Relay for pedal bypass

[knutolai]

> relay already needs a 5v regulator

No it doesn't. Put a resistor in series.

How would I go about calculating that resistor value?

[Seljer]

How would I go about calculating that resistor value?

Measure the resistance of the relay coil (or find the datasheet, but it doesn't hurt to check with a multimeter to be sure). The resistance of the coil is the only thing thats limiting the current after its turned on (the inductance only comes into account when transitioning). Calculate the expected current at the rated voltage, then the limiting resistor to draw the same current at your higher voltage.


For example: 5V relay, 160ohm coil
relay current = U/R = 5V / 160ohm = 0.031A

you need to add a resistor that will drop the 9V supply down to 5V when 0.031A are running through it.
R = U/I = (9V-5V) / 0.031A = 129ohms

So slap a 120ohm in series and it'll the final relay voltage should be close enough: 9V * (160/(120+160)) = 5.14V
The relay itself allows from some leeway regarding over/undervoltage (a typical figure would be 20%) and will still work.

And check the power rating for your resistor: P = U^2/R = (9-5.14)^2/120 = 0.124W
Thats an 1/8th of a watt so you should be fine with normal 1/4watt resistors.

[knutolai]

Thanks. Thats very clear and in depth. Should have known coils had a resistance

The relay itself allows from some leeway regarding over/undervoltage (a typical figure would be 20%) and will still work.

So this is why this approach is acceptable? "9 volt supplies" aren't always 9 volt, but with that much leeway I guess it can tolerate a little variation in both the resistor and supply voltage.

[Seljer]

Inside it just an electromagnet being held back with a spring. The electromagnet doesn't overpower the spring directly at the rated voltage but a good bit before that. It's not a precision electromagnetic balance . Relays are rather spartan, no need for a fancy regulated supply for something this simple! They have to tolerate undervoltage if you ever got a really long cable run too. Or 12V automotive relays where the voltage is all over the place.

The only way to be sure or of what the limits are is to find a datasheet to know what exactly you've got. In the panasonic one you linked to ( http://pewa.panasonic.com/assets/pcsd/catalog/tq-catalog.pdf look at page 3)

Pick-up voltage (minimum to reliably switch on): 75% or less of nominal voltage
Drop-out voltage (how low it has to get to reliably switch off): 10% of nominal voltage
Max. applied voltage (how high it can get before it burns up): 150% of nominal voltage

[knutolai]

Taking a good step back and actually looking at the current draw of the Single Side Stable Relay I've gotta say this is a very current demanding configuration! For the circuit to be active current must constantly flow through the coil (and the series resistor).
9 V/~300 Ohm = 30mA! I can see why the 1/2-Coil Latching Relays are the preferred alternatives.

[J0K3RX]

You really should look at a "one-shot" bistable relay circuit of some sort.

Do a google search for the exact phrase below and you will see an awesome designed AVR based system! It's in another forum, of course

Generic I/O board for pedals, with bistable relay

[knutolai]

Great project! I guess I might land on the MCU route. Thanks for all the effort guys. I feel a lot smarter, or less stupid, now on the Relay-topic. Hopefully other people will find this thread helpful too.

[Seljer]

Yes, the two state stable latching are much better regarding current usage. They are however, slightly less common to find and require more drive circuitry (and they don't offer the "smart" auto-bypass when you lose power)


But it's only an issue with battery powered pedals, it's really much of a hit when using an external power supply

[samhay]

FWIW, I use 12V non-latching relays from time-to-time (with a 9V supply). I like them as they are cheaper and easier to find than bi-stable relays, are simple to use, will give bypass on power-off, and seem to draw ~10 mA when on. Current draw is not great for batteries, but not too bad otherwise, and if you are using a uC to drive them, then you will be pulling a lot more than 10 mA anyway.

[Liquitone]

yeah, I tried bi-stable relays and the circuit and didn't like how the connections to the relay got too cluttered. Although it shouldn't really be a problem with dual layer boards. On the plus side with non-latching relays current draw, it only draws that current when the pedal is not bypassed, which is ok for a wah. Perhaps more annoying with 'always on' type effects like treble boosters and fuzzes, but then you could swap the bypass jumper and effect in/out around to have it 'on' without drawing current.