Relay & fuse/mcb question?

Question

Hi, I would like to ask a simple electrical question.

I need to power a 24 VDC and 3.5 W solenoid valve.
I am using a relay. May I know what specifications of the relay i need to know? e.g. the current for the suitable relay?

Besides, I also need to add a fuse or circuit breaker. I plan to use a 1A fuse. however is circuit breaker a good choice too? any reasons for either choice?

Thanks.

Answer 1

You have not indicated the voltage you will power the relay with, nor have you indicated whether the relay will be normally energized or de-energized during normal operation.



For this reason you will require a single-pole, double-throw (SPDT)contact on the relay (most will come with double-pole, double-throw contacts). The current required for the solenoid valve will be 3.5 / 24 = 0.1456 amperes. Most small relays have contacts rated for 5 amps.



The coil voltage of the relay must be rated for the voltage you will control the relay with.



Incidentally, you should place a diode (wired in reverse to the polarity of the wires across the relay contact, serving the solenoid valve. This will prevent a high voltage spike across the contact, when the solenoid valve is de-energized. This will prevent the contact from burning or welding together.



EDIT

Forgot the fuse/circuit breaker question.



A circuit breaker will be better.It will eliminate the replacing (buying) fuses.



Edit

I would use the relay you have (6 a contact), and place an in-line fuse-holder just downstream of the relay contact. They are cheap (in the USA), and the fuse can be easily and quickly changed.



TexMav

Answer 2

A 24 V, 3.5W device pulls about 150 mA. Depending on the rest of the circuit you might want a smaller fuse.



A circuit breaker is a resettable fuse basically. When it pops it can be reset instead of replacing like a fuse. So which to use depends on wether you expect it to blow often.

Answer 3

agree with the other answerers except that resettable fuses are very cheap now, by the time you buy a packet of glass fuses & a fuse holder you've about paid for the resettable fuse! Look around you can even buy mcb very cheaply that allow you to use them as a switch as well