Experiment - Relay Driver

I am *so* sick and tired of the idiots that 'design' a circuit like this one

that use such a high resistance in the base that it's literally _impossible_ for it to work, especially with a 2N3904. So I decided to try an experiment with both the 2N4401 and the 2N3904, and instead of a relay, use a 100 ohm resistor and a 10VDC source. The usual 12V relay has a coil resistance of 120 to 150 ohms, so it needs about 100 mA at

12V to work at its optimum. Obviously it might work with less, but that's not where it's at.

So I put a 100 ohm 1W resistor as the collector load, and 10VDC, and then a 20k pot with 1k in series to limit the maximum base current. My objective is to get most of the supply voltae to drop across the load resistor with 0.2V or less across the transistor C to E.

I was sadly disappointed by the 2N3904. I had to crank the base current up to 5.75 mA to get the Vce to .2V. Getting the 2N3904 to take 100 mA is really pushing it. The beta or current gain drops off drastically at that current, in this case, the gain is only 17. something. What I'm getting at is that this is a very poor choice for a relay driver transistor, and you couldn't use it in the circuit in that URL above, because there would be only a volt or two across the relay, and the rest across the transistor.

So I put the much heftier 2N4401 in the circuit and did the measurements again. I adjusted the pot for .2V Vce, and it took only .93 mA base current to get the 100 mA Ic. This is a gain of over a hundred at that current, six times better than the puny 2N3904, and might be almost good enough to get the circuit to work in that URL. But that circuit really should have had a base resistor of 4.7k or less instead of 100k; with the 100k, it will never work with either transistor, there's just not enough base current. As a matter of fact, the 741 output can't go close to the negative rail, so it really needs a couple diodes in series with the output to drop a volt or two. A red LED works well, too. Better yet, replace the 741 with an opamp that go to the negative rail.

So don't use a puny transistor pushed to its maximum to drive a relay. Use the right transistor with the adequate amount of base current to get it to drive the relay adequately. The last thing you want is a half pulled in relay.

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