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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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26th Jul 2015, 11:51 pm | #1 |
Nonode
Join Date: Oct 2014
Location: West Midlands, UK.
Posts: 2,181
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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.
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27th Jul 2015, 12:15 am | #2 |
Dekatron
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 5,554
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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.
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27th Jul 2015, 5:47 am | #3 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,222
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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 |
Hexode
Join Date: Mar 2014
Location: Peacehaven, East Sussex, UK.
Posts: 278
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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
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27th Jul 2015, 11:45 pm | #5 |
Nonode
Join Date: Oct 2014
Location: West Midlands, UK.
Posts: 2,181
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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 .
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28th Jul 2015, 12:52 pm | #6 |
Heptode
Join Date: Feb 2006
Location: Roscommon, Ireland
Posts: 732
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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 |
Nonode
Join Date: Oct 2014
Location: West Midlands, UK.
Posts: 2,181
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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.
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