Regulator question.

[HellasTechn]

Hello !

I have a few 78xx and & 79xx Vregulators. I suspect some of them are bad. The 78xx i test with my DMM and bench psu by connection the positive of the PSU to pin 1 , the negative to pin 2 and read output with my DMM from pin 2 to pin 3.

I have an 7915 Vreg that i connect as follows : (since i dont have a psu that would give -V --- GND --- +V) positive lead of psu to pin 1 (ground of 7905) ground of PSU to lead 2 and read output with the DMM between pin 1 and pin 3.

I expected to read a -15V on the DMM with the DMM's gnd to pin 1 of the Vreg and the positive of the DMM to pin 3 of the Vreg
but i only get like -7V (with 24V input)...

What am i doing wrong ? is it just a bad Vreg ?

 

[Audioguru]

Texas Instruments' datasheet for their uA78xx and uA79xx talk about and show an input capacitor and an output capacitor on all schematics and spec's. Your tests have none then they are probably oscillating.

Did you buy cheap fake parts on ebay?

 

[HellasTechn]

No theese are genuine. i just bought a new one and checks out ok the way i mentioned in post 1.

I guess that answers my question about the parts being bad. It is true though that capacitors are required when used on circuit.

Thank you Audioguru !

 

[duke37]

I follow the connections for the 78xx, with the ground at the centre. Did it work?

The connections to the 7915 are different, have you got them correct?
Where is the 7905 connected? Show a schematic.

 

[davenn]

I have an 7915 Vreg that i connect as follows : (since i dont have a psu that would give -V --- GND --- +V)

you don't need one

positive lead of psu to pin 1 (ground of 7905)

correct

ground of PSU to lead 2

ground ?? did you really mean the negative lead ?

I expected to read a -15V on the DMM with the DMM's gnd to pin 1 of the Vreg and the positive of the DMM to pin 3 of the Vreg
but i only get like -7V (with 24V input)...

why would you expect to get -15V out of a -5V regulator ?

 

[duke37]

The 7815 connections are
In (+)
Gnd
Out (+15V)

The 7915 connections are
Gnd
In (-)
Out (-15V)

I think the tag is connected to ground.

 

[HellasTechn]

ground ?? did you really mean the negative lead ?

Yes i meant the negative lead.

why would you expect to get -15V out of a -5V regulator ?

My mistake. I was testing an 7915 regulator.

 

[HellasTechn]

you don't need a negative power supply as the positive one hooked up backwards is a negative power supply

Yes that i fully understand besides current does not really have polarity, it is just movement of electrons. What defines polarity is the direction of the electrons movement.

 

[HellasTechn]

I think the tag is connected to ground.

Tag (metal back of the regulator) is always connected to the center pin. for the 78xx series is ground and for the 79xx series is input.