[ARC5] ARC5 CW Question
Mike Feher
n4fs at eozinc.com
Mon Jun 23 11:56:14 EDT 2014
One obvious thing that has not been mentioned, maybe because it is obvious,
is that the back EMF is of the opposite polarity to the energizing voltage.
That is why diodes work. Otherwise, the diodes would short out the
energizing source. 73 - Mike
Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
732-886-5960
-----Original Message-----
From: ARC5 [mailto:arc5-bounces at mailman.qth.net] On Behalf Of Mike Hanz
Sent: Monday, June 23, 2014 9:28 AM
To: ARC-5 Maillist
Subject: Re: [ARC5] ARC5 CW Question
On 6/22/2014 7:50 PM, Dennis Monticelli wrote:
> When the current in the relay is suddenly interrupted it has to go
> somewhere; it cannot simply be "blocked."
This is not quite correct - let me try to explain why. The collapse of the
magnetic field in an ideal inductor could theoretically produce a voltage
spike of infinite proportions in zero time. But we live in the real world,
and the relay coil has distributed resistance and capacitance. A typical
small 28 volt relay coil will have perhaps a 200 ohm DC resistance in the
winding wire, for example. Absent any other external current paths, the
leading edge rise time of the voltage spike will be lengthened by
inter-winding capacitance, and the coil will self discharge the field energy
into that 200 ohm resistance on the trailing edge of the spike. That's what
creates the pulse of voltage across the coil terminals. It's a voltage
pulse - not a current pulse - at those terminals. It can only become a
pulse of current if you put an external load of some sort on it.
To see this phenomenon, one can do what I did back in the day and build a
simple relay "exerciser", using a mercury wetted relay to get clean makes
and breaks, and an audio oscillator to pulse the mercury relay.
Connect that to 28 volt source and you can drive your RUT crazy for days on
end, as well as seeing the repeating voltage pulse of the collapsing field
on an oscilloscope. That way you can test various snubber techniques with
respect to the external circuit requirements for make and break speeds. If
you want a given relay to release at the absolute maximum fastest rate it
can physically produce, you have to remove every possible current path at
the moment the field producing current through the relay coil is removed.
That's what Steve and I were trying to do for the SCR-274N...it *is* a "CW
capable" set, after all.
> The back EMF will soar as high as it must to release it's stored
> energy in the form of a current spike.
We've already covered that - it is a *voltage* spike across the terminals.
It can only produce a commensurate current pulse if an external resistance
is placed across the terminals.
> That path will likely be a voltage breakdown somewhere within the
> coil, or to the coil frame or via connected components.
Any quality relay manufacturer would strongly disagree with you on the first
two assertions. They *must* build their relays with the foreknowledge that
some applications cannot use snubbing devices (for whatever reason), and
design with the insulating requirements of the particular relay in mind. I
have a lot of machine tools here in the shop with all kinds of industrial
relays that go click and clack in the night, and not one uses a snubber like
the one you are talking about.
As for the "connected components", there is a wealth of possible solutions
for that in the literature, depending on how much you can afford to slow the
relay down. You just have to understand the requirements of your system and
the physics to apply them.
> Putting a reverse diode defines the reverse current path to a very
> safe voltage level but as was pointed out it takes longer for the
> energy to be released, which may or may not be an issue depending upon
> the application. One good way to have your cake and eat it too is to
> define a reverse current path that is much greater than a diode drop.
> For example, one could put a zener in series with a diode across the
> coil. When the diode conducts it does so via the zener's voltage and
> then the relay's energy is released quickly and safely.
At the greater expense of a zener, versus a ten cent diode (or Mac's diode
and resistor combination), yes...but even using a zener (or diode and some
external voltage sink) slows the relay release unless its voltage limit is
above the self discharge limit of the coil - whereupon its not needed
anyway....as the test setup mentioned above clearly demonstrated.
> I usually just use a diode with relays, unless release speed is important.
Yes, that is the conventional approach...but not feasible with what we were
discussing in the SCR-274N.
