Transistor Troubles!

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calvinsykes

Member
I'm trying to drive a BJT using a 5v signal supplied by a PICAXE. The transistor is switching a voltage of 6-8.4V, and I need as close to that Vcc on the output as is possible (this must drive an IRF530 to saturation, so needs to be as high as possible).

Would appreciate some hints on the best circuit design and type of transistor to use.
 
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Dippy

Moderator
Why the 10k? If it's there to prevent floating then it should go from base to ground (0v). As it is it will cause some current to flow through BJT which may not be desirable. Or is there a reason?

I'd have a larger res value to base to limit base current.
You may remove the 100R gate res if pressed for space, though you may have a reason for it being there (?).

Looks like it'll work. It won't allow mega fast PWMing if that's what you're after.

And, as you realise, as your 9V (6 - 8.4V) supply droops then the IRF Rds will increase. A look at the MOSFET data sheet will show you how much. Is it significant with your required load?
 
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calvinsykes

Member
Regarding the 10k, this switch is active low ie when the PICAXE o/p is off then the MOSFET will switch on. The 10K was just to hold the lie high when the PICAXE initialises.
I added the other 100R as a matter of habit, but I guess it's not as necessary for a FET. This will be driving a motor (actually as part of an H-bridge) but just simple on/off; no PWM. the load will be about 8A max.; doesn't seem like it will be a major problem.
 
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