[GreenKeys] Loop Supplies (was: any missing USB-tty adapter orders?)

[Bob Camp]

Hi

The TIP50 is another part that could be used. It costs a whopping 10 cents more. It will handle
twice the voltage and twice the power. The TO-220 package is a bit more common when looking
for heatsinks. It is slightly larger, possibly a drawback if you are building a loop supply in a thimble. 

One cute thing you see on the TIP-50 data sheet - it’s rated for inductive clamping. Simply put,
as long as you don’t overdo it, the transistor is ok with inductive spikes. I’d still go with some sort
of protection. 

With any loop supply, there is one set of numbers that work with a single set of magnets. If you
start to load a single loop up with lots of machines … those numbers go up. Voltages, protection, 
and the like all scale. At some point you *will* need either a crazy high voltage system or 
multiple loops. 

There are high(er) voltage parts out there, my solution was multiple loops. Transformers that will do
> 60 ma aren’t that hard to find. Running multiple resistor or transistor based loops off of a single transformer,
rectifier, cap combo is pretty straightforward. I found it a bit easier with the transistors since you didn’t care
at all about voltage regulation. With a half dozen (as if) loops on a single capacitor, there can be some
level of interaction as all the loops go in and out of service. 

Bob




> On May 19, 2016, at 4:57 AM, Jim Backus <j.backus at jita.demon.co.uk> wrote:
> 
> Hi
> 
> Comments on MJE340 noted. It certainly dates back to 1982 where it is shown in ARRL handbook RTTY section.
> 
> Unfortunately the availability of high voltage semiconductors has been affected by the demise of CRT TVs with their need for high power scanning circuits. A while ago I worked for a company that made solid state modulators for magnetrons and finding devices to switch high current, high voltage fast was a part of the challenge.
> 
> Regards
> 
> Jim
> 
> On 19/05/2016 01:50, Bob Camp wrote:
>> Hi
>> 
>> High voltage supply is whatever you happen to have. Something in the 80 to 100V range is plenty.
>> 
>> Coils all go in series between B+ and the collector of the MJE340. Two neon bulbs in series go across the set of
>> coils.
>> 
>> Bias resistor goes from B+ to the MJE340’s base
>> Zener goes from base to keying input hot side.
>> Emitter of MJE 340 goes to keying input hot side to ground.
>> Keying input cold side goes to the return on the B+ supply.
>> 
>> In practice, I preferred to run a separate keying loop and drive the magnets with a MJE340. The keying loop ran
>> at low voltage.
>> 
>> There is no reason to stick with the 340 these days. It’s a ghastly old part by now. The approach was simple and it did work.
>> I believe it came out of RTTY Journal.
>> 
>> Bob
>> 
>> 
> <rest snipped>
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