[Elecraft] Fw: Inductive relay "kick"
Jack Smith
jack.smith at cliftonlaboratories.com
Tue Apr 7 21:43:04 EDT 2009
Bill:
Are you sure about this? A snubbing diode has to have fast turn on, but
slower turn off is not as much of a concern. A while ago I looked at the
turn on and turn off times for several diodes including the standard
1N4007 and found all sufficiently fast to work well as a relay snubbing
device. Details are at
http://www.cliftonlaboratories.com/diode_turn-on_time.htm
Jack K8ZOA
www.cliftonlaboratories.com
Bill Coleman wrote:
> 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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>
> George William Coleman mail: aa4lr at arrl.net
> Senior Software Developer phone: 770 978-0196
>
> Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr at arrl.net
> Web: http://boringhamradiopart.blogspot.com
> Quote: "Not within a thousand years will man ever fly!"
> -- Wilbur Wright, 1901
>
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