[GreenKeys] Snubber tests

[Peter Gottlieb]

Aaack!  It's late!  That was a 10K, not 100K resistor!!!


Peter Gottlieb wrote:
> I just ran a few tests.  The simplest snubber which still absolutely 
> protects the MJE13005 consists of a diode (I use 1N4007's for just 
> about everything) in series with a 100K 2W carbon resistor across the 
> selector magnet.  The voltage on opening goes up to around 500 volts 
> then shows a nice exponential decay.  The resistor gets mildly warm 
> after steady copy, probably a 1 watt would do fine.
>
> So now I hooked up an optocoupler as the input from the low level data 
> line.  I first tried a TIL111 because I have a bunch of them, no good, 
> not enough current transfer.  The best I could find I scrounged from 
> an old switching power supply board and its datasheet says it has a 
> 200% CTR (Current Transfer Ratio) yet I am not seeing anywhere near 
> that and it is marginal.  I must be missing something.  I am driving 
> at 20 mA and only need 17 mA on the output.  I think I am expecting it 
> should go into saturation and it isn't designed to do that.  I may 
> need additional components, maybe make it like a darlington or use a 
> tiny FET (I used to use the 2N7000 a lot, must have some still around 
> somewhere).
>
> In any case, I did get data from the Dovetron through to the 2B.  
> There was no QRM generated at all, at least not that I could detect 
> even on very weak signals.  However, I was unable to get good copy on 
> any signals.  One problem was that many signals were not 22 msec 
> timing (ie, not 45.45 bauds).  One that was, still no joy, and I note 
> the 2B is not spacing evenly.  It sometimes doesn't space properly, or 
> at all, when cold so I have to get into that part of the mechanism and 
> lubricate, clean and adjust it as necessary.
> Getting there.  Once the opto problem is resolved I need to put one in 
> for the other direction so the keyboard runs the Dovetron.  Then add a 
> full/half duplex switch (hmmm, I wonder if the Dovetron switch does 
> that?) and build the final loop supply/signal converter into a 
> suitably boatanchory looking box.  I did note that the Dovetron 
> "standby" switch does nicely enable/disable the printing.
>
> Peter
>
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