# How calculate and chose the right BJT that comands a relay

I was projecting a simple circuit that, from a digital OUTPUT from an Arduino zero (3.3 V) switch on a BJT that is connected to a relay.

The base of the BJT must have a current of about 5mA or less. Is this configuration enough to run a relay?

I have simulated this circuit and Ic is about 49mA; if I change the relay, Ic increases or it's fixed?

I'm not really good at this thing

simulate this circuit – Schematic created using CircuitLab

## 3 Answers

if I change the relay, Ic increases or it's fixed?

Different relays will draw different coil currents at their designed operating voltage. Check the datasheet for your relay.

50 mA is a reasonable number for a low power 5 V relay, but pretty low if your relay is rated for more than ~2 A switched current.

To maintain your 5 mA base current limit you might need to either use a Darlington configuration, or change from a BJT to a logic-level MOSFET.

A BJT can be considered a "current amplifier", therefore its gain is of interest. In your case (5mA/50mA) this gain is relatively low

A BJT can be considered a "current amplifier", therefore its gain is of interest. In your case (5mA/50mA) this gain is relatively low. For a given amplification, figure out the required base current. The base circuitry is then quite similar to a resistor-LED-circuit.

Note, that Ic is specific for the used relay (datasheet).

• Please don't double post. You can edit your previous answer any time by clicking "edit" below the answer text. Sep 5, 2019 at 16:40
• Sorry, that was an accident... Sep 5, 2019 at 16:47