Digital pin output voltage drops when driving a relay

Question

I'm trying to trigger eight different 5 V relays using an Arduino Atmega 2560. The relays (70Ω coil) are powered by an external 5 V source, and I'm trying to trigger them through a driving stage which contains for each relay an NPN transistor (2N3904) and a 1 kΩ resistor directly connected to digital pin of the Arduino.

The relays in my application are supposed to be driven 2 by 2, not all of them at once.

My problem is that I don't succeed in triggering the relay with digital pins of my Arduino; once I connect it to the resistor, the voltage of the digital pins drops from 5 V to 0.4 V or less. The transistor needs only 7 mA to be triggered, the output current of the Atmega2560 pins is 20 mA max.

I tried with an external 5 V source, it triggers correctly showing 10 mA maximum of current through the driving stage.

Answer 1

Your circuit will no longer work. The original transistor you used was a NPN, you have changed it to a PNP connected somehow as a low side driver. It is important to show the emitter, as it tells us what type of transistor it is, leaving it out as you have done causes many of us to assume you have done it correctly ie the emitter is connected to the ground. Here is a Copy of a circuit from electronics-tutorials that uses a simple MOSFET: https://www.electronics-tutorials.ws/blog/relay-switch-circuit.html Note the 10K resistor from the port pin to ground, that guarantees it will be off during reset and setup.

Answer 2

If the coil is 70 ohms and it's driven by 5V, it should draw about 5/70 = 71 mA. That's pushing the 2n3904 a bit hard, and doesn't even account for inductive effects. (It's a small-signal transistor.) Do you have an NPN power transistor to try, just as a test? If its current gain (beta) is too low, use a Darlington configuration by combining it with your 2n3904. (2n3904 base from digital out resistor, emitter drives power NPN base directly, both collectors are connected together to the bottom of the coil, and the power NPN emitter is the effective emitter of the just-created "super transistor" with high beta and power handling.) Be sure to use an anti-kickback rectifier (1n4001)across the relay coil.

Answer 3

Good Job, you are right on, the transistor is rated at 200 mA which is adequate for the job. The flyback diode will protect your system and the transistor. the drain source drop would be maybe 0.01V on a bad day depending on the RDSon. You will probably need a little lower value base resistor depending on the gain of your transistor. I would use something between 220 and 510 ohm. It should work fine. Using a darlington would hurt you instead of the ~.7 volts drop across the collector it would become about 1.4 volts, leaving the relay with 3.6V assuming you are actually getting 5V. My preference would be a small logic level avalanche rated MOSFET. Use a 50 Ohm resistor in the gate circuit and it will be just fine. Use something like a 10K pull down resistor between the gate and source, that will guarantee that it will be off during reset.