# Need 650mA from 20mA - Which Transistor?

Hey Everyone,

I'm creating a DIY electronic lock with a solenoid lock. I'm using a NAND logic chip to control the state of the lock and the logic chip only outputs a maximum of 20mA current.

My power source outputs 12V with a max current of 2A. The solenoid lock requires 12V and 650mA. What transistor will when applied a 20mA current at the base will allow 12V and 650mA of current to flow from collector to emitter.

Any help would be appreciated. I'm finding it difficult to figure out all these things from the datasheet and tutorials on transistors. Cheers.

Disclaimer: I'm no expert, but....

... my life got a lot easier when I discovered logic level mosfets. Apply 5V to gate and it turns on. No fannying about with currents and gains and other cr@p.

The ones I have happen to be these IRL520, but these IRLZ44 have a much lower "on" resistance (an order of magnitude better).

Yes, whatever works I'll take it! Thanks for these suggestions.

Have a look here for the resistors it needs. Without the pulldown it wont switch off.

Make sure whatever you get is logic level, else it will expect say 10V on the gate (which is mosfet-speak for bjt-speak's base), and won't switch with 5V, hence the name "logic level".

And look for low values of RDS(on), which is its resistance from drain to source (think collector to emitter of npn bjt). It's that resistance that causes the current to convert to heat, so lower the better.

Yes. Use a MOSFET. I like the IRF3708, because it can switch high currents even with 3.3V on the base.

For completeness: If you want to switch higher currents with a BJT, you are looking for a Darlington pair like the TIP120. But I hardly ever find a reason to prefer it to a MOSFET.

ElCaron:
Yes. Use a MOSFET. I like the IRF3708, because it can switch high currents even with 3.3V on the base.

So they evidently stopped the naming convention of IRL for logic level?

As far as I know, they call true logic level MOSFET the one with Vgs of 1-2 V.

No need to use a mosfet for this.
A TIP120 darlington with 1k base resistor (~3.5mA from a 5volt pin) would also work for a 12volt/650mA lock.
There should be 11-11.5 volt left for the solenoid on a 12volt supply, but that's still plenty.
Don't forget the (1N4004) kickback diode across the solenoid.
Leo..

Wawa:
No need to use a mosfet for this.

Well, what is the cost of using the MOSFET in a hobbyist project? The IRF3708 costs 25ct from China. The TIP120 costs 10ct from China, but the I have to have additional TIP120 lying around. With the MOSFET, I also don't dissipate 0.3-0.6W at the transistor (your estimation).

And yes, I noticed the lack of L too, but according to the datasheet, it works much better than most TO-220 L MOSFETs with 3.3V.

Wawa:
No need to use a mosfet for this.

It was me who raised the issue, and I never said a mosfet was required. I just said it makes (my) life easier, and if I keep some handy, they're perfect for hobby use.

As El Caron said, the fact they cost ~3x a bjt, is totally irrelevant. Sure, when making a gazillion of something, parts of cents become significant, but as a hobbyist if I have a drawer full of those I really don't mind tying up 100 units of money rather than 30-40. By the container load, sure, different story.

The best BJT I know as a saturated switch is the ZTX851, it won't struggle at all to provide 0.65A from 20mA drive, but its not the easiest device to source and not the cheapest. http://www.redrok.com/NPN_ZTX851_60V_5.0A_1.2W_Hfe75_TO-92.pdf

chem_eng:
As far as I know, they call true logic level MOSFET the one with Vgs of 1-2 V.

You mean VGS(th)?

That’s the level at edit : below which it’s guaranteed to switch off.