Simple Circuits Question regarding relays

[Sivarias]

Hey, so I'm trying to build an alarm system that turns on after a pulse and stays on until it gets another pulse from a different switch. A magnetic latching relay seems to be the way to go, but all the relays I find dump a massive amount of current when switched over, amd I only need 25mA to power the LED/Piezo combo.

Am I missing something with my mech. engr. backround or am i going to have to find a way to throttle the current?

[factotum]

The amount of current flowing isn't a function of the relay, it's a function of how much power you're providing in the rest of the circuit--a relay is just a switch, so it sounds like you're just using one which is way over-specified for what you're doing. In fact, I'd question why you're using a relay at all--I'm sure there's some sort of circuit that could be set up with a simple transistor that would flip on when asked and then stay on until reset.

[Ninja_Prawn]

Yeah, I'm guessing you're looking at power relays, built for switching things like mains voltage heating circuits and white goods. For low voltage, low current microelectronics (indicator LEDs and buzzers), you should be able to get by with a semiconductor latch of some kind. It's been years since I've done any electronics (I did it at uni but then moved into mech eng for work), but it sounds like the sort of thing a first year student should be able to throw together in an afternoon. :smallwink:

[Sivarias]

The amount of current flowing isn't a function of the relay, it's a function of how much power you're providing in the rest of the circuit--a relay is just a switch, so it sounds like you're just using one which is way over-specified for what you're doing. In fact, I'd question why you're using a relay at all--I'm sure there's some sort of circuit that could be set up with a simple transistor that would flip on when asked and then stay on until reset.

All the transistor examples I see have a switch already there, couldn't I just use a switch in that case?

Also, aren't relays just switches then?

[Gray Mage]

I think the problem isn't the relay itself, but the fact that the ones you've picked use magnetic latches. When the magnetic flux is abruptly severed it causes a reaction on the circuit to try and "maintain" the same magnetic flux, thus the high current (see inductors and similar components that have varying magnetic flux and Faraday's law of induction). Using a non magnetic latch should solve this issue.

-Source, Eletrical Engineer

[Ninja_Prawn]

Also, aren't relays just switches then?

Yes, a relay is a switch that is operated by an electrical signal, rather than human action. The (actual) switches shown in the example circuits might be for turning the system off, testing purposes, or as stand-ins for whatever system you've got generating the pulse. You don't actually have to press them when the circuit is operating normally.

I expect the non-magnetic latches that Gray Mage recommends will be built out of transistors, but I guess that's not really important if you're just plugging a few wires into a little box.

[factotum]

Here, a quick search for transistor latch circuits produced this:

http://www.learningaboutelectronics.com/Articles/Latch-circuit-with-transistors.php

It even mentions using this for an alarm system of some kind.