Using TVS Diode P6KE across relay coil but then also relay coil is getting damaged.

Hi Amol,

The diode is not usually there to protect the coil but to protect other semiconductor components from the voltage spike caused by the coil when it is de-energized. Usually the only way to damage a coil is to put voltage on it that is outside its specifications. Also you might check to see that the relay is designed for AC or DC volts. Using a relay rated for 24 volts AC would get excessively hot if used on 24 volts DC. It would also help to diagnose your problem if you would include a diagram of your circuit and how you have the diode connected in the circuit. Also if you can explain how the coil is being damaged. (heat damage?, shorted?, open?)

John

HI John,

Coil is getting open in between.It is getting damaged near the soldered points.

Not sure but its a burn out, coil has burn between that.

Bidirectional TVS is connected across coil.

I am using 24V DC ,contact rating of 100 Amps.

Connection are as follow,

One end connected to +24V DC through a relay followed by switch and second end directly connected to ground.

Pls refer daigram

Unfortunately the schematic shows a relay and not what is connected on the load side, or how the 24v DC is sourced (ie switchmode supply, battery, etc).

What is on the end of the 100A contacts.? ie a simple resistive load, or a motor.

What is the relay that controls the coil being driven by.

Is that chattering?, or a clean ON or OFF.

How long before the relay is damaged.?

Are you sure the damage isn't being caused by the contacts of the relay.?

Placing the diode across the coil slows down the release time to approx 5 times the relay spec.

This means there may be an arc when the relay opens which will cause large amounts of heating.

I'd suggest carefully pulling apart the relay and examining what is damaged.

You might want to include some high resolution photos of the damage and teardown.

Mark

Unfortunately the schematic shows a relay and not what is connected on the load side, or how the 24v DC is sourced (ie switchmode supply, battery, etc).

What is on the end of the 100A contacts.? ie a simple resistive load, or a motor.

What is the relay that controls the coil being driven by.

Is that chattering?, or a clean ON or OFF.

How long before the relay is damaged.?

Are you sure the damage isn't being caused by the contacts of the relay.?

Placing the diode across the coil slows down the release time to approx 5 times the relay spec.

This means there may be an arc when the relay opens which will cause large amounts of heating.

I'd suggest carefully pulling apart the relay and examining what is damaged.

You might want to include some high resolution photos of the damage and teardown.

Mark

It is connected to 24V 150Ah battery.

It is clean ON OFF
It damaged after 3 hours of operation

The damaged can be also due to contact,the contacts are connected to alternator.

That image helps....

There is no heating apparent from the contact as it seems to be a nice copper colour.

The coil wire appears to have fused.

For any inductive circuit the In-rush/ON current is very high.

You haven't included the model of relay or the coil specs, but whatever it is normally, you can expect 3-5 (or more) times the "at rest" current as it switches on.

Most of these relays have a very high mechanical force, which will add to the ON current.

Unfortunately without the circuit, it is not possible to tell if there is a secondary path that is adding to the problem.

If you have a relay and switch controlling this relay, then I'm not sure why you need the diode across the coil.

It is clean ON OFF

How do you know ... have you bypassed it, or put something across it to check.??

There is obviously something causing it ( as evidenced by the damaged coil wire) so it is a process of eliminating each part to isolate it.

Second guessing and assumptions will not fix it, so you/we need to assume the design isn't working as it's intended.

Include the relay spec sheet, or part number and more importantly the source you are getting them from.

If you simply wire the relay across the 24v without any connection to the alternator, or the other means of controlling the coil, will it last more than 3 hrs.?

This will prove the relay can actually operate in a continuous mode ...

Mark

Relay operates for more then 3 hours without any connections.

Actually relay operates in continuous mode without any problems.

And this relay is used to turn on several motors at one time.

The source is alternator ,does alternator output damages the coil.

Relay coil is energised through feedback from starter switch.

If the coil is being energised by the alternator without any dc rectification then yep it'll generate a lot more heat and fail more rapidly

Hi Amol,

My experience with situations like this indicate that the coil is getting too much current during energization. mcb1 is right in his assesment. Each time this over current occurs the wire that you show in the photo "heats up" as it is exposed in the air and the weakest link. The repeated heating of the wire causes it to change its temper and resistance and become more and more susceptible to failure. I would put a low resistance, perhaps 5 ohms, in series with the the circuit that feeds the coil. If the coil won't pull in with 5 ohms try 3 ohms. This will limit the energizing current and should prevent the failure you are seeing.

John

Hi Amol,

I thought of one other possibility for the cause of this problem. If for some reason the solenoid relay coil is not properly secured to the armature it will move during the actuation. This can cause a mechanical stress or a bending of the leads. Just like bending a wire back and forth will break it, repeated actuation with a loose coil will also break the leads.

John

Thank You ,we just reduced the diode breakdown current.