Electronic – Basic transistor wiring

Finding why a circuit doesn't work remotely and with incomplete information makes life a bit difficult. With any fault finding its a case of isolating where the fault is. I don't think using a different transistor is a problem - it has very similar specs to the 2N2222. My gut feeling is that the relay isn't switching using a 5V supply.

As this is a project in development lets make certain all the bits work at the correct voltages and currents.

Build a simple test circuit.

Is the relay suitable for 5V operation?

Relays come in all sorts switch arrangements and working coil voltages. You can't tell just by their size. Check the specs.

For example a suitable type for 5V operation is the NRP04 ( http://oomlout.co.uk/products/relay-5v-dpdt-with-extras)

Testing procedure

Close SW2. This should operate the relay and the LED should come on. If not the relay may be faulty OR its the wrong type (operating voltage) - either way its a problem.

If all works well the next problem to eliminate is the transistor drive.

With SW2 open, close SW1. This puts a small current into the base of the transistor. You can try different values up to 10K but nothing smaller than about 1K0 or the chip output may not be able to provide the current required.

If the gain of the transistor is too small the transistor may not be switching on fully. Try replacing the transistor with another (NPN) one and test again. If its still not working measure the voltage change across the transistor when SW1 is operated. It should come down from 5V to about 0.2V or less. If it doesn't change by this amount you need more gain so get a different transistor or try two of the 2N5550 transistors connected as a Darlington pair (gains multiply)

Your circuit should now work.

As a final test connect point A (R2 input) to the output pin of the decoder and common the grounds. Test if the output signal turns the LED on and off. (output pin should switch between 0 and 5V)

For switching purposes you should be using the saturation voltage guarantees rather than the DC current gain. For example, the 2N4401 is guaranteed at Ic/Ib = 10, so a base current of ~26mA will typically result in 220mV or so drop at 260mA (eyeballing the below plot).

The reason they say "pulsed" is that they are not accounting for the self-heating of the transistor. 26mA is quite a bit of base current, and the 220mV (typical!) drop will result in about 10 or 20 degrees C heating for the transistor (depending on the package).

If this is a hobbyist thing, you can probably get away with forced \$\beta\$ = 20-30 or so, so a base current of more like 9mA rather than the \$\beta\$ = 10 shown in the plot. I usually use \$\beta\$ = 20 at Ic <= 100mA, which might be a bit on the high side for a microcontroller to supply directly in your case (13mA of base current).

Edit: A "forced beta" of, say, 20 can be achieved as follows. Suppose that the collector current is limited the load resistance to say 200mA. No matter how well the transistor turns on it will never exceed about 200mA (even a dead short). You use a base resistor so the base current is 20mA. The ratio of Ic/Ib is now 20, and we can call that a "forced beta". The gain of the transistor at 200mA would be more like 150, but that's only when Vce is high enough that the gain determines the collector current. The datasheet specifies it at Vce = 5V, which would be a disaster for a switching application.

You can "force" the ratio of Ic/Ib to be as low as you like, but the upper limit is constrained by the transistor characteristics. When you're designing, ideally you'd like to eliminate the exact transistor characteristics, so that the circuit functions provided the transistor meets the minimum/maximum guarantees. The gain should never determine the collector current in a saturated switching application.

A small MOSFET with logic-level gate might be a better choice when you get much above about 100mA. As you can see, the performance starts to go a bit sour above approximately that current, even on the 2N4401.