Diode or relay?

The first one is the right one for you as it has a 5V coil. I'm missing the coil resistance or coil current in the specs and I could not extract a manufaturer either. Only one of the comments stated the coil resistance to be 250 Ohm which means you need 20mA from the power supply. That should be acceptable.

The other is a 12V relay that does not fit in your application.

petrus bitbyter

I dropped in at a RadiShack and checked the specs on the package of the

Specifications: Nornally Open type Voltage: 5VDC Coil resistance: 250 ohms Contact rating: 0.5A at 125VACC Nominal current: 20mA

I guess these specs are OK then, right?

Joe

Yes, they are. (The specs I expect to be available when I'm looking for a relay that suits my requirements. So Radio Shack has something to do to improve their webpages.)

petrus bitbyter

Thanks, I'm going to get it then. I agree that their web pages could use some make-over.

Well, I tried Jamie's solution but that didn't work. So I'm going to try yours as soon as I get a different input terminal block for my circuit board because your scheme uses 3 inputs. BTW, with my limited understanding of circuits, I can't imagine the purpose of that diode there. It seems that its cathode will always be more positive than its anode, so no current will flow through it. Would you explain its purpose?

Well, that diode serves to protect the transistor. When the transistor is switched on, a rapidly growing current flows through it and through the relay coil, building up a magnetic field that activates the contact. After some time the current stabilizes due to the resistance of the coil. Now when the transistor is switched off, the current flow through it stops but the current through the coil cannot stop immediately. The collapse of the magnetic field induces a high voltage on the collector of the transistor that can be several times the voltage of the power supply. That voltage can easily blow the transistor. With the diode in place, the voltage cannot rise but a little bit above the power supply voltage. Then the diode starts conducting, effectively shorting the coil and the current flows harmless back into the power supply.

petrus bitbyter

Wow! Thanks for that very useful tutorial. You have a gift of explaining the complex even I can understand.

You're going to a lot of trouble based on guesswork. What is the problem you can't tell us the make and model of the " new X10 burglar interface device " ???

can't tell us the make and model of the " new X10 burglar interface device " ???

Never mind. Bitbyter's solution works for me now.

There also is a second order effect that can cause grief: The diode delays the relay dropout time.