Hi all,

Please can somebody tell me what this device across the contactor coil is?

 

It is a VDR, Voltage Dependent Resistor, used for suppression purposes.
AKA Varistor

 

INDEED!! The symbol is for a varistor, which usually is a breakdown device, meaning that above some voltage it switches on and conducts quite well, until the voltage is removed and the current stops flowing. The really bad news is that some of them do not shut off when the current supply is switched off.
In some cases a device called a snubber is connected across the coil , which a snubber consists of a capacitor and a resistor in series. That also is used to suppress voltage surges, They have a benefit of not failing in a short circuit mode.

 

Never had a problem with them !
Use a reliable make!!
The R/C version is usually reserved for AC coils, whereas the DC coil uses a Reverse connected Diode.

 

Certainly Max has the correct plan, and the varist0ors used for industrial applications would be a better quality. But the ones protecting consumer electronics DO have the bad habit of becoming permanently conducting. That usually leads to them simply being removed, leaving no protection from the next voltage spike to arrive.

 

I don't recall ever seeing one in consumer electronics?
Most of the latter now use PCB DC relays so invariably have the diode feature,

 

Television sets had them. The solid state color sets all had one to protect the fragile semiconductors of that era. And they would protect ONCE. and that would pop the fuse, and if one did not realize that the device was permanently shorted, it would take the replacement fuse even faster. And the local tv REPAIR SHOPS HAD NO CLUE AS TO WHAT THE PART EVEN WAS.. A shorted disc capacitor they said. So the next spike takes out the regulator transistor for the high CRT voltage.

 

When I first came to Canada I took a job as service manager of a large electronics store service dept, we saw all makes & models of TV including the Philips, the one that separated the men from the boys.!
I do not recall ever in that 6yrs of ever seeing a problem as such as described in any set!!

 

In the area where I lived, in that era, there was enough power glitch activity to cause that sort of problem. MOV protection devices in cheap consumer good still suffer occasionally. The incident I mentioned was 40 years ago.
AND, Some folks are just Lucky!!

 

It is a VDR, Voltage Dependent Resistor, used for suppression purposes.
AKA Varistor

Perfect, cheers MaxHeadRoom

 

INDEED!! The symbol is for a varistor, which usually is a breakdown device, meaning that above some voltage it switches on and conducts quite well, until the voltage is removed and the current stops flowing. The really bad news is that some of them do not shut off when the current supply is switched off.
In some cases a device called a snubber is connected across the coil , which a snubber consists of a capacitor and a resistor in series. That also is used to suppress voltage surges, They have a benefit of not failing in a short circuit mode.

Hi MisterBill2,

Many thanks for the info, and you may be able to help me further.

Snubber for AC and Diode for DC is how I've always known it, but then I've seen many saying the snubber can be used for DC as well, but how can that be if the snubber has a capacitor given it will block DC?

In addition they say the diode should ideally have a resistor to aid the relay turn off time as the diode makes this 'slow' whereas the additional resistor speeds this, however it appears this can raise the back EMF compared to just having the diode. Can you help clarify this?

I have AC and DC relay coils I need to add them on, plus I have inductive loads so I need to add them to the relay contacts in addition to the coils. They say snubbers should be across the contacts and not the load, but why is this if the EMF and thus arcing is due to the load? Why not just put the snubber across the load as this will save the contacts just the same would it not? Is it just easier to install across the contacts than it is across the load and either method is fine?

Cheers in advance

 

The capacitor will conduct on pulses that may occur on power switching, DC.
R/C suppression snubbers are often fitted directly across the load.
In the case of switching high current DC loads, there are relays with arc blow-out magnets close to the contacts themselves for arc suppression..

 

The capacitor will conduct on pulses that may occur on power switching, DC.
R/C suppression snubbers are often fitted directly across the load.
In the case of switching high current DC loads, there are relays with arc blow-out magnets close to the contacts themselves for arc suppression..

Hi MaxHeadRoom,

I can understand how it would reach if an AC component were present when switching, but how does that deal with the DC side?

So RC snubbers can go across either the load directly or the contacts switching them without any pros/cons of either method?

What about the resistor in series with diode, any info on that?

Also, with the RCD version of the snubber, is this limited to DC only given the diode would conduct on the second half of the AC wave?

