[ARC5] ARC5 CW Question

[Mike Hanz]

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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