How do I know the positives and negatives of a 5V reed relay?

Question

I have a 5V reed relay with 4 pins that I got online and it has no markings on it. Does it even have positives and negatives? If so, how do I know where they are?

Answer 1

You look at the data sheet for the relay. Apparently you didn't get one, I'd ask the vendor, as there are lots of other missing info aside from the pin connections.



Check the resistance between pairs of pins. If you find a pair that measures some finite resistance, like 1000 ohms, then those are the coil pins, and the other two are the contacts. If there are no polarity markings on the case, you just have to try both polarities until you find the one that causes the contacts to close (you should be able to hear them click)



edit: reed relays usually have a permanent magnet to bias the contacts. That means the coil is polarity sensitive.

Answer 2

Those are good answers. The reed is normally open, but there could be a permanent magnet that closes the contacts which will open when 2 volts or more are applied to the coil. The reverse polarity will close the contacts harder, which generally has no utility but is not harmful up to about 6 volts. unless the ambient temperature is over about 40 degrees c. If the turn off voltage starts at more than 6 volts, it should drop to 5 volts or a bit less to avoid over heating the coil and demagnetizing the permanent magnet . Possibly the first few micro seconds are at a high voltage to achieve open contact in a few microseconds. Reed relays are faster acting than most other types of relay. If the permanent magnet is partially demagnetized, the contacts may be open, As little as one volt will close the contacts, which is 2 milliamps, making it a very sensitive if the coil is 500 ohms. This was important long ago, but is not now as solid state relays are available with sensitivities of one to several micro amps. With 2 milliamp sensitivity the contacts will likely stay closed until a reverse pulse to the coil resets the contacts to normally open. Likely this is done rarely as reliability is low.

More likely there is no permanent magnet and no provision to install one, but a jury rig may be practical. 5 volts (or more briefly) closes the contacts, which open when the coil voltage drops to 1 volt or less. Reversing the voltage briefly opens the contacts faster, but the reverse voltage will close the contacts again after they are open for a few micro seconds. Tricks like this can produce off time of a few micro seconds to reset an SCR, or other possible applications.

Without a bias magnet, one voltage polarity works the same as the other. Generally ac applied to the coil gives bad results except at the resonant frequency of the reed = 5000 hertz perhaps? Typically the current though the contacts should not exceed one amp for high reliability at either opening or closing of the contacts, but somewhat more current is tolerable though already closed contacts, which is also true of other mechanical switches and mechanical relays but perhaps not most electronic relays.

I would expect that a diode if installed would be visible as it would not be desirable for some applications and a resistor in series with the diode is often desired, but the resistor ohms is dependent on the application. Neil

Answer 3

The polarity does not matter unless it has an internal diode across the coil. By checking with a multimeter set to ohms, you can identify the two connections to the coil as the pair with a resistance reading, however small. The remaining pair will be the contacts and show as open circuit for a normally open relay or a short circuit for a normally closed relay.

Answer 4

Two are for the coil-use an ohmeter to ck.



The contacts are either NO or NC-use ohmeter.



No polarity concerns.