DPDT 2 Coil Relay Problem

Hi all, I set up a test circuit to see whether I could get the DPDT 2 coil relays working I got today, and I can't for the life of me get them working. I intend to use them in a bigger project, can anyone see any obvious issues in this circuit?

Try high side switching with your transistors.

I just tried high side switching on the transistors without any luck, thanks for the suggestion though. I tried pulling out the transistors and it works fine. With 2N2222A's the emitter is where the notch on the casing is right?

I tried pulling out the transistors and it works fine

What is the coil resistance?

The diagram you show IS high side switching. The output voltage is going to be something like 4.5 - .8 or about 3.7V. Did you measure what it actually was??

Try Low Side switching (Emitter to ground, Collector to relay coil, relay coil to +5V) The base resistor can be about 220 ohms and drive the transistor harder.

What is the coil resistance?

167ohms +/- 10% according to the datasheet.

The diagram you show IS high side switching.

My mistake, pretty new to electronics - regardless I tried low side switching (the way that you outlined) when I said I was trying high side.

Tried changing the resistor value like you suggested to no avail.

Got out my multimeter, seems like I'm getting a constant voltage across the coils (around 4.05v.. which is a bit low for the coils but in the actual application they won't be going through an arduino and will be sourced from a wallplug rather than my PC).

The voltage to the base is pulsing like expected. Just to be sure, the 2N2222A's pin layout is like this right? : collecter, emitter (notch), base?

Sorry about the time between the posts, had a power cut.

[ A pinout diagram that doesn't say whether its top view or bottom view is not the most reassuring! ]

bistable relay coils are usually polarised aren't they? Is the current being fed the right direction? You must use common-emitter circuit as the coil needs a full 5V, 4.05V is way too close to the margin (4.0V according to the datasheet - the datasheet also marks pins 1 and 2 as the positive ends of the coils and 15 and 16 as the negative ends).

You almost never use an emitter follower in a digital (switching) circuit, btw.

Hi, This is called a "Latching Relay"

Some of these are mechanical latching. Some are "Magnetic Latching". Which is this??

If it's Magnetic Latching the polarity on the coils IS important. Some such relays have only one coil and you switch polarity to change which way the relay is "magnetically latched".

I used these in Broadcast Transmitter remote control systems during the Enlightenment AKA the 60's...

I agree. Latching types are often polarity specific. Also I have attached my default transistor relay circuit.

This relay does not use alternate polarity for resetting, this is why it has two coils. Applying a voltage to one coil makes it latch from it's default position, applying the voltage to the second coil makes it reset. The polarity I know to be correct, as it works without the transistor. Thanks for the diagrams, I'll double check the pins of the transistors and try again with the low side switching.

I'll keep you all updated when I have a chance to play with this again later today.

I had the same problem with a dual coil latching relay and I switched it to the low side and got it to work that way.

Thank you all for the help, turns out the transistors where the wrong way round, the base and collectors where reversed. I'm also doing low side switching now as suggested.

If I may ask, why is it preferable to do low side switching over high side?


Simple answer is... Because of the way NPN transistors work.

Note: typing on a tablet in an airport... hence... the simple answer.

Simple answer is... Because of the way NPN transistors work.

Haha well that's great and all, but I'm interested in why exactly - a link will do if you can't be bothered typing it up :).

Because with the load in the emitter you can only ever get the voltage on the base minus 0.7 volts on the coil no matter what voltage the collector is going to.

Where as if the load is in the collector you can get a voltage across the coil equal to the voltage to the other end of the coil minus the saturation voltage Vsat of the transistor.

Aha! That makes perfect sense. Thanks.

Is this circuit necessary if I am using 3V relay??

Is this circuit necessary if I am using 3V relay??

What is the coil resistance?


Is this circuit necessary if I am using 3V relay??

What is the coil resistance?

The resistance of the coil is 129 ohms