[ARC5] ARC5 CW Question

Sun Jun 22 20:06:22 EDT 2014

All of the relay coils run on 28V DC. I suppose if solid state diodes were
readily available they would have used them as back emf shunts, although I have never had or heard of a command set keying or antenna relay coil break down.
Sent from my iPhone

> On Jun 22, 2014, at 19:50, Dennis Monticelli <dennis.monticelli at gmail.com> wrote:
> 
> 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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