Transistor base current

A while ago I posted on here for some help with my circuit design. I received some great advice which thoroughly improved the original.

I made the changes to the breadboard design and the circuit ran well.

Then, on one test I left the relay activated for a few minutes by leaving sw1 closed. After a while I found the 2N2222 began to get hot. I thought maybe the resistors R5 4.7k and R6 1k are stopping the transistor being saturated, so I swapped the values round making R5 1k and R6 4.7k. When I ran the circuit again I saw the voltage drop across the transistor was less and it did not get warm at all.

The relays coil resistance is 100ohm and its a normal 12v relay which draws 120mA. The 2N2222 is rated for 800mA so there should be headroom.

Could someone advise if I have the right base resistor setup? The original idea behind the potential divider R5 and R6 was to limit the voltage into the base, as I read in the datasheet the maximum emitter base voltage was 5V. I think I'm misunderstanding something here.

This is as the circuit stands with the resistors in the swapped position so the 2N2222 runs cool.

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Answers 1

  • The AS358 is not a rail-to-rail output; the maximum output is VCC-1.5. Some back of the envelope calculations with a VBE of 1.2V and hFE of 50 indicates that a base resistance of 4.7k is too high for the 120mA coil. The way you have it with 1k is fine. You should expect about a 7 degree Celsius rise in temperature assuming your 2N2222 is in a TO-92 package.

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