using LM3915 to switch relay or motor

Question

I am trying to switch either a 12v motor or a relay with the outputs of a LM3915.

I have found an answer on this site about using a pnp transistor on the outputs to switch multiple leds. the circuit - such as this one:

works for leds but doesnt seem to make a motor spin, the 12v motor or relay just makes a tone.

Is it possible to use this circuit to work with motors or relays?

Any pointers in the right direction much appreciated. Thanks

Depends more on the choice of transistor than the choice of driver. — Hearth, May 22 '22 at 18:10
What output current did you configure for LM3915? What is your R1 and R2? Prefer P-Mosfet here — Jens, May 22 '22 at 21:16

score 1 · Answer 1 · answered May 22 '22 at 13:23

Not exactly

Rules of Thumb for Design

BJT switches are optimized with about 10% of hFE max as a saturated switch
coils will flyback back EMF voltage need some clamp protection V=LdI/dt. RC, R or R+Diode or Zener + diode.
Small relays can use signal diodes rated for 10% of DC current to meet SOA on datasheet with lots of margin
Measure Motor DC resistance DCR to define surge current. It will be 10x the full load (rated) current at full speed and 100x the no-load current +/-20%
- then choose a transistor that can handle this current with margin and not overheat.
- then choose base current to be 10% of hFE max based on Ohm's Law with worst-case supply tolerances
- check that PD in transistor does not exceed 50% of max power spec to achieve a 50'C temp rise on the case ( and more inside)
- It is OK to slightly exceed uC current since VoH is not a logic "1" level.
- Be sure your supply can handle the 10x surge, otherwise ramp up PWM at some frequency to limit current in x seconds.

1 Answers1