[Elecraft] Fw: Inductive relay "kick"

Tue Apr 7 21:10:30 EDT 2009

1N4007 diodes have a notoriously long switchover time. That's what  
makes them useful as a poor-man's PIN diode. They aren't quite so  
useful in back EMF suppression, though.

If you use a fast diode, you should be able to keep the back EMF  
voltage down to the forward-bias voltage of the diode.

On Apr 4, 2009, at 9:52 PM, Carl, WCØV wrote:

>
> ----- 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
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>>
>
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