank: Frequent Contributor
timestamp: April 25, 2014, 04:39:27 pm
content: Well, does the GRD pin show continuity with earth ground? A loose or disconnected ground wire could easily cause all kinds of power supply wonky-ness.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 25, 2014, 04:56:54 pm
content: No, I don't think so. It looks to me like it's isolated through the transformer according to the schematic. I'm going to scope it and see what I get.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 25, 2014, 06:06:38 pm
content: I can't figure it out. The scope is showing a steady ~12VDC on all the output pins. With or without a load.I scoped the transformer output and got the interesting wave form in the attachment. Does that mean anything? Looks like two taps. One at approximately 24 V PtoP and one at approximately 5. Both have the same wave form. Note probe is 10x on Ch 2