[Elecraft] Fw: Inductive relay "kick"
Wes Stewart
n7ws at yahoo.com
Tue Apr 7 22:12:05 EDT 2009
There is a lesson here: before making any pronouncements, check Jack's website for the test results first.
--- On Tue, 4/7/09, Jack Smith <jack.smith at cliftonlaboratories.com> wrote:
> From: Jack Smith <jack.smith at cliftonlaboratories.com>
> Subject: Re: [Elecraft] Fw: Inductive relay "kick"
> To: "Bill Coleman" <aa4lr at arrl.net>
> Cc: elecraft at mailman.qth.net, "WCØV" <wc0v10 at embarqmail.com>
> Date: Tuesday, April 7, 2009, 7:43 PM
> 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
> >>>>>
> ______________________________________________________________
> >>>>> Elecraft mailing list
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> >>>>> Help: http://mailman.qth.net/mmfaq.htm
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> >>>>>
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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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