 

RCD? Do you mean Res,Cap,Diode?
I never use a diode in this configuration, either diode alone or just R/C.
When the DC collapses when turning off a inductive device, the induced power is opposite to the applied power.

 

Actually, the capacitor does not conduct, but it does accept a charge, and that does allow a current in the resistor, which does dissipate much of the transient's energy.
Putting a resistor in series with the suppressor diode also dissipates energy that would otherwise slow the collapse of the magnetic field. It also slows the relay or solenoid release time quite a bit. You can prove this to yourself by putting a diode in series with a DC relay and then powering the circuit with AC. The relay will buzz noisily. Now add a diode across the relay coil to block the current from the other diode. The relay will operate very well with no capacitor at all to smooth the current. It will also release a bit slower.

 

RCD? Do you mean Res,Cap,Diode?
I never use a diode in this configuration, either diode alone or just R/C.
When the DC collapses when turning off a inductive device, the induced power is opposite to the applied power.

Correct on the RCD assumption, however why never use the diode in the configuration as surely it's adding some benefit?

The revering of the DC I can understand, but isn't my question about it being conductive on the negative half of the AC wave still stand?

On the location question again, so it doesn't matter at all if the snubber/diode is placed across the coil/load or the contacts supplying power to the coil/load, it doesn't make a blind bit of difference?

Just on the original question of the symbol which was a Varistor, so this wouldn't be used for the EMF suppression like the snubber/diode then? Can they be used in conjunction with the snubber/diode? I'm assuming not as they would surely interact/affect each other when operating?

 

Actually, the capacitor does not conduct, but it does accept a charge, and that does allow a current in the resistor, which does dissipate much of the transient's energy.
Putting a resistor in series with the suppressor diode also dissipates energy that would otherwise slow the collapse of the magnetic field. It also slows the relay or solenoid release time quite a bit. You can prove this to yourself by putting a diode in series with a DC relay and then powering the circuit with AC. The relay will buzz noisily. Now add a diode across the relay coil to block the current from the other diode. The relay will operate very well with no capacitor at all to smooth the current. It will also release a bit slower.

Ok, so because it can charge the capacitor this is the same as shunting it through a diode, so why use the RC snubber on DC rather than a diode (with or without the resistor). Or should the snubber only be used on DC as a last resort and if possible stick to the diode?

What I don't get about the resistor is how this speeds it up because surely the diode is essentially a direct short once conducting bar a few mOhm, or is that the problem, because it's a low resistance it doesn't drop the voltage too much so it runs round the coil again, keeps it up, then through the diode and drops a bit more etc until the point the drop is too much and the field can't maintain itself and collapses? So the resistors is to drop the voltage as much as possible and the diode is just for blocking conduction when operating normally?

 

The diode is used because it is MUCH SMALLER than an adequately rated capacitor, and also because it costs a whole lot less. AND a diode can be hidden inside a small relay housing, which can give an unknowing person a very hard time getting a circuit to function correctly.

 

The diode is used because it is MUCH SMALLER than an adequately rated capacitor, and also because it costs a whole lot less. AND a diode can be hidden inside a small relay housing, which can give an unknowing person a very hard time getting a circuit to function correctly.

But the capacitors on the snubbers aren't that big, unless that's because they are not adequately rated? Yes the diode is smaller, but I've seen very small snubbers encapsulated into small packages before? I mean from that are we saying the packaged snubbers used on AC coils/contacts you buy from say RS are not actually rated properly? Also why does the snubber include that diode in the RCD setup?

So is my theory of why the resistor is included with the diode correct?

I need to read through the documentation you guys have supplied, but is there a rule of thumb to use when specifying/designing the component values for the Diode+Resitor and Snubber circuits? I've always used 1n4001 or 1n4007 diodes when just using a diode but I want to add the resistor in now. I need to protect both AC and DC coils+contacts, the AC being 230V (very low nominal current) and the DC is 24V (again very low nominal current)

 

Consider that the diode across a DC coil reduces the inductive spike ONLY by providing a current path for the energy to dissipate in the resistance of the coil and diode loop. Like I have stated quite a few times, in many posts, the result is a delay in the relay releasing, and a slower opening of the contacts. And so when an added resistor is in series with the diode and the coil, power is also dissipated in the resistor. That may speed the release of the relay quite a bit, depending on the value of the resistance. So the series resistor is an option when release time matters. But usually no resistor is used because of convenience issues. Adding two components is a lotmore complex.