Drive BJT via Esp32 DAC

Question

I've an ESP32 with a DAC pin that outputs a variable voltage between 0 and 3.3 V.

I need to drive a 12v motor using only a NPN power BJT like mj802g (or BUTW92, mj1403, NTE29, BUV21G, ...) powered typically at 12v (+ or - 1 volt)

My requirements is that motors is stopped at 0v, 50% of power at esp32 1.65 out, 100% of power at esp32 3.3v out: my idea is that motor's power is linear with esp32 output

What is the best way to achieve this ? I know that the power dissipation of the BJT will be a problem in this configuration, but I don't want any pwm drive or mosfet for now.

Answer 1

You didn't specify the motor, but here is a general idea of a circuit that uses an auxiliary 24V supply and drives the BJT with a TIP120 darlington. The motor is represented by the 1Ω resistor so the maximum current would be 12A (of course you can't get all the way there because the BJT has some voltage drop).

The purpose here is to show you some idea of the power dissipation in the various components:

The top (violet) trace in the top plot pane shows you the motor voltage, which tops out at around 11V.

The gray trace in the middle plot plane shows you the current from the 24V supply, which is maximum at a bit more than 2A

The bottom pane shoes you the dissipation in the various power components.

The blue trace is the pass transistor dissipation, which is maximum at 6V on the motor, as you would expect, at a bit less than 35W.

The red trace is the dissipation in R4 which is maximum at maximum output voltage- at more than 21W.

The green trace is approximately the dissipation in the TIP120 which peaks at at almost 9W.

All in all, a pretty ugly combination- requiring heat sinks and massive parts as well as an inconvenient 24V supply at substantial current.

A PWM solution in this power level could require minimal heat sinking.

BTW, a brushed DC motor will have no-load RPM proportional to ESP output with this kind of circuit, however there will be minimal braking since the circuit shown does not sink current.

Answer 2

As I understand it ESP32 does not have analog outputs, only PWM as such the most of the extra heat will go into the motor instead of the transistor.

My requirements is that motors is stopped at 0v, 50% of power at esp32 1.65 out, 100% of power at esp32 3.3v out: my idea is that motor's power is linear with esp32 output

Your requirement is unclear: motor power is dependant on the mechnical load.

But you should get something like what you want with the naiive connection of output pin to transistor base through a suitable current-limiting resistor.

^{simulate this circuit – Schematic created using CircuitLab}

However if you want direct speed control or torque control it the gets more complicated because then you need to sense or control the current or voltage going to the motor.