Flyback Diode

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fifty60_4802GND

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I am looking for what I believe is called a flyback diode. The normal diode kit I would use is a Veeder Root diode kit 846000-022. However, the Veeder Root, like a cheesburger at McDonalds these days, is experiencing extremely long lead times.

The diode kit will be used on 24VDC solenoid valves. The controller that required the diode is recommending a Suppressor Diode with Ratings: Output Current of 1.0 Amp, Maximum reverse voltage 420 Volts

I am having trouble even finding a good vendor to ask questions to. Any suggestions on where to look?
 
fifty60_4802GND said:
I am looking for what I believe is called a flyback diode. The normal diode kit I would use is a Veeder Root diode kit 846000-022. However, the Veeder Root, like a cheesburger at McDonalds these days, is experiencing extremely long lead times.

The diode kit will be used on 24VDC solenoid valves. The controller that required the diode is recommending a Suppressor Diode with Ratings: Output Current of 1.0 Amp, Maximum reverse voltage 420 Volts

I am having trouble even finding a good vendor to ask questions to. Any suggestions on where to look?
Click to expand...
p/n: 1N4005

1A @ 600V max reverse voltage.

Mouser electronics will sell you a single diode or an entire reel. Less than $0.50 a piece in single quantities. When I looked, Mouser had over 8,000 in stock.


We typically use 1N4003 diodes for our 24V relays and contactors; exact same spec but with a lower max reverse voltage of 200V. Not sure why the higher reverse voltage is spec-ed with only a 24V coil, but trust the controller user manual.



SceneryDriver
 
Here is what the original diodes from Veeder look like. The solenoid itself gets installed inside of an Explosion proof enclosure. It looks like the diodes are shrink wrapped with wire leads attached. Is this something you have seen before? Do you see any issues shrink wrapping the 1N4005's in this manner. It is my understanding that once inside an explosion proof enclosure, non-hazardous wiring methods and components can be used.

Below is the diode and also the solenoid. They are the Type Q4-230 soldnoids.



1631188468437.png
1631188557737.png
 
fifty60_4802GND said:
Here is what the original diodes from Veeder look like. The solenoid itself gets installed inside of an Explosion proof enclosure. It looks like the diodes are shrink wrapped with wire leads attached. Is this something you have seen before? Do you see any issues shrink wrapping the 1N4005's in this manner. It is my understanding that once inside an explosion proof enclosure, non-hazardous wiring methods and components can be used.

Below is the diode and also the solenoid. They are the Type Q4-230 soldnoids.



View attachment 2557774
View attachment 2557775
Click to expand...

I suspect the shrink wrap is simply to insulate the bare leads that are typical of diodes.

As long as the enclosure is properly rated for the environment, conventional internal wiring is fine.
 
We originally used 1N4004 which were more common and less expensive than 1N4003. I concur with the lack of need for anything over a 50V rating on 24V systems; it should never be a concern. The higher tech is something like a TransZorb (tm) which typically, in 24V systems, will clip at something around 30-50V. I believe (it's been 40 years since I was directly involved) that we later used the Vishay SA33CA-E3/54. Mouser shows them at 50 cents.
 
The reason you want to have a higher voltage rated diode than the voltage level of the coil is because when you shut the voltage off to the coil there is still energy stored in the magnetic field of the coil. It will generate a pulse of electrical energy of opposite polarity as the field collapses. I generally use 1N4005 diodes because they are good for just about any DC voltage I would ever see and the leads are sturdier than the diodes rated for lower Volts or Amps.
 
petersonra said:
The reason you want to have a higher voltage rated diode than the voltage level of the coil is because when you shut the voltage off to the coil there is still energy stored in the magnetic field of the coil. It will generate a pulse of electrical energy of opposite polarity as the field collapses. I generally use 1N4005 diodes because they are good for just about any DC voltage I would ever see and the leads are sturdier than the diodes rated for lower Volts or Amps.
Click to expand...
But with a correctly installed diode, the current pulse from the stored energy of the magnetic field will be going in the forward direction of the diode. If there is a high capacitance in the circuit somewhere, there might be ringing of opposite polarity, but that would have to be at a much lower voltage level when the voltage in one direction is clamped.
 
We use the STTH2R06 which is a 2 amp 600v ultrafast diode. Good for flyback diodes since they stop conducting reverse current about 50 times faster than a standard rectifier diode.
 
All good suggestions here. Please consider the diode polarity is reversed wrt the relay coil voltage. When the coil is on, it has 24V across it so a diode with a PRV rating of >50V is OK. When the coil is turned off, good old Lentz says the coil voltage reverses to maintain current flow through the coil in the same direction. But now the diode is forward biased and its PRV doesn't matter.

To dive deeper, the physical turn off rate of the coil depends on the L/R time constant; R being the coil +diode resistance and L of course is the coil inductance. With a normal diode, R is low and the coil will turn off slowly compared to the use of a Zener diode which will stay off below its Vz and hence give faster turn off.

There are circuit configurations one can use to eliminate the flyback diode, but that is the path of the penny pitching rathole.
 
Thanks for all the great insight information to build my understanding and intuition.

Does it matter where the diodes are installed? I know they have to be installed across the solenoid coil, but does this have to be directly at the physical coil location?

The controller for the coil is located around 50 ft away from the actual coil itself. I would prefer to locate the diode at the controller, which is completely out of the hazardous area (Which is always my preferred protection method, though it opens the pandora's box for wiring and conduit in and out of hazardous areas).

Does it matter if I install at the physical valve, or 50ft away back at the Valve controller? I think electrically it is the same, though there is a physical 50ft difference in actual placement.

50ft is a lot to the caveman, but not a lot to the electron. Relatively speaking of course.
 
210911-1007 EDT

fifty60:

Consider you circuit. The load is an inductor with internal resistance, and there is shunt capacitance across the coil. For the purposes at hand you can probably ignore the capacitance. If your point of switching is some distance from the coil, then you can probably just lump the the inductance and resistance of the wiring into that of the coil.

Whether you put the diode directly at the coil or at the switch point the diode will operate the same.

The voltage seen across the switching element will be slightly different. If the diode is directly at the coil, then the inductance of the wiring from the switch point to the inductor can produce a slight arcing at the switch point, but probably of no importance. Now move the diode to just after the switching element, and that removes any load inductance between the load and the switch. But this does not eliminate inductance before the switch, So possibly it is better to put the diode across the switch.

I can not tell you what is the best diode location. May not make much difference. Try some experiments. It all may depend upon the values of residuals

.
 
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