Surge current protection for relay output

[karkip]

Hello,

I m working on a project where a relay is used to cut off input of a power supply. Just basic switching ON and OFF of power supply using relay.
In some cases, i found the relay got stuck in closed position. After opening the relay, i found contacts got welded. Not just stuck, but actually welded.
I m assuming this can be the cause of surge current, so trying to figure out how can we avoid this.
Possible solution , i m assuming a RC Snubber or NTC. please suggest what should i use

 

[KlausST]

Hi,

Find out what switching current, continous current you need.
Then choose the correct relay.

Without current values, AC or DC, relay type, contact material...no one can give you detailed help.

Klaus

 

[FvM]

Need to consider the "power supply" characteristics. If it's a switched mode supply, inrush current of the input filter capacitor is most likely causing contact welding. A NTC current limiter can possibly help, but using an oversized relay or contactor with sufficient surge current capability would be a safer method.

 

[karkip]

It is used with SMPSs, approx 35 watt.
Assuming the surge current the reason, will be implementing a NTC . Will test this.
Already using Relay 250VAC/10amp .space is constraint so Oversized relay is not I want to go for.
And after exploring more, I found a RC snubber in parallel with MOV is also used for same.
how will it be working for surge protection?

 

[FvM]

RC snubber and MOV are good to protect relays switching inductive loads, but they have effects against contact welding caused by capacitive loads.

 

[karkip]

The relay used is RELAY 12V MPR-S-112-A, 10Amps. Switching load is LED Drivers mostly. Will be from 15-75watts.
Tried adding NTC 10D11 in series with Live and Relay contact.
But still the relay contact stuck and mostly permanently welded. found out after opening one of the relay.

 

[KlausST]

Hi,

some pictures show "3A" others show "5A", I´ve not seen "10A".

my simple recommendation:
no detailed datasheet --> don´t use it.

NTC 10D11:
It says "Zero power resistance" = 10 Ohms. Thus it limits the switching current to 230V/10Ohms = 23A
It´s a bit more than 3A!!!

A 75W load (with a guessed power factor of 0.8) will draw only 75W / (0.8 x 230V) = 0.41A steady current.
--> Better choose a more suitable NTC.

Klaus

 

[karkip]

Hi,

some pictures show "3A" others show "5A", I´ve not seen "10A".

my simple recommendation:
no detailed datasheet --> don´t use it.

NTC 10D11:
It says "Zero power resistance" = 10 Ohms. Thus it limits the switching current to 230V/10Ohms = 23A
It´s a bit more than 3A!!!

A 75W load (with a guessed power factor of 0.8) will draw only 75W / (0.8 x 230V) = 0.41A steady current.
--> Better choose a more suitable NTC.

Klaus

the seller provides the same relay in 10amp also. and it worked fine until now with normal resistive loads.
agree to your recommendations, but it was already in use .

i chose ntc 10ohms, considering its ability to protect against surge , as calculated above 23A.
Can you brief what are other factors to be considered while choosing a More suitable NTC ?

 

[karkip]

Ok. just read more and found 230/10= 23a, will be the current which will flow through relay contacts. so NTC resistance has to select to limit the current value , which can pass through relay contact. since relay contact has max 10amp, 230v/10a=23ohm, so NTC should be 23ohm.
So if i think , i can go with NTC 30ohm. I will have to 10ohm NTC in series, since i have them in hand. Will this help ?

 

[podericof]

Hello,

I m working on a project where a relay is used to cut off input of a power supply. Just basic switching ON and OFF of power supply using relay.
In some cases, i found the relay got stuck in closed position. After opening the relay, i found contacts got welded. Not just stuck, but actually welded.
I m assuming this can be the cause of surge current, so trying to figure out how can we avoid this.
Possible solution , i m assuming a RC Snubber or NTC. please suggest what should i use

Hi there,
if the contact of your relay is welded, it probably means you have selected the wrong relay.
without knowing more details about your application is very difficult to make suggestions.
If the contact is welded, it means that there is a high current that probably became a high current arc or plasma, and is this plasma that eventually welds the contacts.
For this reason, I don't believe an NTC or a snubber will fix your issue.
If you share more details I'll have a look and I can make some suggestions.

Good luck!
Francesco

 

[KlausST]

so NTC resistance has to select to limit the current value

That´s the basic idea.

