Using 2 transistors as relay

GopherT

Joined Nov 23, 2012
8,012
Hi
Will try this instead of Relay.
Can it be done, and what value for resistor ?
Your circuit will work in concept. The BC337 can carry up to 500 mA (keep it below 300 mA to be safe (R = 39 or more). The problem is that your TIP42 can only carry 6 amps for a continuous current (10amps pulse with short duty cycle). Also a problem with TIP42 as a switch means the gain is too low. The TIP42 has only 15x gain when over 3 amps. That means you need base current of one amp or more. Most people would suggest even higher for a fast switch.

For a 10 amp motor, you would be best off with a P-channel mosfet. MOSFETS operate differently and are commonly used as a relay replacement. And they generate much less heat.

Use something like a irf9540. Or something more modern. You can drop it right into the drawing I posted below to replace the PNP. It will not work in your drawing.

This is more typical.

image.jpg
 
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GS3

Joined Sep 21, 2007
408
TIP42 will not stand 10 A collector current. And 100 ohm base resistor might be too high for a minimum gain of 15 or less at high collector current.
 

Thread Starter

FroceMaster

Joined Jan 28, 2012
544
i will not fried anything, so i need to know if this setup will Work for output up to 16A 12-15V
max drawn current at input must be 10 mA
 

Thread Starter

FroceMaster

Joined Jan 28, 2012
544
everything is possible, have asked here before, but no help.
Need help to calculate the value of resistors and help chosing the right transistor and mosfet.
only thing, to-92 or similar to transistor and TO-220 to mosfet, and do i need cooling ?

the input is a constant 12-15v not a pulse.
 

GS3

Joined Sep 21, 2007
408
i will not fried anything, so i need to know if this setup will Work for output up to 16A 12-15V max drawn current at input must be 10 mA
I think you will find that people are more willing to help those who are trying to learn than those who just want the work done for them. This is very basic stuff about transistors, gain, etc.
 

GopherT

Joined Nov 23, 2012
8,012
everything is possible, have asked here before, but no help.
Need help to calculate the value of resistors and help chosing the right transistor and mosfet.
only thing, to-92 or similar to transistor and TO-220 to mosfet, and do i need cooling ?

the input is a constant 12-15v not a pulse.
The BC337 is a TO-92 package in my post #8, the MOSFET is a TO-220 and the best option I see is
http://www.st.com/web/en/resource/technical/document/datasheet/CD00002632.pdf
As the P-channel. It can handle 80 amps and should not need cooling (heat sink) at 15 amps unless you are pulsing it (PwM).

Good luck
 

Roderick Young

Joined Feb 22, 2015
408
If you can afford to switch the negative terminal of your load, may I suggest
4110.gif
  • single TO-220 device
  • won't need a heat sink at 10A, if not switched quickly
  • can be driven directly with 12-15 volts (but don't exceed 20)
  • static current draw on input essentially zero (much less than 1 uA)
  • will withstand transients of 100V on the drain (if this is an automotive application)
 
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