PDA

View Full Version : Which Relay ?



Carlinux
09-09-2008, 07:18 AM
Hi,

I know that has been discussed many times in other threads but I didn't find the exact answer I'm looking for.

I'm using a BS2px to PPT a small TalkAbout radio. The PINx is connected to a transistor who is switching on/off a micro relay (275-240) www.radioshack.com/product/index.jsp?productId=2062480&cp= (http://www.radioshack.com/product/index.jsp?productId=2062480&cp=) to jump 2 wires from the radio, but, since there is no voltage in those 2 wires I was thinking about using an optocoupler like the 4N26.

Do you think this could work ? Do you have any ideas ?

Thank you for your help.
Carl

Mike Green
09-09-2008, 07:39 AM
Unless you have a schematic of the radio, I'd stick with the relay. Maybe you could make it work with a 4N26 and maybe you can't. Unless you know just what you're switching and how it's connected inside, you're better off substituting one switch closure (the relay) for another (the PTT switch). You can get a smaller, quieter relay by using a reed relay. Radio Shack sells a nice 5V coil reed relay that you can just substitute for your existing relay.

Carlinux
09-09-2008, 10:05 AM
Thanks Mike !

So with the small reed relay (275-232 and/or 275-233) do I need a transistor or I can connect direct to the BS2 (see the attachement) ?

275-232 : www.radioshack.com/product/index.jsp?productId=2062478&cp= (http://www.radioshack.com/product/index.jsp?productId=2062478&cp=)
275-233 : www.radioshack.com/product/index.jsp?productId=2062479&cp= (http://www.radioshack.com/product/index.jsp?productId=2062479&cp=)

Thanks,
Carl

Mike Green
09-09-2008, 10:54 AM
If you use the 5V relay (#275-232) you should be able to connect it directly to a BS2 pin. You'll still need the reverse connected diode across the coil. You would not use a series resistor. The relay coil current is about 10mA which is well within the capability of a Stamp I/O pin as long as you don't try to drive too many at once (1-4 ok). I would connect the other end of the coil to +5V and use LOW to turn the relay on.

Carlinux
09-09-2008, 04:24 PM
Mike,


Posted 8/28/2008 11:48 AM Mike said...
Most reed relays are fast. They were originally developed for use in telephone exchanges. You will need to look at the datasheet for the specific reed relay you want to use to find the on and off switching times. You probably want a 5V coil with a coil current of 20mA or less. That would involve a coil resistance of 250 Ohms or more. These are readily available. You will also need a diode like a 1N4001 or 1N4148. You would connect one end of the relay coil to a Stamp I/O pin and the other end to ground (Vss). You connect the cathode (white or black banded) end of the diode to the Stamp I/O pin and the other (anode) end of the diode to ground (Vss). A HIGH statement will turn the relay on and a LOW statement will turn it off. At the beginning of your program, put a LOW statement to make sure the relay is off.


I think I'll try it like you said in that post. Set the pin to low at the beginning and then high to turn on the relay.

But what exactly is the role of the reverse connected diode across the coil ? If I use a Crydom lc241 (attached file) do I need a diode too ?

Thanks,
Carl

Mike Green
09-09-2008, 07:57 PM
This is a SSR (Solid State Relay) designed specifically for controlling AC power. It won't work in your situation, but there's no relay coil, so you don't need the diode.

With any inductance like a relay or solenoid or motor, when you switch off the current, the magnetic field collapses and, as it cuts across the coil winding, generates a reverse voltage that can become higher than the original activation voltage for the device. This can destroy the switching device if not dissipated. That's the function of the reverse connected diode. It short circuits the reverse voltage so it's dissipated in the resistance of the winding and that of the diode.

Carlinux
09-10-2008, 04:24 AM
Mike, thank you for your explanation on the diode !

I'm heading to radio-shack today to get the reed relay.

Another question off topic: I live in northern California (Tracy) and I'm looking for a good electronics store. The only place I know it's Fry's but they don't carry a lot of electronics components. Do you know a good place in the Bay Area or between Sacramanto and San Jose ?

Thanks,
Carl