I will have to 10ohm NTC in series

You can do so.
But it
* results in higher power loss.
* means your 10 Ohms resistor should be some kind of surge proof", becaus the initial current my ab above 10 Ohms which means a peak (over 10 ms) power of 1000 W.
The short max peak current may be sqrt(2) * 230V / 20 Ohms = 16A.

The expectable continous power dissipation is: (with the assumption above) ==> (0.41A)^2 * 10 Ohms = 1.7W
This is a lot!

Klaus

--- Updated Apr 3, 2022 ---

For this reason, I don't believe an NTC or a snubber will fix your issue.

I don´t agree.
Voltage is given, steady current is way below specified relay current.
Thus external circuitry may keep voltage as well as current within relay specifications.

For sure it needs to be well chosen.
And yes - there is a lot of unkown. Maybe there are (inductive, capacitive..) filters installed, or other circuitry.
Thus I recommend to verify the protection circuit by watching voltage as well as current of the relay contacts with a scope.

Klaus

 

[karkip]

That´s the basic idea.


You can do so.
But it
* results in higher power loss.
* means your 10 Ohms resistor should be some kind of surge proof", becaus the initial current my ab above 10 Ohms which means a peak (over 10 ms) power of 1000 W.
The short max peak current may be sqrt(2) * 230V / 20 Ohms = 16A.

The expectable continous power dissipation is: (with the assumption above) ==> (0.41A)^2 * 10 Ohms = 1.7W
This is a lot!

I didn’t get the power dissipation issue .
I m actually planning to use 3 NTC in series of 10 ohm, NTC 10D11
or should I specifically go for a standard 30 ohm NTC

 

[KlausST]

I m actually planning to use 3 NTC in series of 10 ohm, NTC 10D11

Ah, I thought you wanted to add a standard resistor in series to the NTC...

Klaus

 

[karkip]

The relay used is RELAY 12V MPR-S-112-A, 10Amps. Switching load is LED Drivers mostly. Will be from 15-75watts.
Tried adding NTC 10D11 in series with Live and Relay contact.
But still the relay contact stuck and mostly permanently welded. found out after opening one of the relay.

These are the known conditions

Hi there,
if the contact of your relay is welded, it probably means you have selected the wrong relay.
without knowing more details about your application is very difficult to make suggestions.
If the contact is welded, it means that there is a high current that probably became a high current arc or plasma, and is this plasma that eventually welds the contacts.
For this reason, I don't believe an NTC or a snubber will fix your issue.
If you share more details I'll have a look and I can make some suggestions. So

Load is within rating of relay contact but inrush current makes the relay contact stick . NTC suitable value is target. Load is led driver , which is not always of same model but ratings is 15-75 watt

--- Updated Apr 3, 2022 ---

Ah, I thought you wanted to add a standard resistor in series to the NTC...

Klaus

My bad . 3 number of NTC in series , 10ohm value of each . Will this be suitable ?

 

[KlausST]

My bad . 3 number of NTC in series , 10ohm value of each . Will this be suitable ?

I would not use it in a series production, but for a test it should work.

Klaus

 

[karkip]

I would not use it in a series production, but for a test it should work.

Klaus

ok. I should test it with this series combination, because this is what all I have in hand . What can be the side effects if used in series arrangement?
I plan to order NTC 30 ohms , which will take some time . So till then , I have to use 10ohm NTC .

 

[KlausST]

Hi,

Increased cost, increased heat dissipation, increased voltage drop,

Klaus

 

[karkip]

Hi,

Increased cost, increased heat dissipation, increased voltage drop,

Klaus

i see. Cost and voltage drop are ok, since we are ready to bear this much to solve this issue. But heat dissipation will again effect the performance. I was assuming more number of ntc might help in heat dissipation better since heat will be dissipated through three bodies, instead of one.
Now the main plan is to go with single 30ohm NTC. For current testing, will also be using series combination of 3 ntc. will try both. will update after this.

--- Updated Apr 4, 2022 ---

Need to consider the "power supply" characteristics. If it's a switched mode supply, inrush current of the input filter capacitor is most likely causing contact welding. A NTC current limiter can possibly help, but using an oversized relay or contactor with sufficient surge current capability would be a safer method.

was thinkng like the NTC are capable of handling inrush current, but they need time to cool down once the system is OFF and should not to be turned ON before it reaches its rated resistance , in able to protect against another inrush. Or it can fail.
So with Inrush capable relays, will there also be this kind of limitation ?