evilkitty
Posts: 370
Joined: Tue Apr 15, 2014 11:39 pm

Base resistor value

Sun Feb 14, 2016 9:46 pm

I am planning on using a 2N3906 and a 2n2222a transistor
Image
i don't know what size resistor i should use
i and read and watched video on how to calculate it but keep getting confused with the terminology

i would just use a single tip120 and some random high value resistor but i don't want to redo more wiring than necessary
my load is 35cm of cool white smd 3528 led tape (3 leds every 5cm)

i was very new at this kinda thing and used a relay board the 1st time
My Pi Server: http://imgur.com/a/6xIUI | Thermostat: http://imgur.com/a/4LVnT

User avatar
Tage
Posts: 287
Joined: Fri May 24, 2013 2:29 am
Location: St Thomas, Ontario Canada

Re: Base resistor value

Mon Feb 15, 2016 3:59 pm

the 2N3906 is too small to drive this load, it is rated at 200mA max.
you could try to drive the load with the 2N2222A instead, but to avoid that the transistor heats up you have to provide at least 50mA into the base. This is a bit too much for the GPIO output, so you may need a second transistor to drive the base of the 2N2222A. the 2N3906 can be used. connect the base to the GPIO with 1k resistor, the emitter to 3.3V and the collector to the base of the 2N2222A through a 50 ohm resistor. The values are not critical but you need enough base current into the npn transistor that is driving the load to keep its collector-emitter voltage from rising, as it does not take much to overheat it. Voltage drop multiplied by load current must not be more than 0.5W so you cannot afford more than 1.3V voltage drop in the 2N2222A.
the best thing would be to use an n-channel MOSFET that has Rdson specified at 2.5V. unfortunateley you can only buy these in surface mount packages, so it takes some soldering.
evilkitty wrote:I am planning on using a 2N3906 and a 2n2222a transistor
Image
i don't know what size resistor i should use
i and read and watched video on how to calculate it but keep getting confused with the terminology

i would just use a single tip120 and some random high value resistor but i don't want to redo more wiring than necessary
my load is 35cm of cool white smd 3528 led tape (3 leds every 5cm)

i was very new at this kinda thing and used a relay board the 1st time

evilkitty
Posts: 370
Joined: Tue Apr 15, 2014 11:39 pm

Re: Base resistor value

Mon Feb 15, 2016 11:34 pm

since those 2n3906s are the only onhand pnp i have, ill just redo some wiring and use a tip120, and recommend resistor value or should i just use a 10k-ish one
thanks
i assume a tip120 can handle that without a heat sink
https://www.fairchildsemi.com/datasheets/TI/TIP120.pdf
My Pi Server: http://imgur.com/a/6xIUI | Thermostat: http://imgur.com/a/4LVnT

User avatar
Tage
Posts: 287
Joined: Fri May 24, 2013 2:29 am
Location: St Thomas, Ontario Canada

Re: Base resistor value

Tue Feb 16, 2016 4:31 am

you could use the TIP120, at 360mA load it should work fine with a base resistor of 1k to 2k when driven from 3.3V
evilkitty wrote:since those 2n3906s are the only onhand pnp i have, ill just redo some wiring and use a tip120, and recommend resistor value or should i just use a 10k-ish one
thanks
i assume a tip120 can handle that without a heat sink
https://www.fairchildsemi.com/datasheets/TI/TIP120.pdf

evilkitty
Posts: 370
Joined: Tue Apr 15, 2014 11:39 pm

Re: Base resistor value

Tue Feb 16, 2016 11:44 pm

Thanks, ill just use a couple 3k resistors in parallel, next best thing i have is 1k or 2.2k
My Pi Server: http://imgur.com/a/6xIUI | Thermostat: http://imgur.com/a/4LVnT

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