0
\$\begingroup\$

I have a 12v led strip and have tested the current draw at ~500mA per colour.

I want to drive this via my Wemos GPIOs which are 3.3V @ 12mA max.

For testing I have been using 2N2222 transistors with 330Ω resistors and it seems to work well, in that the current draw is similar as via 12V directly. Ideally I should aim for 12mA base current right, so 220Ω resistor?

Although measuring the base current only gives 4mA which is less than I would have expected given (3.3 - 0.7)/330 = ~8mA, any explanation for this?

Also with a transistor gain of 500/8 = 62.5 (or 500/4 = 125?) I think the transistor is not fully switched on, and it does start to get hot quite quickly so this is probably true.

I suspect that gain is too high so I need to cascade 2 transistors to increase the gain in two steps. Like 5mA -> 50mA -> 500mA. So it would be something like GPIO 3.3V via 510Ω to give (3.3 - 0.7)/510 = 5mA, then 12V via 220Ω to give (12 - 0.7)/220 = 51mA

I also have some IRL540N mosfets which I tried but these don't fully switch on at 3.3v so would need a transistor to switch these anyway.

edit: Looking back at my original order for these transistors and these are what I bought https://www.aliexpress.com/item/2N2222A-Free-shipping-100pcs-in-line-triode-transistor-NPN-switching-transistors-TO-92-0-6A-30V/32816748799.html they say 2N2222A on the packaging but are not the old style metal case ones, so not entirely sure what they are now MSP2222 maybe?

This is what I had in mind for the circuit

\$\endgroup\$
4
  • 1
    \$\begingroup\$ At half voltage and half current --- 6v and 0.25 amp---- any 2N2222 wiii burn up. Unless you use duty cycle to modulate the average intensity. \$\endgroup\$ Mar 25, 2019 at 3:43
  • \$\begingroup\$ What do you mean by half voltage, half current, half of what? \$\endgroup\$
    – trapper
    Mar 25, 2019 at 4:18
  • \$\begingroup\$ For a switch (or linear element, such as bipolar wires as linear amplifier) in series with a resistive load (we'll assume the LEDs behave like this), the maximum power dissipation is at half power and half voltage. \$\endgroup\$ Mar 25, 2019 at 10:49
  • \$\begingroup\$ I don't understand where you got 6V & 0.25A from though, half of what exactly? \$\endgroup\$
    – trapper
    Mar 25, 2019 at 10:58

1 Answer 1

1
\$\begingroup\$

I would highly recommend using a mosfet rather than a transistor to drive LED lights from a microcontroller such as a Wemos D1. The transistors I've tried get hot when conducting that level of power. Use a RFP30N06LE (available from Amazon for less than $1) instead for much better results as it can handle 30A@60V. Please see my blog post on this subject for further details.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

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