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Relay coils and diodes

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recapen(at)earthlink.net
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Posted: Fri Sep 05, 2008 8:56 am Post subject: Relay coils and diodes

My experience - shared for mine and other's education and offering of opinion.



This past weekend, I was working on the wiring of my flap position sensor for my two-speed elevator trim circuit.  The two-speed trim circuit essentially switches a low speed set matronics governor for a high speed set one via a pair of 4PDT relays whenever the flaps come off the upstop.



As I was testing the detector switch - I got 'bit' by something that felt like house current (done that too!).  Not knowing what was going on, I repeated the test...with identical results.



OK, break out some of Bob's literary work - read-up and put in some diodes across the power terminals of the coil.



Re-reading made me think that in my case, the diodes may be even more important...in Bob's writings (Paraphrased) "the absence of the diode presents the opening switch with the spike"; in my case, I have a couple of things wired up within the master switch...  I'm thinking that in my case the master switch would 'see' the spike...along with anything else that is left on when I turn off the master.



I know that the 'real' airplane stuff should be able to live in this environment and should not produce this environment either - however in the OBAM world, I can put in non tso'd trim servos and indicators (of which I have had a couple of them mysteriously blow one of their indicator lights) which are pretty good gear as far as I'm concerned - they just haven't spent the money and gone through the certification process.



Got the diodes and will be putting them in this weekend.  Is it possible that I'm allowing a spike to wander around my electrical system by turning off the master with some of these relays energized?



Your thoughts please,

Ralph Capen

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nuckolls.bob(at)cox.net
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Posted: Fri Sep 05, 2008 2:17 pm Post subject: Relay coils and diodes

At 12:54 PM 9/5/2008 -0400, you wrote:

 	  Quote: 	
		  

<recapen(at)earthlink.net>



My experience - shared for mine and other's education and offering of opinion.



This past weekend, I was working on the wiring of my flap position sensor 

for my two-speed elevator trim circuit.  The two-speed trim circuit 

essentially switches a low speed set matronics governor for a high speed 

set one via a pair of 4PDT relays whenever the flaps come off the upstop.



As I was testing the detector switch - I got 'bit' by something that felt 

like house current (done that too!).  Not knowing what was going on, I 

repeated the test...with identical results.


	


   Yeah, been there, done that. In my case I got the

   fingers across the contacts of a cub-scout electric

   buzzer project that operated from 3 volts (two flashlight

   cells). The field collapse from the small buzzer

   solenoid coil was sufficient to give ME a buzz which

   caused me to drop the project thus damaging it.



   Unfortunately it was not my project and my schoolmate was

   pretty unhappy about it. Neither of us understood what

   happened or why . . . but he got a personal demonstration

   of the effect a few days later. Even after my Navy veteran

   electrician father explained it to us, we didn't really

   understand.

 	  Quote: 	
		  OK, break out some of Bob's literary work - read-up and put in some diodes 

across the power terminals of the coil.



Re-reading made me think that in my case, the diodes may be even more 

important...in Bob's writings (Paraphrased) "the absence of the diode 

presents the opening switch with the spike"; in my case, I have a couple 

of things wired up within the master switch...  I'm thinking that in my 

case the master switch would 'see' the spike...along with anything else 

that is left on when I turn off the master.



I know that the 'real' airplane stuff should be able to live in this 

environment and should not produce this environment either - however in 

the OBAM world, I can put in non tso'd trim servos and indicators (of 

which I have had a couple of them mysteriously blow one of their indicator 

lights) which are pretty good gear as far as I'm concerned - they just 

haven't spent the money and gone through the certification process.



Got the diodes and will be putting them in this weekend.  Is it possible 

that I'm allowing a spike to wander around my electrical system by turning 

off the master with some of these relays energized?


	


   No. First, while the voltages generated by magnetic

   field collapse on a relay or contactor can be pretty

   spectacular, the ENERGY they contain is small. Given

   that the voltage is proportional to rate-of-magnetic

   decay, it stands to reason that maximum rate occurs at

   minimum current or widest opening of contacts on the

   controlling switch. This is a long winded way of stating

   that the source impedance of the relay coil as an energy

   storage device is big . . . too big to propagate significant

   pulses of "spike" energy out onto the bus.



   Most accessories have some amount of capacitance right

   across the power input leads as filters. This capacitance

   alone is a huge energy sink to inductively stored spikes.

   Risks to anything other than the controlling switch due

   to magnetic field collapse has been greatly exaggerated

   and poorly understood.



   There is no reason to be concerned beyond the effects

   on the controlling device . . . even if the diodes are

   not present.



   Bob  . . .

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recapen(at)earthlink.net
Guest

Posted: Sat Sep 06, 2008 4:11 am Post subject: Relay coils and diodes

Thanks Bob.....

---

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