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.



P.S

If I change the Electromechanically relay with a SSR that has a load voltage of 0-400VDC and load current of 40 amp, will I still need to install the diode ?

 

Hi Guys

P.S

If I change the Electromechanically relay with a SSR that has a load voltage of 0-400VDC and load current of 40 amp, will I still need to install the diode ?

No, but check the manufacturer's DATASHEET and application notes to be sure of all requirements. I prefer the SSR option.

 

I prefer the SSR option.

Well I like the SSR option as well, but what I don't know is if it will run fine with the coil pumping current when its begin disconnected

if the solenoid causes the contacts on the electromechanical relay to get welded, what would it do to the SSR, will the SSR work fine without a diode across the solenoid ?

 

Check the manufacturer's DATASHEET. They will have recommendations (there may be a built-in diode or they may recommend adding one. Check it out.

 

checked the datasheet, but nothing was found there only the diagram showing a diode on the load side (terminal), I cant see how the diode will work when its not across the load ?

 

SSRs can handle AC or DC loads.

DC loads
This type of load should be considered inductive, and a diode should be placed across the load to absorb any surges during turn-off.

 

DC loads
This type of load should be considered inductive, and a diode should be placed across the load to absorb any surges during turn-off.

Any idea what type of Diode I should use for a 900W Solenoid running on 120 VDC ?

 

The same current that flows through the coil will flow through the diode when the circuit is opened. So 900W/120V=7.5A. Your diode should be rated AT LEAST 120V/7.5A. Getting something rated a little higher than that wouldn't hurt.

 

A coil diode won't stop the contacts being welded together.

You need an RC snubber across the coil contacts, to reduce arcing when the contacts open. Or a relay rated for enough DC contact current with your type of load.

 

A coil diode won't stop the contacts being welded together.

You need an RC snubber across the coil contacts, to reduce arcing when the contacts open. Or a relay rated for enough DC contact current with your type of load.

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.

The transient voltage will be less than typically generated by an RC snubber. Of course the effect of using a diode is that it slows the release time of the solenoid as compared to an RC or zener diode snubber. So, if speed is important, you will want to consider using one of those instead.

 

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.

 

1500W Transient Voltage Suppression (TVS) Diode. Like this one from Littelfuse:
Part number: 1.5KE150A, Unidirectional, 1500W, Vr=128v

creakndale

 

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.

 

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.

 

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.

 

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.

 

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

 

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

 

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.

***************************************************

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).

 

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.