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Are you SURE they're sometimes called freewheeling diodes?? I thought that was something else?
Relay question
Oct 21, 2010
57 Replies
I've always called them "freewheeling diodes". What is your something else?
It's a good thing I paid attention to the fuzz on that memory and put the "I think" in there, then.
In the US they're called "funny round things" -- we're not technologically advanced enough to use 'em.
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
Now that we got that cleared up!
So Travis, Has the suggested diode addition or a snubber circuit cleared up the circuit upset that you were experiencing?
Michael Karas
Carousel Design Solutions
http://www.carousel-design.com
I have to say I'm very impressed by the discussion my problem has spawned. I will definitely return here with future questions and I may be able to help with one or two issues that arise here as well. :-)
Anyway, to follow up, this thread has helped me quite a bit. The solution I've found that works best for my circuit is a varistor placed in parallel with my shutter. I tried several different versions of a 'snubber', each one seemed to improve performance, but not totally remove my problem. However a 26V (DC) varistor cleaned things up nicely. Before this problem arose, I had never even heard of a varistor, but I am familiar with the theory of its operation.
Anyway, thanks again to all who helped with this one, I very much appreciate your input! :-)
Not to be a spoilsport, but a varistor isn't the best thing to use in
that application.
The reason is that every time it takes a hit it'll be irrevocably and
continuously degraded in that its leakage current will increase. How
much it increases will depend on how much energy it has to absorb and
how often it has to absorb it, but the bottom line is that eventually
the MOV will be taking more current than the shutter will when you
operate the shutter.
An ideal solution for that problem would be to use a couple of hefty
Zener diodes in series-opposition across the shutter's coil since
they'll do the same job, but they'll never degrade.
1N5360's should do it.
John,
Thanks for that input. I read a couple things about varistors degrading over time, but, what is the timeline on that (days, months, years?), or is it more likely a function of usage, like how many times I activate my shutter in the course of daily operation?
Nah - use Transzorbs. They're available in bipolar versions:
formatting link
Cheers! Rich
It depends on how hard you hit the MOV and how often.
Here:
http://www.iaei.org/magazine/?p=2545
They're just Zeners with the bipolar ones being a pair connected in
series opposition, but good find!
I tried Digi-Key and couldn't come up with anything, so went for the
dicrete Zener fix.
The general name is "transient voltage suppressor diode", or TVS diode. Search at Digi-Key for "TVS", then select the "TVS - Diodes".
When selecting parts, pay attention to the reference currents in the standoff voltage, breakdown voltage, and clamping voltage specs. The breakdown voltage is similar to zener voltage, and the standoff voltage is the maximum voltage that you want to pass through without flowing much current through the TVS.
The manufacturers are rather quiet about the actual construction of TVS diodes, but they seem very similar to "zener" diodes that have been design-optimized for transient protection.
Jay Ts
Those numbers look familiar. We put 1.5KE24's across some of our coils. (Just to reduce the arcing and sparking when the students unplug them at full current.)
George H.
Did you search for "TVS Diode"? Digikey shows 27,693 matching records. ;-)
Yes,seems each hit does a little damage, TVS diodes are better, as they're like zeners with a documented dynamic resistance and max Volts for some very high Amps rating. So a 1500W peak power TVS is like much like a 5W zener with defined overload current pulse handling.
Heh - they're zeners, but they're not JUST zeners ;-) They're designed to take an incredible hit, and dissipate the power at leisure. It has to do with thermal stuff or something.
I once worked at a place that used MOVs (Metal Oxide Varistors) all over the place, and they cost a fortune to replace on warranty. Then one day the Transzorbs showed up in a trade mag (this was the mid-1970s), and we issued ECOs for everything. Threw away the MOVs, stuck in transzorbs, and service calls plummeted.
Cheers! Rich
Zeners on steroids. ;-)
And they come in bipolar versions.
Cheers! Rich
Breakdown voltage: 26V
Power: 600W
Polarization: Bidirectional
Search results: "No records match your search criteria"
I get 44 hits on two pages (lots of duplications - packaging options) with those specifications (standoff voltage = 26V rather than breakdown).
The first one:
formatting link
I think you just needed to specify a range of breakdown voltages.
Look for:
Vishay: P6KE24C, P6KE24CA, P6KE27C, P6KE27CA, P6KE30C, P6KE30CA
And also check other manufacturers for similar devices.
Get a datasheet for Vishay's P6KE series, take a careful look.
There isn't a model at the 26V level, but if you check the specs for the above devices, you may find one that fits your application.
If you compare TVSs to MOVs, you will find that TVSs have much sharper cutoff than MOVs. I expect that the P6KE27CA or even a P6KE30CA will do nicely for you.
The difference between Vishay's A and plain models is that the plain have a 10% tolerance, and the A models are 5%, and that is around an average value of breakdown voltage that does not appear explicitly on the datasheet. ;-) Instead, they specify minimums and maximums. Also, make sure you specify the C devices; those are bidirectional.
Jay Ts
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