[GreenKeys] Re: TTY-Connect transformer questions

[gil smith]

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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