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  #11  
Old 07-01-2013, 11:50 AM
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strantor strantor is offline
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I also had beef with RB's statement, but I know who the bigger expert is. So assumed he was right and I was wrong. I wanted to do the digging and pull some references but I didn't have the time to find anything that spelled it out. So I'm glad you spoke up, let me know that my understanding isn't fundamentally wrong.
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  #12  
Old 07-01-2013, 01:17 PM
creakndale creakndale is offline
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1500W Transient Voltage Suppression (TVS) Diode. Like this one from Littelfuse:
Part number: 1.5KE150A, Unidirectional, 1500W, Vr=128v

creakndale
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  #13  
Old 07-01-2013, 02:09 PM
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Originally Posted by creakndale View Post
1500W Transient Voltage Suppression (TVS) Diode. Like this one from Littelfuse:
Part number: 1.5KE150A, Unidirectional, 1500W, Vr=128v

creakndale
That one seems a little ill equipped, with a peak surge current of 1mA.

Here's a list of other TVSs that have voltage ratings >130V and amperage ratings >8A.

Note: I've never used a TVS diode for this purpose; I usually use a plain ol' rectifier diode.
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  #14  
Old 07-01-2013, 04:47 PM
ian field ian field is offline
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[QUOTE=qitara;623449]Hi Guys

I got a solenoid that is getting the relay that is driving it going bad due to the contacts getting welded, so now its time to install a diode across the solenoid, only thing is that I am not a pro at sizing the correct diode, So i would be thank full if some of you guys could help


the Solenoid is a 900W DC solenoid running on 110VDC.

There is a slight problem with a solenoid/relay back emf diode - as it clamps the spike it causes current to circulate round the diode/coil circuit, that current delays contact opening and draws a longer arc - the ragged contact surfaces weld easier!

For lower voltage method is to use a clamp zener in series with a regular diode to prevent forward conduction is better than blindly shunting the spike.

For a 110V solenoid you might be better off with a MOV spike suppressor.
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  #15  
Old 07-01-2013, 06:02 PM
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Quote:
Originally Posted by ian field View Post

There is a slight problem with a solenoid/relay back emf diode - as it clamps the spike it causes current to circulate round the diode/coil circuit, that current delays contact opening and draws a longer arc - the ragged contact surfaces weld easier!

For lower voltage method is to use a clamp zener in series with a regular diode to prevent forward conduction is better than blindly shunting the spike.

For a 110V solenoid you might be better off with a MOV spike suppressor.
Again, I believe this incorrect. If adding a diode actually aggravates the arcing problem, then why is it such a common practice to add the diode? Why can I buy relays and solenoids that have the diode installed from the factory?

The reason coils cause arcing across the contacts because they are inductors, and inductors oppose changes in the flow of current. When you suddenly open the circuit, the inductor will keep the current flowing at whatever voltage is necessary (up to infinity, theoretically) until the field collapses. So with no diode, the voltage just spikes until whatever voltage is necessary to arc across and complete the circuit. When you install the diode, that current now has another option of where to go, and it does choose to take the path of the diode, not the open contacts. With the diode installed and the new current path option in place, the coil does not need to increase it's voltage to bridge any gaps and complete the circuit, so no spike occurs. Upon opening of the contacts (@T0), the voltage across the contacts should be 0V, and rapidly approach source voltage (120V). So if upon opening the contacts the voltage across them is 0, then why are you saying that this will make the arcing worse? Can you provide a published reference that supports your claims? Because I have not heard this before you & (to an extent) RB said it. The way I have just explained it is the way that I understand it, and if I am wrong I welcome the correction.
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  #16  
Old 07-01-2013, 08:09 PM
ian field ian field is offline
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Quote:
Originally Posted by strantor View Post
Again, I believe this incorrect. If adding a diode actually aggravates the arcing problem, then why is it such a common practice to add the diode? Why can I buy relays and solenoids that have the diode installed from the factory?

