0
\$\begingroup\$

I am developing an ATMEGA382 project meant to automate certain aquarium maintenance functions. I need to apply power to 12volt solenoids and pumps. I thought I would use an NPN transistor like a switch with a 3.3v base and 12v collector. I understand this is incorrect. I do not necessarily need high speed switching. What would be a better option? I looked at an IBGF transistor, but that seems to have the same issue I am already facing.

Should I just dump a bunch of 3.3v into both base and collector with a boost converter on the emitter?

Thanks!

\$\endgroup\$
  • 1
    \$\begingroup\$ MOSFETS would be better, or RELAYs if it's pretty low duty cycle and you need the isolation. \$\endgroup\$ – Trevor_G Jul 14 '17 at 19:47
1
\$\begingroup\$

Your description sounds like:

schematic

simulate this circuit – Schematic created using CircuitLab

In this circuit the transistor is an emitter follower. The emitter voltage will be 0.7 volts less than the base voltage.

You should instead do:

schematic

simulate this circuit

With this circuit the transistor will act as a switch, and will have about 0.2 volts between emitter and collector when the base is pulled up by the ATMega output.

\$\endgroup\$
0
\$\begingroup\$

You're going to want to look at some sort of isolated component. Relays are mechanically isolated - your 3.3V would drive the coil and the switch would handle the 12V side. Optoisolators are the solid state version of a relay (in the crude sense) - they use a light signal generated by your 3.3V to drive an optically sensitive phototransistor. For this application with low switching frequency, I think a relay is going to be the way to go, but you will most likely need a transistor to interface your controller logic level with the relay coil current, which is in the 100mA range - take this one as an example.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.