Re: {Collins} Concerning DC relay coil for KWS-1, please read.



I have gotten lost  a bit  on  this.
 
We are   getting the remaining pieces to get 
our  KWS-1  complete here at the museum.
 
We have had the kind  gift of a antenna  relay
has the coax connectors  plus  many contacts on the   
side.. but the coil is  is 110 AC... now  to use  this on 110  DC
what  shall we  do?  
 
( Now, just the hose, the HV  cables  and working finals to  go!)
 
Thanks Ed Sharpe archivist for SMECC  _www.smecc.org_ 
(http://www.smecc.org)  
 
 
In a message dated 11/16/2013 12:37:03 P.M. Mountain Standard Time,  
ve3tou@xxxxxxxxxxxxxxxxxxx writes:

I agree.  But another few bits of relay lore may be worth stating.

Basically,  when you try to cut off the current in an inductor (such as a 
relay coil)  suddenly, the current initially does not change. Instead, 
(in an extreme  case), the voltage builds up until something breaks -- 
which is why you  can get visible arcing, etc.

Connecting a diode across the coil limits  the inductive kick to the 
diode forward drop, which is less than 1 volt.  The diode sees an initial 
current equal to the coil current; this decays  exponentially with a time 
constant L/R, where L and R are the inductance  and resistance of the 
coil (actually a bit more slowly because of diode  drop). The diode never 
sees a large inductive kick because this kick never  happens. But I'd 
still use a 1N4007 with 110 VDC, to allow lots of  margin.

So a simple diode suppressor is in fact ideal from all points  of view 
except one -- since the coil current decreases slowly, the relay  will 
hold in for a significant time. This may be unacceptable in some  
circuits. The only way to overcome this is to allow a controlled amount  
of voltage to build up and buck the current; this is achieved with   
bewildering combinations of resistors, capacitors, diodes, varistors,  etc.

73
Bob


On 2013-11-16 1:31 PM, Dale wrote:
>  Billy,
>
> Wiring the coils in parallel should work just  fine.  Each coil will draw 
its design current based upon its DC  resistance.  Since this is a DC 
circuit, impedance (which is an AC term)  is not a factor.
>
> As for a protection diode - good idea!   Two coils with 110VDC on them 
will create one heck of a back-emf when the  circuit opens rather suddenly.  
One diode should do it.  Be sure to  observe polarity so that the diode does 
not present a short to the applied  voltage.  I'd use at least a 1N4007 - 
maybe even a higher PRV device if  you have one.  The current is not a big 
issue, PRV is.
>
>  Have fun.
>
> 73, Dale
> WA9ENA
>
>
>  -----Original Message-----
>> From: William Yates  <veniceamplifier@xxxxxxx>
>> Sent: Nov 16, 2013 12:11  PM
>> To: "collins@xxxxxxxxxxxxx"  <collins@xxxxxxxxxxxxx>
>> Subject: {Collins} Concerning DC  relay coil for KWS-1, please read.
>>
>> Greetings to the  list
>> I am preparing an Aemco 110 vdc coaxial relay without the  external 
contacts,
>> to accept an outboard 110 vdc relay that I  fastened to the body. The 
two coils
>> will be wired in parallel in  order to activate the RX muting etc...
>> The Aemco coil meaaures 5k,  and the PB unit measures 11K.
>>
>> Question: Is there any  convention or concern I should consider here 
with regards
>> to  impedance? Shunt diode across terminals?
>> This pertains to the  GDT's that belong to K6KVI who is preparing for 
the 2nd annual
>> GDT  challenge coming soon. This is a significant pair as they once 
belonged  to
>> UC Berkley and has a rather storied background. Getting them on  the air 
for the
>> challenge is paramount.
>>
>>  Come to think of it, ALL GDT's have a storied background, so much the  
better!
>> Thanks in advance.
>> Billy N6YW
>>  AC12-12703
>>                 
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