[GreenKeys] snubbery

[Peter Gottlieb]

Thank you.  I did not think about the bias caused by no snubbing, but of 
course you are right.  The diode-resistor snub appeals to me since I can 
easily set the max voltage and amount of snubbing.  The minimum amount I 
would want is enough to keep the switch transistor from exceeding its 
Vce rating (with some margin to account for dV/dt effects) but as you 
point out I may need more to match the turn on pull-in delay. 

The selector magnet has a good bit of inductance.  My supply is 
rectified 120 from an isolation transformer so open loop there is 170 
VDC yet on turn on I can see the constant current transistor in 
saturation for many milliseconds before dropping back to current 
limiting.  It is a little hard to tell where the exact pull-in occurs 
(and then there is mechanical inertia to consider) but even with a 700 
volt or so avalanche limit it holds there for a couple msec.  Since I am 
all set up for it on the bench perhaps I will take readings with 
different amounts of snubbing, and also determine the pull-in and 
drop-out currents.  Unfortunately I do not have a spring scale so 
everything will have to be considered approximate but it should be 
interesting data nonetheless.

Peter


jhhaynes at earthlink.net wrote:
>
> By the way, I've just discovered a wonderful device for switching
> loop current.  Fairchild HSR312 electronic switch, a couple of
> bucks at Mouser.  It's optically coupled so you get ground isolation,
> and in one connection it doesn't matter which direction current flows
> in the loop.  So you get a four-terminal box with two terminals to
> turn it on, the photodiode, and two terminals that can go in series
> with a loop anywhere.  It will need snubbing, and after looking at
> MOV devices I decided to use a Zener diode or two instead.  The
> Zeners are pretty precise about the breakdown voltage and the wattage
> they will handle - the MOV devices seem to be a lot more fuzzy, but
> would probably work OK.
>
> Don't worry too much about the snubber affecting the waveform.
> The current rises at a rate limited by L/R, so there is some delay
> between closure of the switch and the selector magnet pulling in.
> It you didn't snub on opening the switch the current would fall to
> zero instantly (and the voltage would rise to infinity) so you would
> get a bit of spacing bias.  Ideally you would snub at twice the voltage
> in the loop supply, so that the current would decay at the same rate
> it rises and there would be no bias.  Of course this is complicated
> by other things, like the difference between pullin and release
> current of the selector magnet, so maybe you don't want the current
> to decay at the same rate it rises.  But the point is just not to snub
> too severely.
>
> A diode in series with a resistor across the magnet would be another
> way to snub.  You shouldn't get ringing with a capacitor because
> the series resistor damps the tendency to ring.
>
> Note that even when they used relays for loop switching they used
> contact protecting networks, R and C in series, across the contacts
> and those are effectively snubbers.  80 ohms and 0.25uf were typical
> values, but you need to watch with an oscilloscope to see what
> sort of snubbing you are getting.
>
>
> jhhaynes at earthlink dot net
>
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