[ARC5] ARC5 CW Question

Sun Jun 22 19:50:17 EDT 2014

When the current in the relay is suddenly interrupted it has to go
somewhere; it cannot simply be "blocked."  The back EMF will soar as high
as it must to release it's stored energy in the form of a current spike.
 That path will likely be a voltage breakdown somewhere within the coil, or
to the coil frame or via connected components.  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.

I usually just use a diode with relays, unless release speed is important.

Dennis AE6C

On Sun, Jun 22, 2014 at 3:26 PM, Mike Hanz <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>
>> To: Jay Coward <jcoward5452 at aol.com>; arc5 <arc5 at mailman.qth.net>
>> Sent: Sun, Jun 22, 2014 1:00 pm
>> Subject: Re: [ARC5] ARC5 CW Question
>>
>>
>> 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>
>>> To: J Mcvey <ac2eu at yahoo.com>; ARC-5 List <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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