7812 voltage output too high? - Page 1
Forum section: electronics / beginners

author: Zuul
author_rank: Contributor
timestamp: May 05, 2019, 11:01:44 am
content: HelloThis is a power supply for a Philips NMS8220 I am trying to get sorted. The computer seemed to be working fine but of course I just had to open it up and poke around in there. Measuring +12v at the motherboard connector and it was way high.I swapped out all the caps, the 7812 and 7912 regulators, but I am measuring over 17 volts out of the 7812 on the pin.. the voltage in is around 19.5vI have swapped out the 7812 again and it's still about the same. It is a 7812CT and the original one is in the trash so I couldn't tell you the numbers on that...The fuse that is missing in the pic was at some point in time been "fixed" with aluminium foil by the previous ownerWhat should I try to get this fixed? Please treat me as a total noobie because, you know, I am!Excuse the fuzzy pics.

author: mikerj
author_rank: Super Contributor
timestamp: May 05, 2019, 11:04:45 am
content: Measure between the centre pin and output pin of the 7812. There may be something that lifts the center pin voltage above ground, either accidentally or deliberately.

author: wraper
author_rank: Unknown
timestamp: May 05, 2019, 11:07:37 am
content: There might be resistor in parallel to it. Probably R8. In that case increased voltage without load present is normal.

author: Zuul
author_rank: Contributor
timestamp: May 05, 2019, 11:09:21 am
content: Thanks for replyingOK, it measures steady 17.03v on the pins

author: Zuul
author_rank: Contributor
timestamp: May 05, 2019, 11:10:52 am
content: Right, is it safe to plug it in? Where should I measure next? At the connector?

author: wraper
author_rank: Unknown
timestamp: May 05, 2019, 11:18:45 am
content: Circuit is quite strange. Looking in the bottom of PCB. GND pin of 7812 goes to plus pin of rectifier, input and output cap of vreg and nothing else.It's apparently 7912 connected to minus pin.

author: Zuul
author_rank: Contributor
timestamp: May 05, 2019, 11:27:42 am
content: Quote from: wraper on May 05, 2019, 11:18:45 amThe service manual is too large but I took a screenshot of a page

author: wraper
author_rank: Unknown
timestamp: May 05, 2019, 11:30:09 am
content: You'd better post schematic rather than PCB layout.

author: wraper
author_rank: Unknown
timestamp: May 05, 2019, 11:35:08 am
content: Looking on PCB layout from manual, R8 actually is connected in parallel with input-output of 7812. So my first guess was right. If you test this PSU connected to load, then the problem is with too small load (faulty circuit), not PSU.

author: Zuul
author_rank: Contributor
timestamp: May 05, 2019, 11:37:32 am
content: Right..

author: Zuul
author_rank: Contributor
timestamp: May 05, 2019, 11:51:53 am
content: So I could've mentioned that I have an identical machine for parts that I just took out the power supply from and connected it to the same transformer and it measures 13.21v without load on centre pin and v-out on 7812. I belive it says AN7812 on the regulator...does that matter at all?

author: wraper
author_rank: Unknown
timestamp: May 05, 2019, 12:20:46 pm
content: Quote from: Zuul on May 05, 2019, 11:51:53 amAN7812 likely has higher quiescent current. Does not matter.

author: bdunham7
author_rank: Super Contributor
timestamp: May 05, 2019, 02:45:25 pm
content: "the original one is in the trash so I couldn't tell you the numbers on that"Since you are a self-admitted noob, let me give you some old-timer advice.Never throw the old parts away until the job is done!Just follow that rule without thinking about it.As for your problem, measure the voltage (to ground) on all 3 legs of your 7812, then do the same thing on the other unit. Then put a small load (500-1K) on the output and do it again to both. More measuring and less soldering!

author: Zuul
author_rank: Contributor
timestamp: May 05, 2019, 04:04:29 pm
content: So I've looked at the schematic and made some measurements:power supply 1: transformer 31.5VAC --> 1B4B1 rectifier 19.6VDC measured at v-in on 7812CT regulator, 17.08VDC out. I measured R8 to be 56 Ohms.power supply 2: same transformer ------> 1B4B1 rectifier 18.2VDC measured at v-in on AN7812 regulator, 13.38VDC out. R8 is 54.5 Ohms. This one working OK. (or should I say, I'm not afraid to plug this in to the motherboard)Now, looking at the schematic I noticed capacitors C4 and C6 are on +12V and took a look on both boards... C6, according to the schematic, is supposed to be 10uF but is 1uF on both boards....Does it matter? I don't know.. Equally, C9 should be 10uF on -12V but is 1uF on power supply 1.Well I swapped them out for 10uF and no change, infact v-out on 7812 PSU1 is now 17.78V, 7912 out is -13.3V.Next to try is putting the 7812 from PSU2 into PSU1......and now I have 13.6V out from the old 7812.

author: Zuul
author_rank: Contributor
timestamp: May 05, 2019, 04:17:58 pm
content: Quote from: bdunham7 on May 05, 2019, 02:45:25 pmThanksabout that small load.... 500-1K sounds like a resistor to a noob.IDK it sure feels like these 7812CT regulators are garbage (totally subjective opinion)....at least for this power supplyEDIT: I bit the bullet and connected the power supply to the computer, booted it up, and measured voltages... +12.5V, -12.5V, +5.00VSo all is well I guess..Thanks for your time!

author: wraper
author_rank: Unknown
timestamp: May 05, 2019, 05:41:42 pm
content: Quote from: bdunham7 on May 05, 2019, 02:45:25 pmDoing so is useless. There is no issue with PSU. Not to say 1k won't do much, there is 56 OHm resistor in parallel with vreg.