Help me understand NPN transistors, circuit does not work

Question

I'm working on a project where I need a transistor. I want a Attiny85 mcu to turn of and on a relay. But the current in that chip is to low so I figured out I have to use a 2N2222 NPN transistor as a switch so the relay could draw current from the source and not thru the mcu.

I'm using the tool over at http://www.falstad.com/circuit to help me set up the circuit, but I'm having some problems.

To understand NPN transistors I've tried to take the example at falstad and update it to my settings, using 5V for power, adding a switch. But it does not work. I really want to test it before I get to solder it together so the tool was really nice.

It won't work as you want without a relay coil. They're not telepathic, they need coil current to operate. — Andy aka, Jul 20 '17 at 07:51
Related: https://electronics.stackexchange.com/a/25676/35022 — MarkU, Jul 20 '17 at 07:53
Possible duplicate of Driving a relay directly with a microcontroller? — Harry Svensson, Jul 20 '17 at 08:09
Very basic rule - to turn on an NPN raise the gate voltage more than 0.6V above the emitter voltage, usually through a resistor like you already have. BTW, in the above diagram you would burn it out instantly. Put the load in the collector side — Dirk Bruere, Jul 20 '17 at 08:49
Also, do not forget the protection diode across the relay coil — Dirk Bruere, Jul 20 '17 at 08:50

score 2 · Answer 1 · answered Jul 20 '17 at 08:07

You've made a mistake when using your falstad simulation.

In the top right corner is a 'Run/Stop' box. What I suspect has happened is that you've taken the NPN transistor example. Changed the voltage to 5V without adding a base resistor first, exceeded the max current which has caused the simulation to stop.

If you press the 'Run/Stop' button again now that you've added your base resistor then it should simulate correctly. As a general rule to remember, if there are no yellow dots (which represent current flow) then your simulation has most likely stopped.

Edit: Just for completeness, here's the current flowing through the transistor with no voltage on the base, notice how the current values are the same as your image?

Help me understand NPN transistors, circuit does not work

1 Answers1