24V DC relay circuit

Thread Starter

corrado33

Joined Oct 22, 2009
8
Hi guys. I have a quick question. I'm building a project with a 24V DC power supply, however I want there to be an ON/OFF switch (obviously). But I want to do this with a VERY small switch and relay. However, I can't seem to find a good way to implement this.

I need to use a very small contact switch to control a relay which controls power to 4 motors at 3 amps a piece and some other small things, but I only have 24V DC power. Most if not all small switches can't handle 24V DC. How can I get around this? Could I build a small voltage converter? Maybe using a small MOSFET transistor? Since the relay would only draw very little amps could a MOSFET handle a large voltage drop? Say to 5v or less?

Thanks!
 

Thread Starter

corrado33

Joined Oct 22, 2009
8
I have about a very small area to fit my switch in. I haven't looked up the switches yet because I'm not sure if the circuit is possible. Basically I have a door that I'm shutting and it's going to activate this switch when it's closed. The door is about half an inch think (or tall, however you want to look at it.) So the switch could be at most an inch tall. Basically I'm looking for a tiny switch like this... http://www.radioshack.com/product/index.jsp?productId=3190382 But if I can't get it to work I can DEAL with bigger ones.
 

Thread Starter

corrado33

Joined Oct 22, 2009
8
I am not sure to understand your problem. 24 V is ways below the minimum isolation that any switch can handle! You have to worry for the current if the switch in question is very small, but you are saying that your relay need only few milliAmps, so I realy don't see where the problem is. Just connect your switch between 24 Volts and your relay without any fear.

Alberto
Ok so if the relay coil pulls let's say 50-100mA, is that too much for small switches?


Oh and I liked the magnetic switches, but they're a bit big.
 

Papabravo

Joined Feb 24, 2006
21,159
Ok so if the relay coil pulls let's say 50-100mA, is that too much for small switches?


Oh and I liked the magnetic switches, but they're a bit big.
Give us an example of what you consider a "small" switch. It does not sound like it should be a problem.
 

SgtWookie

Joined Jul 17, 2007
22,230
Here is one way that you could use that low voltage, low current switch with a Darlington transistor to drive that relay, and stay within the switch specifications.



R1 limits current to <1mA. D2 is a 5.1v Zener diode; however with the very low current flow, it's breakdown voltage will be less than 5v.

S1 is that 5v 1mA switch you found. When it closes, it feeds about .7mA to the base of the Darlington transistor, which turns on the relay.
 

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