[ARC5] 274N Paper

[Mike Hanz]

Brian Clarke wrote:

>What you have provided is one of the solutions to a standard problem with
>the switching
>of relays - handling the back emf. However, the ARC-5 and SCR-274N series
>were designed to run off 22 to 30 V dc. By putting a diode in series with the
>relay coils, you
>may just have marginal operationality when the battery gets to 22 V dc.
>

Notwithstanding the fact that silicon diodes didn't exist when ARC 
designers were worried about 22 volt battery levels, I guess I don't see 
the problem here, as the 0.6v drop across the diode is lost in the 
noise.  Does anyone operate these sets today at 22 volts?  I do run mine 
at 24vdc, but that's primarily to extend component life, and 28 volt 
relays normally have guaranteed pull-in voltages around 13-15 volts to 
make sure they work reliably at lower battery voltages.  Many of them 
even work well with 12 volts applied to the coil, at the expense of a 
penalty in "make" or "pull-in" time.

>Another solution that many designers use, that doesn't require lifting any
>wires, and
>that gets around the series Voltage drop, is to connect your diodes across
>the relay coils. 
>

This is truly a 50 year old standard for eliminating the back EMF pulse 
that the relay coil generates when the field collapses - the technique 
was used with selenium rectifiers back before the advent of solid state 
components.  Unfortunately it has the *opposite* effect of what Steve 
was trying to correct, and that was to /minimize/ drop-out delay and get 
some coordination of the timing among all the paralleled relays.  I've 
seen drop-out times of hundreds of milliseconds with large armatures and 
a parallel diode (only one diode is actually necessary with a set of 
paralleled relays unless there are concerns about pulse radiation and 
damped ringing due to lead inductance .)  In cases where the relative 
timing between contacts isn't a problem, it's certainly the best way to 
eliminate voltage spikes running around the circuit.  With parallel 
diodes I suspect one would be lucky to get 5WPM out of the set and you 
will have a perceptible delay between release of the key and the sound 
of the relays falling out.  With the series diodes, it might do 15WPM.

With regard to the reversibility of the modifications, it seems to me 
that the series diodes in the cables most closely match the ideal of 
lowest modification impact.  After all the work Steve had put into 
making those cables and the fact he was tearing his hair out trying to 
get a hundred other things ready for the special event, I was afraid to 
broach it since I'm getting too old to bend down and check the bottom of 
my car for IEDs...  :-P

73,
Mike