Subject:
Re: [AMPS] re: Radio Relay
To: <amps@contesting.com>
Date:
Sat, 22 Mar 1997 17:46:22 +0000
From:
Ron Hooper <ab4ru@stc.net>
Organization:
Is there life after Contesting?
Diode protection:
How the general purpose diodes are suppose to work accross relay coils
is that are intentionally put on in reverse to arrest the voltage
generated caused by back EMF, which is the opposite polarity from the
voltage applied. This back EMF can slow the pick time of the relay when
energizing. Back EMF is at its highest when current is high and drops
to nothing when the relay is fully picked. Now when you open the relay
the diode does nothing to arrest the collapse of the magnaetic field.
We have use a commerically available device called a snubber
manufactured by several companys designed specifically for thier
devices. This is nothing more than a resistor, capacitor in series that
goes accross the coil.
I have seen is some cases relays would stick or delay opening because
the diode was not accross the coil. I think it helps to keep the pole of
the relay from becoming magnitized in certain types of relays also. This
would appear to be the same problem as stuck contacts and found to not
be the case. When I get back to work I will see what Siemens are using
on thier DC output modules for protection, which are designed
specifically for inductive loads. They dont require the snubber circuit
so something must be built into each output.
Ron W4WA
Tom Rauch (W8JI) wrote:
>
> Bob Morris K2RK wrote:
>
> > I assume the diode is the standard spike protection, provide
> > outboard.
>
> That's what I was curious about. It is impossible to add eternal spike
> protection with a diode, unless the diode is a zener or a biased diode
> clamp.
>
> The reason is this...
>
> The back pulse coming out of the amplifier, as the relay line is opened,
> is the same polarity as the initial keying voltage. If you have a plus
> 12 volt line, the backpulse will be a positive voltage of many volts.
>
> Assuming a 12 volt line, the only way to externally clamp the backpulse
> voltage is to use a conventional diode with its' cathode tied to a
> positive twelve volt (very stiff) source and anode to the relay line
> (clamping the relay line positive voltage spike at 12 volts), or to use
> a 12 or 14 volt zener clamp to ground.
>
> A cathode to keying line NON-zener diode, or a series diode, does
> nothing.
>
> > ps... The 3-500 AC wiring...in the ARRL Handbook. I couldn't even
> > figure out what they were saying to comment on it...
> >
> > Quote---- 1995 ARRL Handbook - 1995, page 13.36, fig 13.37
> >
> > "Chassis ground is not equivalent to the ac-line ground.
> > The ac neutral wire is connected to chassis ground, so the chassis
> > must be isolated from the equipment cabinet, which must be connected
> > to the ac ground conductor"
>
> They obviously were aware the 3-500 amp was unsafe because of the
> warning, but how do you isolate the chassis of the amplifier from the
> grounded cabinet?? That seems even more unsafe! Can you imagine
> completely isolating the SO-239's for ac voltage while not upsetting RF
> grounding??
>
> Seems like a lot of work when proper primary wiring would cure the
> problem.
>
> >Not just ac but there's 3 0r 4 kv going to this box....
> >
> > Get an extra copy of the Handbook, so your widow can give it to
> > her lawyer....
>
> There are also two 2000 volt plus supplies that have the neagative leads
> improperly clamped (only via a reistor), and have no chassis safety
> grounds. If either supply would develop a primary to secondary
> transformer short, or if a lead should break or a high voltage fault
> occur, the chassis or negative leads could "fly up" to full high
> voltage.
>
> Not a very safe thing for the hamshack.
>
> 73 Tom
>
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