I'm aware of how quickly the subject line can become diffused in these
internet discussions, but it seems appropriate to put the alligators aside
every now and then and pay homage to the original problem of draining the
swamp. :-)
- Mike KC4TOS
> On Sun, Jun 22, 2014 at 3:26 PM, Mike Hanz <aaf-radio-1 at aafradio.org
> <mailto:aaf-radio-1 at aafradio.org>> wrote:
>
> On 6/22/2014 5:24 PM, Jay Coward via ARC5 wrote:
>
> Well put Mike, I get a better picture of what's happening.
> Now, what is the back emf voltage on l those relays? Depends
> on voltage/ inductance,yes ?
>
>
> Indeed! I don't recall the voltage range for the 274N relays -
> couple hundred?...I disremember. You can laboriously calculate it
> you can characterize the equivalent circuit of the inductor with
> its distributed resistance and capacitance, but it's easier to
> just measure it empirically. The pulse width was only a few
> microseconds if you didn't try to shunt it with another device
> like a diode or capacitor.
>
> Would a 1N4004 be adequate for subject relays?
>
>
> More than likely, but for the price, why not just jump to a
> 1N4007. They're cheap enough...:-)
>
>
> I suppose Steve's solution would be called EMF Blocking
> Diodes? The forward voltage drop is insignificant but the main
> concern would be reverse breakdown spec of the diode.
>
>
> Different people call it by different names. It was just another
> technique I stashed in my memory bank way back when I first
> graduated from college and was doing a lot of transistor driven
> relay work. I was surprised still recalling it, frankly. It was
> only after puzzling over Steve's problem that the light went on,
> thus the suggestion to try it.
>
>
> Are other sets prone to this problem?
>
>
> Any that have paralleled relay coils for keying would likely have
> the same issue. I couldn't tell you offhand which ones those
> might be. You'd have to do a check of each one - particularly
> those which offer break-in capabilities, where you have to key the
> antenna relay for each dot or dash.
>
> 73,
> Mike
>
> -----Original Message-----
> From: Mike Hanz <aaf-radio-1 at aafradio.org
> <mailto:aaf-radio-1 at aafradio.org>>
> To: Jay Coward <jcoward5452 at aol.com
> <mailto:jcoward5452 at aol.com>>; arc5 <arc5 at mailman.qth.net
> <mailto:arc5 at mailman.qth.net>>
> Sent: Sun, Jun 22, 2014 1:00 pm
>
> Excellent question. If you want to *short* the back emf, yes.
> Unfortunately, circulating the energy in the collapsing
> magnetic field
> back through the coil also significantly lengthens the time
> the relay
> hangs in there after turning off the power. That's not a good
> thing for
> a keyer...unless you enjoy sending at 5 WPM or less...:-)
> You have the
> same problem with using a capacitor across the coil. By using a
> *series* diode on *each* coil, you prevent that energy from
> feeding back
> into the other coil and thus keeping it alive for a longer
> period...and
> vice versa. If you put a scope across the key, you'll see a
> fast pulse
> of a few microseconds across it - the current has no place to
> go, so the
> field in each relay collapses very quickly.
>
> - Mike
>
> On 6/22/2014 2:42 PM, Jay Coward via ARC5 wrote:
>
> I always thought you put the diode across the coil to
> short the back emf. (?)
> Jay
> -----Original Message-----
> From: Mike Hanz <aaf-radio-1 at aafradio.org
> <mailto:aaf-radio-1 at aafradio.org>>
> To: J Mcvey <ac2eu at yahoo.com <mailto:ac2eu at yahoo.com>>;
> ARC-5 List <arc5 at mailman.qth.net
> <mailto:arc5 at mailman.qth.net>>
> Sent: Sun, Jun 22, 2014 11:33 am
>
> On 6/22/2014 11:12 AM, J Mcvey via ARC5 wrote:
>
> In summary, the only relays that should be clacking
> when keying is the
>
> antenna relay in the BC442 antenna switch unit and K52 in the
> MD7. Is this
> correct?
>
> That is correct. Steve KB4DMF discovered the chirp
> described in my
> first e-mail after putting together an entire two
> transmitter/three
> receiver set in its original complete form. That's not
> typically how
> hams used/use them, so we had never heard of any anomaly
> like that
> before. Ferreting out the reasons behind the chirp and
> fixing it was an
> interesting exercise.
>
>
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