Low current Relays?

[Transmogrifox]

Does anybody know of any existing DPDT relays that can operate on less than 7 V and 5 mA?  10 mA may do...I have an idea for "emergency" switching in a battery operated effect with a relay, but I'm thinking this is the reason that effects usually use FETs or mechanically-activated true-bypass switching.  Either way, this application I have absolutely requires switching from an electronic signal and I would like to avoid using FETs unless there's no way around it.

[Nasse]

:oops: I dont know such relays for sure though I have one 30 years old 6 volt relay in a junk box. Sometimes component catalogs and retailers announce special relays that have two latching positions. You can chance switching state by control voltage pulse, and after that it consumes no current. You need two switches or some electronics to control them. I bought two of them once because they were cheap but have not tried them. Probably they are 9 or 12 volts anyway. If posiible I can check one link if there is some info.

A have a faint memory of a circuit in some magazine or book but I dont remember which one and I have massive "archive", but anyway but in this story was something like a big electrolytic capacitor storing energy for relay changeover startup energy hungry pulse, and voltage for that cap and relay coil was trough some resistor that reduced current consumption somewhat.  Relay stays "on" with less voltage and current than it is needed for make reliable contact.

[R.G.]

Try reading the info at GEO. There's a lot of stuff there on relays.

You are correct that relays eat a lot of power if they're held on by current, probably too much for a battery to hold up. You also sound like you're trying to invent the capacitive discharge relay flipper, which has been done a couple of ways.

To answer the immediate question, no, to my knowledge there are no 7V/5ma DPDT relays available. Relays take a certain amount of power to flip, and you'll find that the voltage rating on the coil times the current that the coil lets flow from the rated voltage is essentially constant for a relay family. That is, if you halve the voltage to drive the coil, you need to double the current to get the necessary mechanical power to flip the relay. It is possible that superultralow power relays like you're describing exist, but they're not common, and in this business, that means way expensive.

The common values of relay coil voltage are 4v, 5v, 6v, 9V, 12v, 24v, 48v, 120v(ac). Of these, you will only find 5v, 12v, and 24v available as other than special order. I always get a bit miffed that 9V ones are not easily available, but they are minimum order things.

Here's another alternative. Use latching relays, per the article in GEO on the latching relay driver. Latching relays only use power to flip the relay to a new position, then you can turn the power off and it stays where you put it. No continuous power drain. If you use a 5V relay, you can load up a capacitor to whatever voltage you have available, say, 7V to 9V, and then "fire" the relay from the capacitor with some adaptation of the latching relay driver at GEO. You'd have to wait for the cap to charge up each time before flipping again, but it would work if you calculated the cap value to store enough energy to flip the cap each time.

It's a fairly straightforward calculation. You just have to ensure that you have stored in the cap enough CVsquared/2 energy to provide the min current long enough to ensure the relay has latched in the opposite position. The relay specs should tell you what you need. I recommend the NEC miniature series that includes the EA2. Good relays, that.

R.G.

[Transmogrifox]

That's kinda what I figured from scrolling through a DigiKey catalog.

I did read the Geo page and have decided the latching relay is the way to go.  I'm actually not making a firing cap device (in case you are interested), I have a circuit that I'm working on that goes unstable under certain conditions, and since the instability depends on properties of effects pedals outside the circuit, I cannot simply compensate or change parameters to keep the circuit stable.

Therefore, I have designed an oscillation detector that sends a logic high or logic low output depending on whether the circuit is stable.  I want to use the relay to bypass the circuit and also connect a head-toe diode clipping set-up to protect the external circuit from a high amplitude signal when this happens.  

I may resort to CMOS, but the "True Bypass" concept seems worth the effort. With a latching relay, I must design some extra logic to ensure that the relay is in the "bypass" state as long as the detector provides logic "high" levels.

Any suggestions to simplify my approach?  My oscillator detector is a very simple circuit, works beautifully, so the need for simplicity is in the translation from logic signal to bypass.

[mattburnside]

I've got some Aromat relays that are rated at 9V, but IIRC they will switch lower than that (down to the point where, if the battery is dead/dying, the switching will work, but the effect won't). I put a small-value resistor in series with the coil, and (again IIRC) it will stay switched with around 10mA. I don't have the part number handy, but I'll have a peek when I get home. If they do what you want, I can send you a few.

[Transmogrifox]

Thanks for your reply.  I have decided to ditch the idea of a non-latching relay.  10 mA will drain a battery faster in bypass mode than in 'on' mode, so I designed an oscillator and some logic circuits to strobe a latching relay until it's in the correct position....a heck of a lot less current than keeping one on all the time.