Transistor Base Resistor

geneshults

Hello to All

How does one size the Base Resistor for a switching Transistor?
I have read the Nuts and Volts of Basic Stamp Column #6, Silicon Steroids for the Stamp.
They have referred to three different switching components.
2N2222 RB of 390 ohms
ZTX689B RB of 390 ohms
ZTX605 RB of 3.3k ohms
The project I'm working on requires a higher amp draw than the listed components.
I'm looking at a 5 amp draw and was wondering how does one determine the base resistor needed?
Does anyone have a suggestion on a component this will be suitable at a 5 amp draw?

Leon

What is the supply voltage?

stamptrol

Every transistor has a datasheet which explains its parameters. Just put the transistor part number into Google.

For instance you say you want a 5 amp "draw", which is the Ic (collector current) in the datasheet. You'll need a transistor with a 5 amp or greater Ic.

Also in the datasheet, you will find the maximum base current (Ib) or a graph showing how much base current is needed to allow a certain amount of Ic to flow.

If we assume a NPN transistor in the traditional arrangement (emitter at 0v ), the base current is controlled by the series base resistor. The base current can be calculated by: (Vbase_supply - 0.7)/ Rbase.

If your Vbase_supply is from a microcontroller pin at 5 volts, your maximum base current will be limited to 15 - 20 mA.

Cheers,

Dr_Acula

Another very useful value in the datasheet is the Hfe - which is the gain. For instance, a 2n2222 has a gain of about 100. So if you put 1ma into the base, you can switch 100mA. Of course, you can't put 1A into the base and switch 100A because that exceeds other specifications.

5A is more than a signal transistor can switch.

You can do this in two stages, using one transistor to amplify (say) 1mA to 100mA, and run that current into a power transistor with a gain of 50 to turn 100mA into 5A. If you do that, it works out easier to swap the type of transistor with each stage, ie NPN switches PNP which switches NPN.

Or you can use a darlington transistor where the two transistors are in one package.

Or you could use a mosfet, which can do this with one device. There are mosfets that can be driven with 5V on the gate and can switch 60A.

Or you can use a relay.

Or you can even chain relays. I have a microcontroller with 1mA on the output which drives a small signal transistor, which then drives a relay which then drives a contactor relay which drives a pump. 5mW controls 3600W which is a gain of 720000. That would not be possible with one device but you can do it with 3 in a row.

What are you wanting to switch?

Beau Schwabe

Check this application note that I put together... Even though it references a motor in the schematic, a Relay, Light bulb, etc. could just as easily be substituted.

Hope this helps: Don't get HOT choosing the wrong transistor

tonyp12

This mosfet looks like a good contender:
up to 30amp with a low 3.15v on the gate

http://www.mouser.com/Search/Refine.aspx?Keyword=NTD5865NL
http://www.onsemi.com/pub_link/Collateral/NTD5865NL-D.PDF

if you want something smaller (smt) 7amp at 3v gate
http://www.mouser.com/ProductDetail/International-Rectifier/IRLML0030TRPBF/?qs=sGAEpiMZZMvECErq9cesgNhJTPOXHpv3%252be4oxUUNV%2fQ%3d

These are low side N channel, you should put the motor/lamp/solenoid above it going in to drain pin and source pin connected to Gnd.
If plan is to be left on continuous design with large copper areas as heat sink.
for over 5 amp i would also add these plus large copper areas
http://www.mouser.com/ProductDetail/Wakefield/217-36CT6/?qs=sGAEpiMZZMttgyDkZ5Wiuh6vt2S17U8%2f%2fPVea3mahwg%3d

If while prop is booting up and it's important that mosfet stays off, connect at 47k ohm resistor from gate to gnd.

geneshults

Thanks To All...
You all have been most helpful. In answering my question to the point. :thumb: