[GreenKeys] TTY-Connect pictures and 160V vs 80V info

[Bob Camp]

Hi

Very nice batch of documentation on the web site !!!

After looking at all your *very* neat pictures of the unit another heat  
issue comes to mind.

At 160 volts and 60 ma each loop is going to generate ten watts of  
power. You really have four high voltage loops in the box when both  
sides are set to full duplex. Since two loops are 30 ma and the other  
two are 60 ma that's 15 watts per.  With a 70% efficient power supply  
(pretty good for a little transformer) and the rest of the stuff in the  
box you are close to 50 watts of heat. Depending on the box size and  
what is stacked on top or bottom of it this may be a problem. Sticking  
a nice noisy fan on the back of thing would help a lot.

Another, far less noisy solution would be to mount the loop resistors  
in the cabinet of the TTY. If you go with big old ceramic wire wounds  
then mounting becomes fairly easy since you don't require a heat sink.  
The cabinets are all so darn big that a few more watts won't have much  
of any effect on them.

One other caution - I have seen more than a few machines that have a  
ground path on the keyboard side. It's not common to see it on the  
magnet side but it can happen. It's probably a real good idea to have  
people ohm out their machines before hooking the board up to them. A  
ground short on either side will smoke the board (also a heat issue  
....) or at least blow a fuse.

Given the number of hands these machines have passed through you can  
never be sure of what's in there. Most older setups had the loop  
resistor in series with the transformer. In order to keep the magnets  
in mark you just grounded the "bottom" end of them. A lot of setups  
worked sort of the same way with one side of the keyboard contacts tied  
to ground. The net result would be a machine with one or more magic  
switches on it that kill the board when you close them or even when you  
just plug in the machine.

	Take Care!

		Bob Camp
		KB8TQ






On Jun 30, 2004, at 12:57 PM, gil smith wrote:

> Hi folks:
>
> Some of you wanted to see pictures of TTY-Connect, so I put some up:
>
> http://www.vauxelectronics.com/gil/tty-connect/
>
> Also, the high-voltage loops may be configured for either 80VDC or  
> 160VDC.  To use 80VDC, the transformer primaries are wired in series,  
> and to use 160VDC, the transformer primaries are wired in parallel.   
> The power resistor values change too, of course.
>
> I really recommend the 80VDC option, since things run much cooler.   
> With the heatsinks listed in the parts list, the unit set to full  
> duplex (which burns additional power), and the loop set for 60-mA,  
> temperatures read:
>
> config       xfrm           heatsink
> ----------------------------------------------------------------------- 
> --
> 80VDC     100 degrees      125 degrees
> 160VDC    110 degrees        175 degrees
>
> 175 degrees (F) is just too darned hot, in my opinion.  If you really  
> must use a 160V loop, it should have a larger heatsink and/or fan.  If  
> you use 80V, the heatsink listed in the parts list should be fine with  
> vertical mounting and simple convection cooling.
>
> Or, if you configure the loop (and your machines) for 20-mA, the 160V  
> configuration will run much cooler.  But most folks have 60-mA  
> machines, so I still recommend 80V.  I really doubt that anyone will  
> notice the slight-to-none reduction in selector range margin.
>
> thanks,
>
> gil
>
>
> ;---------------------------------------------------------------------- 
> ---
> ;  vaux electronics, inc.             480-354-5556
> ;  www.vauxelectronics.com  (fax: 480-354-5558)
> ;---------------------------------------------------------------------- 
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>
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