[Elecraft] Fw: Inductive relay "kick"

[Carl, WCØV]

----- Original Message ----- 
From: "Carl, WCØV" <wc0v10 at embarqmail.com>
To: <K2TK at att.net>
Sent: Saturday, April 04, 2009 8:50 PM
Subject: Re: [Elecraft] Inductive relay "kick"


> Good point. The diode will conduct the back EMF minus internal voltage 
> when the supply voltage to the coil is removed. You do need a diode with a 
> high enough PIV rating. But even Radio Shack has silicon diodes with 1000 
> PIV rating, 2 amps forward current rating at five for a dollar.
>
> We make electric actuators where I work, motors with internal rollerscrews 
> and often they get equipped with electric safety brakes (apply when power 
> is removed, release when power is applied). We normally supply 1N4007 
> (1000 PIV rated forward bias) diodes with brake equipped motors.
>
> 50% of those you ask would do that. The other 50% would use a very fast 
> switching diode as you suggest. Both work just fine since the only job is 
> to conduct the EMF coming out of the coil when the actuating voltage is 
> removed. BUT you do need a diode with a high enough PIV rating. Some of 
> our bigger unsuppressed motor brake coils can spit out over 600 vdc when 
> the supply is removed.
>
> And yes some of our customers hook up the diode or the brake supply 
> backwards and blow the diode (if they don't blow the supply first). 
> Protection is gone, and the back EMF from the relay coil is free to blast 
> wherever it wants to go.
>
> I am sure the Elecraft relays are way below that. My point to Don was that 
> simply doubling the voltage rating of the coil to get the back EMF is way 
> off. A scope will tell the real tale.
>
> BTW I got yelled at by a medical tech who pointed out that the pain 
> (real - I tried the relay wire experiment myself) is the voltage going 
> right across your heart. To some, 450 vdc may be a very bad day.
>
> 73, Carl WC0V
>
> ----- Original Message ----- 
> From: "Bob" <K2TK at att.net>
> To: "WCØV" <wc0v10 at embarqmail.com>
> Sent: Saturday, April 04, 2009 2:07 PM
> Subject: Re: [Elecraft] Inductive relay "kick"
>
>
>> Hi Carl,
>>
>>           What am I missing here?   Using a regular silicon diode that 
>> has a fast switching time, or even better a Schottky diode wouldn't they 
>> clamp at the forward bias of the junction .7 volts of less.   If not how 
>> long could you expect the diode to survive that abuse of 75v across the 
>> forward biased junction.  If the diode then failed open you would never 
>> know your protection was gone.
>> 73,
>> Bob
>> K2TK
>>
>> Carl wrote:
>>
>>>Don, seriously disagree about the magnitude of the "kick". For years I 
>>>taught a course on industrial installations and demo'ed the "kick" issue 
>>>by soldering two bare wire to the coil of a small Potter and Brumfield 
>>>"ice cube" 12 vdc relay. I would pick the biggest guy in the class to 
>>>hold the wires while I touched them to a nine-volt transistor radio 
>>>battery. And then watch as he would throw the relay across the room. A 
>>>reverse diode would vastly reduce the effect.
>>>
>>>Well, I got yelled at and was told "Hey, Stupid (and stupid wasn't the 
>>>word they used)! Put that on a storage scope to see what you're messing 
>>>with." The scope showed a spike of 450 vdc! The diode reduced it to less 
>>>than 75 vdc.
>>>The magnitude is due mainly to the large inductance of the coil, but 
>>>until some measurements are taken, some caution is advised. The 
>>>inductance of any coil inside a K2 or K3 is bound to be much smaller. But 
>>>the phenomenon is the same.
>>>
>>>73, Carl WC0V
>>>______________________________________________________________
>>>Elecraft mailing list
>>>Home: http://mailman.qth.net/mailman/listinfo/elecraft
>>>Help: http://mailman.qth.net/mmfaq.htm
>>>Post: mailto:Elecraft at mailman.qth.net
>>>
>>>This list hosted by: http://www.qsl.net
>>>Please help support this email list: http://www.qsl.net/donate.html
>>>
>>>
>