[GreenKeys] Current loop not providing (enough) current

[Jordan Spencer Cunningham]

I'm certain that's the problem. I've done some testing on my Models 15 and 19 in the past, and if the 26 magnets are in the same realm of tolerances, they'll start to become useless at around 50mA, give or take. 25mA is much too low a current to have any effect.

The keyboard is keying the loop successfully since that's just mechanical, and the board can pick up signals even lower than 20mA, but the selector magnets can't.

The question is why. Looks like you expected 96 volts and so calculated 1600 ohms resistance to get 60mA. However, it sounds like there's more resistance in your loop. A few hundred ohms is expected, but you've got an additional 2.2k ohm and change. 

I'm willing to bet the loop may already have a 2k ohm resistor in series somewhere-- maybe installed within the machine itself if you have nothing else on the loop. Try measuring resistance in the full loop with and without your extra resistors, and I'd guess you'll see the same thing. As long as you see roughly 2000 ohms on the loop without your extra resistors, it will be safe to power it without the extra resistors. At 96 volts, however, you'll get a current of only around 40 - 45mA, which probably won't reliably trigger the magnets. But who knows, maybe it'll work fine-- it's always worth trying.

If not, you'll either need to locate and replace that 2k resistor with your 1600, or you'll need to boost the voltage.

-
Jordan Spencer Cunningham
teletype.net


On Sat, Sep 21, 2019, at 9:37 AM, Zach Zarzycki wrote:
> Well - I have my 26 hooked up though the volpe board and have managed to get as far as when I type on the teletype's keys the terminal window on the laptop accurately displays what I've punched up.
> 
> However, typing on the teletype doesn't cause the keys to punch paper, and typing on the laptop keyboard won't cause the teletype to do anything.
> 
> I'm set up with a boost converter and a 12v 3amp power supply that SHOULD be supplying roughly 60mA at 97 volts though the loop after being boosted and then ran though about 1.6k wire wound resistor (600 ohm and a 1kohm in series, both 25W rated). 
> 
> The problem is when I read the voltage out of the booster, it shows 97v, but the most I've seen any amperage on any test point is 25ma, a far cry from 60ma. I'm wondering this this is possibly the reason for the computer keyboard not being able cause the keyboard close the magnet and select a letter then type. 
> 
> 
> My gut says maybe? But I find it very odd though that I can make the greenkeys act as a keyboard, so maybe I'm missing something else.
> 
> Zach 
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