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Old April 15th, 2019, 03:17 AM   #1
ASF
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OT: Flyback diode selection

Hi all,

I have a couple of solenoids I'm using on a very low temperature application, and the documentation notes that they don't have any integral transient protection (e.g. flyback diode), and that the manufacturer recommends they be fitted externally. I figured I'd just solder a diode across the solenoid plug with the wiring, but I realised I have no idea what specifications I need to consider when selecting a diode.

The solenoid valves I'm using are attached, is anyone able to give me any pointers on what rating I'll need for my diodes? I'm using the 24VDC coil (which isn't explicitly described in the attached data sheet, but the stats are basically identical)

Thanks!
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File Type: pdf 222LT Cryogenic Solenoid Valves.pdf (124.4 KB, 32 views)
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Old April 15th, 2019, 05:23 AM   #2
Mad_Poet
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Not sure how to specify the rating for the diode . . .
But I believe the idea is not to protect the coil, but to protect the device
driving the coil. Being as that coil might get ultra cold, I'd put the diode
at the terminals closest to the (PLC output?) that drives the coil.

Or maybe a varistor?

Poet.
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Old April 15th, 2019, 05:41 AM   #3
BryanG
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These are a handy and simple retrofit:

https://www.productsforautomation.co...ov-p/asa24.htm


https://uk.rs-online.com/web/p/din-4...ctors/1746622/
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File Type: jpg DIN43650 retrofit.jpg (22.7 KB, 167 views)
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Old April 15th, 2019, 07:18 AM   #4
EICS
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Look up specs for 1N4004 diodes from memory
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Old April 15th, 2019, 08:37 AM   #5
James Mcquade
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An alternate solution would be to purchase terminal blocks with the diodes built in. better option in my opinion.
Allen Bradley and Automation Direct make the terminal blocks.

you may also want to look up the specs on a 1N4001 diode as well.

its been a while, but I believe the reverse breakdown voltage is 1,000 volts

james
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Old April 15th, 2019, 09:32 AM   #6
tarik1978
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you can find connectors with integrated flyback diode like this one:

https://www.mencom.com/shop/solenoid...9-opening.html
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Old April 15th, 2019, 09:51 AM   #7
PLC Pie Guy
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1N4001 is a common number that will work for most solenoid and relay coil applications. (Low Voltage)
I just bought a pack of 50 for less than 5 bucks.


I just spent the last week retrofitting an Arbro stroke harvester that is attached to my excavator. I had to learn all about diodes and how to protect the little computer outputs that run my hydraulic solenoid valves. I added an interposing relay to each solenoid as the OEM solution had the solenoids powered directly from the computer outputs. 1.3 Amp per solenoid.... That made me nervous. So using the relays, I took that amount of current away from the outputs. Now the outputs see the coil amperage and the inductive kickback from the relay coils. I use the diodes I ordered to snub the kick back from the relay coil itself.

It does have me wondering how small the coil needs to be before fly back protection is not needed. Based on my research, its the high voltage produced from the collapsing of the magnetic field upon de-energizing that causes the damage. One source i viewed provided an example with an oscilloscope that showed a kickback of over 250V across the transistor output, caused from the coil kickback when using only 24VDC coil voltage. The same kickback was reduced to an insignificant voltage on the scope with the fly back installed. This was on a very small relay to.

One thing I did learn. Dropout of a coil that is using a flyback diode only will have a noticeable but slight delay on drop out upon de-energizing.There are more preferred methods of snubbing if this slight delay will cause issues. One solution has a resistor in series with the diode. Other solution showed a resistor only across the coil without the diode. Though my electronics theory is rusty, it does seems there are a few ways to skin this cat.

Its very common to see systems without fly backs installed. Now I'm curious as to when its OK to NOT use them as it seems any solenoid or coil inductor will produce a kickback voltage upon de-enrgizing.

I think this 1N4001 will work fine for you.
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Old April 15th, 2019, 10:11 AM   #8
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Quote:
Originally Posted by James Mcquade View Post
An alternate solution would be to purchase terminal blocks with the diodes built in. better option in my opinion.
Allen Bradley and Automation Direct make the terminal blocks.

you may also want to look up the specs on a 1N4001 diode as well.

its been a while, but I believe the reverse breakdown voltage is 1,000 volts

james
FYI - The 1N4007 is 1000 volts, 1N4001 is only 50 volts
https://www.vishay.com/docs/88503/1n4001.pdf
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Old April 15th, 2019, 11:48 AM   #9
James Mcquade
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Lynx777,

THANK you for the correction !!
its been a long time since I worked with diodes.

I looked it up and learned something
1N4001 - 50 volts
1N4004 - 400 volts
1N4007 - 1000 volts.

Again, thanks for the correction.

I would go with the 1N4004 or 1N4007 diodes.

james
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Old April 15th, 2019, 12:07 PM   #10
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Quote:
Originally Posted by James Mcquade View Post
Lynx777,

THANK you for the correction !!
its been a long time since I worked with diodes.

I looked it up and learned something
1N4001 - 50 volts
1N4004 - 400 volts
1N4007 - 1000 volts.

Again, thanks for the correction.

I would go with the 1N4004 or 1N4007 diodes.

james
James.... Just curious. Why would you say the 1N4004 and not the 1N4001 that is rated for 50V. His coil voltage is 24 volts so 2X coil voltage = 48, 2 more for safety factor = 50V. I thought rule of thumb was 2 X Coil voltage.

Not criticizing, just learning!

I know any of these listed would work but curious of why one would select the diode with the higher voltage rating?

Thanks...
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Old April 15th, 2019, 12:22 PM   #11
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Because the peak inverted voltage generated is higher than 50 volts.
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Old April 15th, 2019, 12:28 PM   #12
Chilliwack Murray
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It's a general practice to always pick the component with the highest voltage or current rating when there is no practical difference, it's just hedging your bet and there is no price or size difference in these diodes.

That said, the breakdown voltage rating applies to the reverse direction which would have only 24VDC applied in this case so anything higher than the maximum expected output voltage is fine regardless of the expected flyback voltage. Remember the diode will block the 24VDC output but short the reverse polarity flyback voltage from the coil as it will be a forward bias to the diode.
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Old April 15th, 2019, 01:57 PM   #13
James Mcquade
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the magnitude of the reverse coil voltage can be 4 times the coil voltage or more. adding to that, why not use the highest rated voltage diode? sure, its way over kill, but why risk it?
in my experience, build a Sherman tank, not a tinker toy.

james
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Old April 15th, 2019, 01:57 PM   #14
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When I was in the service we stocked the 1N4007 just for this sort of use. Because it's rated for 1000 volts we could use it with virtually any DC coil from 6 volts up to 200 volts, even higher depending on the coil size. "One size fits most."
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Old April 15th, 2019, 02:14 PM   #15
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I learned the hard way, 10 times is ok.
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