Voltage control using 78xx - UK Vintage Radio Repair and Restoration Discussion Forum

Oldcodger · 26th Jul 2015, 11:51 pm

I seem to remember on some electronic office equipment ,the standby function was via a toggle control on the common of a (in this case ) 7805, and I think ,via a transistor used as a switch with C-E in the common to gnd path. This was over 20/25 years ago, and on redundancy I had a ceremonial burning of all my workshop manuals for that range. Anyone dabbled with this as a way of switching a PSU on/off.

Refugee · 27th Jul 2015, 12:15 am

You would have to switch the ground connection to a negative 5V supply in order to bring its output down to zero.

TonyDuell · 27th Jul 2015, 5:47 am

I don't quite get this. If you open-circuit the common connection to a three terminal regulator (certainly a 78xx or 79xx) the output voltage rises to essentially the input voltage (less a saturated transistor drop?). Anyway, the output most certainly does not go to zero.

This was a common problem with the TO3-canned 78xx's. The common connections was the case. If the mounting screws came loose then the output would go sky-high and kill ICs all over the instrument.

tony brady · 27th Jul 2015, 8:13 am

You need to switch the input to the regulator. While you can use an NPN it's better to use a PNP series transistor with an NPN controlling it. This allows control from a low voltage source such as a processor

Oldcodger · 27th Jul 2015, 11:45 pm

As said, I can only remember parts of the circuit as it was a long time ago . As I remember it the transistor was in the common , and switched the 7805 ,but as said I only remember the circuit in passing. The need was for a in car 12 supply when the ignition was off, and the idea of using a transistor instead of a relay ( which would need to be operated when ignition on ,although at a lower current if a series resistor was switched in ), appealed in terms of circuit space saving and relay being on .TBH, I 'd not tried simply sticking a 7805 in a supply and disconnecting the common, as I've not had much to do with this component.In most of my professional life ,I've dealt with SMPSU, or series pass regulators .

FRANK.C · 28th Jul 2015, 12:52 pm

Would it not be more likely to be a LM317?
In the LM317 data sheet there is a circuit showing it been switched using a transistor driven by TTL.

Frank

Oldcodger · 29th Jul 2015, 11:20 pm

Possibly ,Frank, as I only glanced at this part of the circuit, and thought it could be useful in future. But I remember the supply line was 5v from one IC,with no other components. Circuit was used on a lot of Dictaphone desktop machines in the mid 80's, hence my vagueness about the circuit.

26th Jul 2015, 11:51 pm	#1
Oldcodger Nonode Join Date: Oct 2014 Location: West Midlands, UK. Posts: 2,181	Voltage control using 78xx I seem to remember on some electronic office equipment ,the standby function was via a toggle control on the common of a (in this case ) 7805, and I think ,via a transistor used as a switch with C-E in the common to gnd path. This was over 20/25 years ago, and on redundancy I had a ceremonial burning of all my workshop manuals for that range. Anyone dabbled with this as a way of switching a PSU on/off.

27th Jul 2015, 12:15 am	#2
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: voltage control using 78xx You would have to switch the ground connection to a negative 5V supply in order to bring its output down to zero.

27th Jul 2015, 5:47 am	#3
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,225	Re: voltage control using 78xx I don't quite get this. If you open-circuit the common connection to a three terminal regulator (certainly a 78xx or 79xx) the output voltage rises to essentially the input voltage (less a saturated transistor drop?). Anyway, the output most certainly does not go to zero. This was a common problem with the TO3-canned 78xx's. The common connections was the case. If the mounting screws came loose then the output would go sky-high and kill ICs all over the instrument.

27th Jul 2015, 8:13 am	#4
tony brady Hexode Join Date: Mar 2014 Location: Peacehaven, East Sussex, UK. Posts: 278	Re: voltage control using 78xx You need to switch the input to the regulator. While you can use an NPN it's better to use a PNP series transistor with an NPN controlling it. This allows control from a low voltage source such as a processor

27th Jul 2015, 11:45 pm	#5
Oldcodger Nonode Join Date: Oct 2014 Location: West Midlands, UK. Posts: 2,181	Re: Voltage control using 78xx As said, I can only remember parts of the circuit as it was a long time ago . As I remember it the transistor was in the common , and switched the 7805 ,but as said I only remember the circuit in passing. The need was for a in car 12 supply when the ignition was off, and the idea of using a transistor instead of a relay ( which would need to be operated when ignition on ,although at a lower current if a series resistor was switched in ), appealed in terms of circuit space saving and relay being on .TBH, I 'd not tried simply sticking a 7805 in a supply and disconnecting the common, as I've not had much to do with this component.In most of my professional life ,I've dealt with SMPSU, or series pass regulators .

28th Jul 2015, 12:52 pm	#6
FRANK.C Heptode Join Date: Feb 2006 Location: Roscommon, Ireland Posts: 732	Re: Voltage control using 78xx Would it not be more likely to be a LM317? In the LM317 data sheet there is a circuit showing it been switched using a transistor driven by TTL. Frank

29th Jul 2015, 11:20 pm	#7
Oldcodger Nonode Join Date: Oct 2014 Location: West Midlands, UK. Posts: 2,181	Re: Voltage control using 78xx Possibly ,Frank, as I only glanced at this part of the circuit, and thought it could be useful in future. But I remember the supply line was 5v from one IC,with no other components. Circuit was used on a lot of Dictaphone desktop machines in the mid 80's, hence my vagueness about the circuit.