[SOLVED] What's the purpose of power relay in DC power supply?

[tajiknomi]

Whats the purpose of power relay in DC power supply ?

Someone brought me an old power supply (for repair) DC 12v, he use it to charge the automobile battery. When i opened it, I diagnosed that it has some broken rectifying diodes so i replace those diodes and now it works fine. But another thing i noticed is; There is an power relay in it, i didn't understand what's the purpose of that so i removed it from the supply.

Here is the supply image

The power relay JQX-76F

 

[KlausST]

Re: Whats the purpose of power relay in DC power supply ?

Hi,

This is no true power supply, it is a battery charger.
* Power supplies usually are not useful for battery charging
* Battery chargers are not useful to supply some load.

Relay:
How can we know? We need to guess: reverse polarity protection?

Klaus

 

[d123]

Re: Whats the purpose of power relay in DC power supply ?

Hi,

What's the rest of the circuit in that charger? Do you see/think it serves to disconnect the charger when the battery voltage is sensed to be fully charged, if there's anything in there that may be used to sense it, or some kind of (other) safety function?

 

[schmitt trigger]

Re: Whats the purpose of power relay in DC power supply ?

After you removed the relay, will the charger still work?

Most likely it won't.

 

[tajiknomi]

Re: Whats the purpose of power relay in DC power supply ?

@KlausSt: Yes, this is battery charger and not a power supply.

@D123: I should have mentioned that in the first place, The charger only contained Transformer, Rectifying diodes, Power relay(with cap to it terminal, maybe to avoid debouncing of relay) and connectors. It was dissassembled and i had to assemble it again so i couldn't figure out why power relay is used in this circuit, therefor i kept it out.

@schmitt trigger: I don't have battery right now but the DMM readings are showing DC ~15v (OFF-Load). I test its drop out voltage and at around ~2A resister load, it's voltage drops to 11v and i thing that's not a good indication. But the battery aren't resistive load, i think they behaves like capacitor so the dropOut won't be that much when connected to automobile battery, correct me if i am wrong here.

I would like to know the purpose(s) of this relay in this kind of circuit, and i know that without schematic it should be harder to guess but i will like to know the different possiblities of its existance in this kind of ciruit.

 

[tajiknomi]

Re: Whats the purpose of power relay in DC power supply ?

@KlausSt:

We need to guess: reverse polarity protection?

If it was used for reverse polarity protection then the Power relay should have some diode connected to it but it doesn't have any.

 

[RobertFalbo]

Re: Whats the purpose of power relay in DC power supply ?

It might be a disconnect when the battery voltage gets high enough. Energizing the relay would disconnect the charging. The relay coils are on the battery side.

 

[tajiknomi]

Re: Whats the purpose of power relay in DC power supply ?

It might be a disconnect when the battery voltage gets high enough. Energizing the relay would disconnect the charging. The relay coils are on the battery side.

I don't think that's the case, because the minimum switching voltage of relay is 6v and this charger is used for 12v. What's more probable is that it might be used for reverse-polarity protection.

 

[FvM]

Re: Whats the purpose of power relay in DC power supply ?

What's more probable is that it might be used for reverse-polarity protection.

Possible, but you'll need at least a diode to detect reversed connection. Contradicting your previous statement

The charger only contained Transformer, Rectifying diodes, Power relay(with cap to it terminal, maybe to avoid debouncing of relay) and connectors.

It would be so easy to sketch a schematic before disassembling the device...

 

[tajiknomi]

Re: Whats the purpose of power relay in DC power supply ?

It would be so easy to sketch a schematic before disassembling the device

When i got the device, connections were missing, I have to connect that from scratch. There i couldn't understand where can i plug the relay and for that i had to find its purpose first

 

[FvM]

Re: Whats the purpose of power relay in DC power supply ?

When i got the device, connections were missing

How do you know that some electronic parts weren't missing too?

 

[tajiknomi]

Re: Whats the purpose of power relay in DC power supply ?

How do you know that some electronic parts weren't missing too

I don't know that whether the parts were missing or not. I opened the enclosure and find the components which i had stated above. I connected those components except the relay.

 

[RobertFalbo]

Re: Whats the purpose of power relay in DC power supply ?

It might be a disconnect when the battery voltage gets high enough. Energizing the relay would disconnect the charging. The relay coils are on the battery side.

Another possibility is over current protection, the coil would be across a resistor in series with the output. When the voltage across the resistor was high enough, (like 6 Amps, on a 1 ohm 36 watt resistor) the relay would energize, disconnecting the charger's output and at the same time latching the relay by switching the power to the coil.

 

[tajiknomi]

Re: Whats the purpose of power relay in DC power supply ?

I have connected the relay for reverse polarity protection now. I think that's the critical thing to be avoided.