Thanks kubeek, thought it was OK like that, and the circuit is wired exactly like last pic checked it many times.

 

...............the circuit is wired exactly like last pic checked it many times.

There seems to be only three choices:
1) Error in wiring
2) Faulty device
3) Incorrect part type

 

There seems to be only three choices:
1) Error in wiring
2) Faulty device
3) Incorrect part type[/QUO
Would you use 78 and 79 regs in this type of power supply?

 

78 and 79 type regulators should work fine for this application.

 

Although with 27V on the input and 12V output you'll be wasting a lot of power even at modest currents.

 

Hi all, after all the problems with the power supply I think the reason why the 7912 regulator wasn't outputting -12v was down to the high voltage in of -27v dc. I changed transformer to a 8v 4.5va and both + and - voltages are correct.
Does anyone know why the 7812 could reduce the 27 volts down to 12 but the 7912 couldn't? To my knowledge these regulators can handle that size of voltage, and although there is alot of energy lost reducing the voltage from -27v to-12 it should be capable.

If I get a replacement transformer at 9v 10va , after rectification will voltage be around 15 or 16v dc?
I think 15 or 16v going into regulator would be suitable as less energy wasted, also am I correct in calculating current supplied from the transformer would = 10va/12v dc =0.83mil/amp.
As always appreciate any help.

 

Power-dissipation induced thermal shut-down of the 7912?

 

Power-dissipation induced thermal shut-down of the 7912?

Would thermal shut down stop any voltage from outputting from 7912?, there was 18 v on output could never drop to 12 tho.

 

Could someone please tell me are these calculations for working out dc current and voltage values for a power supply including a transformer and bridge rectifier correct.
Transformer sec rating 9v 10va
10va/9=1.11amps
Dc equivalent voltage = 1.41*9=12.6v.
Current= 0.62*1.11=0.68ma
Also does all transformers output a higher than rated voltage when not underload?

 

Hi all, still having problems with power supply output voltage.
Having learnt that bypass capacitors are needed across vin and vout of the 7912 I ordered some 35v 2.2 and 1.5mf tantalum type caps, as recommended in wiring diagrams that were posted through link on this site.
Anyway having received caps today I fitted them but unfortunately they have not reduced the voltage , does anyone have any ideas why this still doesn't work? As always I have double checked all my connections and I made sure the caps were poled correctly as shown in diagram I have included.
Also whilst experimenting with different value electrolytic caps I found that having a single 10mf connected from ground to vout on the 7912 brought the voltage down to the correct -12v, but it will only do this when the - of the cap is connected to ground and the + to vout, which is reversed polarity to that shown in wiring diagram, again any ideas why this would be?
Appreciate any replys.

 

Hello,

Did you isolate the regulators from the heatsink?
On the LM79XX series the tab is also connected to the middle pin wich is the input voltage:



Bertus

 

Hi there bertus, I have the circuit built on a breadboard so heatsinks not in place. Its same old problem 7912 will not output -12v. I had been advised about adding caps at vin and vout of 7912, and having placed them in circuit there was no change.
Strangely what I have found is that whenever a 10mf electrolytic cap is placed across the ground an vout of 7912, the voltage drops gradually to -12v. I found this strange as the cap has to be connected -to ground and + to vout for the voltage to begin to drop to -12v which I thought had to be the reverse as it is a - voltage, and is connected that way on diagram. Also some caps have popped when fitted - to ground and + to vout yet the 10mf remains ok and drops voltage to -12v, any ideas what's going on with it?
The only other solution I can think of is replace the transformer with 12v at 14.7 no load, instead of the 15v at 20v no load that I have. By my calculations V peak=1.41x14.7v= 20.7v and V average=0.90x14.7v=13.23v.
Having proved that the 7912 will output correctly at around 20v on vin then this could be a solution. Also I would like to have 500ma available on +12 and -12, total of 1amp dc current. If I used a transformer rated at 12v 20va then by my calculations
20va/12=1.66ac amps which = 0.62x1.66=1.02/2 =around 500ma per 12v output.
Hoping you could confirm if its worth changing transformer or have I missed something with the capacitors.
Thanks

 

Hello,

Could you post a photo of the breadbord set-up?

Bertus

 

Hi bertus, here is photo of breadboard. Blue is + voltage white is - .black wire is 0v

 

Hello,

The grounding connections are not clear.



Do you have other colors of the BB wire?
Could you make the ground white, + red and - blue?

Bertus

 

That looks like a complete mess and I can´t see half the connections. Also your output caps are WAY too far from the regulators to have any effect. You are supposed to place them as close as possible, which means right next to the regulators.
Also, why is pin 2 on the left regulator connected to pin 1 in the right regulator and nothing else?

 

Hi all I rewired with red = +12, white = -12 and black = ground. Kubeek ignore that photo links are crossed over and hard to identify. Hope new photos are better, included tantalum caps across both regulators but still no change in 7912 voltage output.

 

 

It's a bit hard to tell, but it appears the output-to-ground cap on the 7912 is not is the same row as the output pin.

 

Good spot, have corrected the capacitor, was in a hurry last night. No Change to output voltage still close to -19v.
Sorry for the messy wiring in previous posts, it may look bad but they are all correct apart from one mistake, and that was because I was in a rush. Have had this circuit built many times on breadboard and on pcb with same results, tried different components and no change. The only solution I have found is by reducing the voltage through a variable resistor the 7912 begins to output correctly when the input voltage is at 20v, instead of the 27v. Please note that both 79 and 78 regs have been swaped although they are of the same manufacturer. Might get different manufactured regs will wait and see.
Was thinking of changing transformer as the no load voltage is quite high at 20v which = 27v after rectification, could someone confirm that my calculations are correct for new transformer,
VP= 1.41x13.7v no load=19.3v @ vin
I= 0.62x917ma=568ma
Transformer spec 2x12v 2x917ma no load 13.7v. Went with lower voltage and higher current output, va=22
Cant see what else to try apart from reducing voltage or trying voltage regs from a different manufacturer. If anyone can think of anything else please let me know, thanks