[GreenKeys] Loop power supplies

[Bob kb8tq]

Hi

The “cute trick” with a transistor constant current setup is to organize it so the 
current limit transistor also can key the loop. Back when Poly Packs wanted lots
of money for transistors, that was the way to go,. 

With a 100V loop voltage, the transistor will see about 6W on a 60 ma loop. Anything 
in a TO-220 package will handle the power. A heatsink of some sort is a good idea. 

The voltage is the bigger issue. If you *do* use it as a keying transistor, it will have to 
handle a reasonable inductive spike (a couple hundred volts) to get the magnets to 
de-energize quickly. A pair of neon bulbs was the state of the art spike suppressor 
back in the 1970’s. 

If you don’t use it as a keying transistor, you just need something who’s DC safe
operating area includes 60 ma at a bit over the loop voltage. A protection diode
or two also goes in if it’s not keying. 

The KSC5502DTTU would work fine. Mouser sells them for 86 cents each in singles or 73 
cents if you buy ten. No reason to use them instead of any of a few thousand other
parts. They just came up first on a quick sort. Their current gain has a nice peak
just past 60 ma …

A zener diode and a resistor are about the only other parts involved. Dead bug 
wiring them on the part leads isn’t all that hard.

Bob

 




> On Mar 4, 2018, at 1:58 PM, Jack <wa2hwj at att.net> wrote:
> 
> United Press used a small transistorized constant current gadget. It might been the NR unit. Maybe we could get some PC boards made up? We’d have to find current suitable transistors. 
> Jack K2TTY
> 
> 
> Sent from my iPhone
> 
> On Mar 3, 2018, at 6:05 PM, Sheldon Daitch via GreenKeys <greenkeys at mailman.qth.net <mailto:greenkeys at mailman.qth.net>> wrote:
> 
>> Here is a link to the Northern Radio Company's loop current control manual:
>> 
>> http://www.tmchistory.org/NorthernRadio/Manuals/ib_238-1-2_4_8_66.pdf <http://www.tmchistory.org/NorthernRadio/Manuals/ib_238-1-2_4_8_66.pdf>
>> 
>> 73
>> Sheldon
>> 
>> 
>> From: Jim Haynes <jhhaynes at earthlink.net <mailto:jhhaynes at earthlink.net>>
>> To: Ralph Mowery <rmowery28146 at earthlink.net <mailto:rmowery28146 at earthlink.net>> 
>> Cc: 'Green Keys' <greenkeys at mailman.qth.net <mailto:greenkeys at mailman.qth.net>>
>> Sent: Saturday, March 3, 2018 11:36 AM
>> Subject: Re: [GreenKeys] Loop power supplies
>> 
>> On Sat, 3 Mar 2018, Ralph Mowery wrote:
>> 
>> 
>> > 
>> > I and others have tried it, and 24 volts and under does not work even on
>> > short loops ( like maybe 5 or 10 feet of wire)  with machines like the model
>> > 15 printer.  It really needs more than 50 volts to do the job, close to 100
>> > or 150 volts to really work well.
>> 
>> > 
>> I haven't tried it, but there are circuits that use the "constant current"
>> properties of a transistor rather than a fixed resistor, and are said to
>> give equivalent results with lower voltages such as 48V.  For instance
>> QST, Jan 1972, p. 40.  Maybe someone who knows how can do a SPICE
>> simulation to compare such a circuit with a plain resistor.
>> 
>> 
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