The reason coils cause arcing across the contacts because they are inductors, and inductors oppose changes in the flow of current. When you suddenly open the circuit, the inductor will keep the current flowing at whatever voltage is necessary (up to infinity, theoretically) until the field collapses. So with no diode, the voltage just spikes until whatever voltage is necessary to arc across and complete the circuit. When you install the diode, that current now has another option of where to go, and it does choose to take the path of the diode, not the open contacts. With the diode installed and the new current path option in place, the coil does not need to increase it's voltage to bridge any gaps and complete the circuit, so no spike occurs. Upon opening of the contacts (@T0), the voltage across the contacts should be 0V, and rapidly approach source voltage (120V). So if upon opening the contacts the voltage across them is 0, then why are you saying that this will make the arcing worse? Can you provide a published reference that supports your claims? Because I have not heard this before you & (to an extent) RB said it. The way I have just explained it is the way that I understand it, and if I am wrong I welcome the correction.
I was referring to the relay contacts opening more slowly - this happens because the catch diode shunts the current back through the coil.

Maybe I just read more application notes than you do.
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  #17  
Old 07-01-2013, 08:25 PM
ian field ian field is offline
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Quote:
Originally Posted by strantor View Post
Again, I believe this incorrect. If adding a diode actually aggravates the arcing problem, then why is it such a common practice to add the diode? Why can I buy relays and solenoids that have the diode installed from the factory?

The reason coils cause arcing across the contacts because they are inductors, and inductors oppose changes in the flow of current. When you suddenly open the circuit, the inductor will keep the current flowing at whatever voltage is necessary (up to infinity, theoretically) until the field collapses. So with no diode, the voltage just spikes until whatever voltage is necessary to arc across and complete the circuit. When you install the diode, that current now has another option of where to go, and it does choose to take the path of the diode, not the open contacts. With the diode installed and the new current path option in place, the coil does not need to increase it's voltage to bridge any gaps and complete the circuit, so no spike occurs. Upon opening of the contacts (@T0), the voltage across the contacts should be 0V, and rapidly approach source voltage (120V). So if upon opening the contacts the voltage across them is 0, then why are you saying that this will make the arcing worse? Can you provide a published reference that supports your claims? Because I have not heard this before you & (to an extent) RB said it. The way I have just explained it is the way that I understand it, and if I am wrong I welcome the correction.
************************************************** *

http://educypedia.karadimov.info/library/13c3264.pdf
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  #18  
Old 07-01-2013, 09:33 PM
creakndale creakndale is offline
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Quote:
Originally Posted by strantor View Post
That one seems a little ill equipped, with a peak surge current of 1mA.
Perhaps it is better to check the actual datasheet instead of relying on Mouser's typographical error?

creakndale
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  #19  
Old 07-01-2013, 10:24 PM
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strantor strantor is offline
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Quote:
Originally Posted by ian field View Post
I was referring to the relay contacts opening more slowly - this happens because the catch diode shunts the current back through the coil.

Maybe I just read more application notes than you do.
Quote:
Originally Posted by ian field View Post
************************************************** *

http://educypedia.karadimov.info/library/13c3264.pdf
Ah I see the confusion, you're talking about suppression on a relay coil, and everybody else is talking about suppression on a solenoid (which is what OP asked about).
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  #20  
Old 07-02-2013, 04:49 AM
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Quote:
Originally Posted by crutschow View Post
Au contraire. Don't understand why you say that. A reverse-biased diode across the coil will most definitely minimize the problem of the contacts being welded. The diode will carry the coil current when the contacts open, keeping the voltage transient voltage no higher than the forward voltage drop of the diode and essentially eliminating any contact arcing.
...

Whoops! You are correct, I missed in post #1 that he was using a relay to drive a relay, and thought there was only one relay with both coil and contacts being very separate!

Please ignore my previous post.
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