Electronics > Beginners
7812 voltage output too high?
<< < (3/4) > >>
Zuul:
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?
wraper:

--- Quote from: Zuul on May 05, 2019, 11:51:53 am ---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?

--- End quote ---
AN7812 likely has higher quiescent current. Does not matter.
bdunham7:
"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!
Zuul:
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.
Zuul:

--- Quote from: bdunham7 on May 05, 2019, 02:45:25 pm ---"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!

--- End quote ---

Thanks  :-+ about 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 supply



EDIT: I bit the bullet and connected the power supply to the computer, booted it up, and measured voltages... +12.5V, -12.5V, +5.00V
So all is well I guess..

Thanks for your time!
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod