[ARC5] ARC5 CW Question

[Mike Hanz]

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.
>
>