# How to increase the current?

1. Jul 27, 2012

### medosoft

How to increase the current?!!!

I'm designing a drive circuit for a 12V DC motor
I'm using a (2N3055 BJT) as a switch to control the output, and I'm using a (74121 monostable oneshot) to control the BJT switch.
So, the output of the 74121 is fed into the base of the BJT.

Here is the schematics for my circuit
http://img594.imageshack.us/img594/7411/etpcircuit.jpg [Broken]

According to the datasheet of the BJT, β=100. Which means that the current supplied to the base should be at least 1/100 of the current that is supposed to run through the collector. The motor needs about (2-3) Amps to run at a proper speed, meaning that the base current should be at least (20 - 30 mA).
The problem is that the output current of the 74121 ranges from (4 - 16 mA).

Is there anyway to increase the current?
I can sacrifice some of the voltage

Last edited by a moderator: May 6, 2017
2. Jul 27, 2012

### uart

Re: How to increase the current?!!!

You could add a base drive circuit like in the attachment.

Or you could just replace the 2n3055A with a suitable n channel logic level mosfet and you wouldn't need any auxiliary drive components.

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• ###### etpcircuit1.jpg
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Last edited: Jul 27, 2012
3. Jul 27, 2012

### gnurf

4. Jul 27, 2012

### jim hardy

Re: How to increase the current?!!!

indeed the good ol' 3055 is not the greatest for current gain. hfe may drop to more like 10 in the Ic=amperes range.

TTL sinks better than it sources so your design is needing a tweak or two

uart's MOSFET is one solution, so is his darlington

a 555 in between the '121 and the '3055 is another, but the 555 inverts so use the '121's Qnot output.
Tie TRIG and THRESH together and 555 becomes a schmitt trigger buffer with substantial drive capability. That's a handy trick .

glad you're experimenting , it's the best teacher.

old jim

Last edited: Jul 27, 2012