[GreenKeys] TTY-Connect transformer questions

Wed Jun 16 21:16:48 EDT 2004

Hi

This is getting a bit further along than I'm really at but here it goes  
anyway:

A 75 volt resistive loop is probably no worse than what Teletype did  
with their own transistor based keying circuit so certainly it will  
work. One trick *seems* to be to put a capacitor across the magnets so  
they release slower. That way the slow pickup is balanced out and the  
net distortion is less.

A 75 volt constant current loop will do quite a bit better than a  
resistive loop at the same voltage.

A modern constant current loop can *probably* (like I said ...) be made  
with a LMV431, a 22 ohm resistor and a high voltage TO-220 package  
MOSFET. You still have the power dissipation in the FET but it also  
acts as the loop keying transistor. It's the same amount of power you  
would have in the loop resistor, it's just coming out of the FET. Total  
cost of the keying parts is probably less than a buck and certainly  
less than two bucks.

Now for the really weird stuff ....

Take the same constant current keyer, add another MOSFET, a ten or  
twenty micro henry toroid, a diode and a little stuff on a PIC. There  
may also be an op amp or a few resistors but who's counting the small  
stuff ...

Poof ... you have a switching loop supply. The loop bias runs directly  
off of a12 volt supply most of the time and only ramps up to high  
voltage when it has to. One pin on the PIC does the pulse drive for the  
FET. One or two analog inputs finish out the resources.

What you do is wait for a space to mark transition to come along. Since  
you know in advance when this will be you fire up the switcher  
something like 10 ms before the transition. It charges up the output  
cap and then stops charging before the event. Since the loop is in the  
space condition the switcher only drives a cap but not the loop  
magnets. When the constant current keying transistor switches on the  
capacitor supplies the voltage to drive the magnets. With a good model  
for the magnets you can size the capacitor and the charge voltage so  
there is just enough energy to do the job but no so much that you just  
throw it all away.  With the analog inputs to the PIC it can learn what  
the optimum voltages are for the magnets you have in the loop and the  
capacitor on the board ....

You run about 60 ma from the 12 volt supply all the time. You are  
either supplying loop current directly or supplying the switcher as it  
boosts the voltage. It seems kind of weird that you get away with that  
little draw off of 12 volts but at lest the models say it's true.

The net result is that you have a very modest dissipation in the  
constant current / switch transistor. It only has high voltage on it  
and current through it for about a mili second each time it switches.  
You get about 20 switches a second so that's not a lot of on time. If  
you have a 12 volt supply most of the time and about four volts on the  
magnets then you get a half watt from the low voltage part of the  
process. The switcher boost transistor isn't running all that long  
either so there may not be a whole lot of power in it either. You may  
not need a heat sink on either part ...

What we just have built is pretty much a modern florescent lamp  
ballast. The MJE340 keyer transistor used to be the king of the hill  
for this sort of stuff and now the FETS seem to be the way they go. The  
net result is that the parts don't cost an awful lot of money. I  
suspect that the cost is still under two bucks. It's certainly under  
three ....

The only real problem with the idea is that you use a lot of PIC clock  
cycles to generate the drive for the switcher. This is driven by trying  
to keep the switcher inductor small and the ramp up fast. If you  
dedicate an eight pin part to the task then you would add about buck to  
the total cost. If you can fit it into a part you already have then  
that part of it is free. Since you are replacing the keying transistor  
and loop resistors you already have the total delta cost might not be a  
whole lot ....

Of course none of this is proven with a breadboard at this point. It's  
also a little late given that the pc boards are already in the fab  
process .... sorry about that.

	Enjoy!

		Bob Camp
		KB8TQ

On Jun 16, 2004, at 12:25 PM, gil smith wrote:

> Hi Ken:
>
> The schematic is not ready yet.  I tried a smaller VA 115:115 xfrm in  
> my previous TTY232 project, but it got pretty hot.  This one still  
> gets hot, but the higher VA rating can't hurt reliability, and it's  
> about the same size/price anyway.  I have actually tried two different  
> xfrms of about the same rating from different manufacturers, and they  
> both seem to work well.
>
> The power resistors get pretty hot too, even though they are mounted  
> on vertical finned heatsinks with some convection cooling.  The  
> heatsink could be bigger, or a fan could be added, if desired.
>
> I am going to try different resistors and use only half of the xfrm  
> output (for 75VDC in the loop instead of 150VDC).  A 75V loop will be  
> just fine for local tty machines and things will be much cooler.  The  
> xfrm has dual 115 primaries, and dual 115 secondaries, so this is  
> easily done by wiring the primaries in series, and wiring the  
> secondaries in parallel, for a 2:1 step down.  Folks can build the  
> loops either way.  When I get the schematic and parts list finalized,  
> I will point out the option.
>
> Yes, you can use an xfrm as you did, to supply both HV and LV  
> windings.  I kinda like to keep the LV stuff separate (and a small LV  
> xfrm is only a few bucks).  If the HV windings ever shorted to the LV  
> windings, the board could be toasted, and safety compromised.  Whether  
> a short is physically possible depends on how the xfrm is wound  
> (separate bobbins...), but since the HV xfrm will get hot, I'd leave  
> it on its own.
>
> Another thing that no one asked about yet is using a  
> dual-115-secondary to drive the two HV loops.  Yeah, again, now a much  
> hotter xfrm.  I like to keep things reasonably cool when possible.
>
> thanks,
>
> gil
>
>
> At 03:48 AM 6/16/2004, you wrote:
>> Hi again,
>>
>> Check going out in this morning's mail. Will be using the HV loops to  
>> start with but will be populate the low voltage loop also. Junk box  
>> here is very well stocked so expect that I will need to purchase very  
>> little to complete the project.
>>
>> Notice that recommended HV transformers secondary current is several  
>> times required output current. Is that for a safety factor or is  
>> there other circuit current requirements that I am missing. Don't  
>> have a copy of schematic if available.  I have several dual primary /  
>> dual secondary 48 VA power transformers available that I plan to use.  
>> Used same in ST-6 with normal secondaries supplying low voltage and  
>> second 115v primary winding also used as secondary. Was able to do  
>> this as total supply output draw including hijacked primary winding  
>> was considerably less than 50% of the transformer rated VA.
>>
>> Thanks for doing this board project. Nice looking board and excellent  
>> manual.
>>
>> Thanks again,
>>
>> Ken
>
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