Driving a relay using 2n3904
 

Hello folks

I need to drive a relay using one of the BoB's output - the relay is the one similar to the one's we use on the proximity circuits (pcb mounted)

the resistance on the relay is like 30K, I have 2n3904's lying with me which I want to use to drive the relay

the schematic of the BoB is here http://www.sunrom.com/files/1117-schematic.pdf

I needed to know what value of current limiting resistor I should use on the base of the transistor.

also kindly post any formula to be used

RGDS
IRfan

some things I downloaded
 

here is some of the stuff i downloaded on the relay stuff

Irfan,

I used this information to construct a circuit to control relays from the PMDX-122 BOB. Works fine.

John,
Looks like your link is referenced to your e: drive on your local machine. :)

Thanks Heath. I fixed it, I hope.

Thanks Folks!

One of the problems with the 2N3904 transistor is that it is an NPN device . The base on an NPN transistor requires a positive voltage to turn on the transistor. A PNP transistor has a base that turns on the transistor when the voltage goes to ground. Many TTL outputs are ACTIVE LOW, meaning that they work best with PNP transistors. If the TTL output is ACTIVE HIGH, then it is best suited for use with NPN transistors.

Mike if you have seen the Bob layout - do you think it can drive the 3904

Ifran,

Most small general purpose transistors can easily handle a beta of 10X. That means if you want 30mA between the collector and emitter on the transistor, you have to have 3mA at the base. The circuit in the BOB layout shows that the collector of the output is 470 ohms, so you have roughly 10mA available to drive the base. Without an additional resistor the 3904 would saturate, meaning only that turn-off times would be a little longer than if an additional resistor were used to limit the current to the base.

Put a 1k resistor in series with the input to the base and see what happens. If the transistor always stays on, then replace the 1k resistor with a 2k resistor. When I'm playing with low cost, general purpose components, I rarely go to the trouble of calculating the ideal resistance, instead, I just substitute values until I get the expects results. It's much faster. The cost is minimal. If you blow out a $0.20 transistor, just put another transistor in the circuit and keep trying.

A very highly recommended designer showed me that starting with a reasonable guess and then using a "clicker box" (a box that had lots of internal resistors that could be selected, one at a time, by turning the "clicker switch") was much more cost effective than spending a lot of time computing the ideal resistance. Every transistor varies a little from every other transistor. Find a value that works with several transistors and you'll probably be just fine. Just use a "plug board" so that you can plug in the components, one after the other, without soldering.