[MOVED] Low Ib current NPN transitor for PIC, Ic=1A

[T3STY]

I have a heavy load to drive with a PIC and I want to use an NPN transitor with load on Collector.
I'm looking for a transistor that starts conducting with a very low current on base, low enough to be driven with a PIC (TTL out, 25mA max). I also need high current on Collector (around 1A).
To better explain the way I'm going to use it, here's the schematic:


Does somebody know any transistor I could use?

 

[betwixt]

Re: Low Ib current NPN transitor for PIC, Ic=1A

Ib = Ic/hfe so for 1 Amp collector current, the base current will be 1/hfe. In theory, any transistor with a collector current rating of at least 1 Amp and a hfe of at least 40 could be driven by the PIC. There are thousands of such devices on the market but not all may be avaialble locally to you. Please note that those a re MINIMUM ratings, higher ratings will do no harm and a transistor with higher hfe will need less current to drive it. If you want the smallest current, consider using a darlington transistor, they will typically switch 1 Amp with only 1mA of base current but beware that they tend to have a slightly higher voltage demand on their base pins and a poorer satuation voltage.

Brian.

 

[T3STY]

Re: Low Ib current NPN transitor for PIC, Ic=1A

I don't need really low current transistor like the darlington you suggested. I was going to use a BD435 transistor but then I looked at the datasheet and it says Ic=4A, Ib = 1A. I thought I needed 1A for making it switch. Math says the hfe for this transistor is 4 (still out of range for a PIC). In this case with 25mA on Ib I could get 100 mA on Ic. Is that right?
I'm now searching on the web for hfe=40 transistors, I'll let you know what I found ASAP.

EDIT
I found the BD135/137/139 transistor datasheet which should have hfe=40 (min), and the collector current Ic=1.5A. I can easily find this transistor in stores near me. If I well did the math it should be fine for my purpose. What do you think?

 

[sarge]

Re: Low Ib current NPN transitor for PIC, Ic=1A

I found the BD135/137/139 transistor datasheet which should have hfe=40 (min), and the collector current Ic=1.5A. I can easily find this transistor in stores near me. If I well did the math it should be fine for my purpose. What do you think?

Why not NMOS? If you need a switch it is better way to place N-channel MOS transistor (but don't forget about input capacitance and miller effect).

P.S. Why is this question is in analog IC and layout section ?

 

[betwixt]

Re: Low Ib current NPN transitor for PIC, Ic=1A

Those transistors should work OK. The idea of using a MOS transistor is a good one because the drive current requirement, at least at low frequencies is very much less than a bipolar transistor needs but you must be sure to use a type that can fully saturate with the gate voltage your PIc can supply.

Yes - this is in the wrong section of the Forum. I do not have privilege to move posts, perhaps one of the moderators could do it for us.

Brian.

 

[T3STY]

Re: Low Ib current NPN transitor for PIC, Ic=1A

Why not NMOS?

Because I have very little knowledge about BJT transistor, and absolutely no knowledge about NMOS (and other) type of transistors
I trust you when you're saying an NMOS would better suit my needs, but having no knowledge makes it very difficult to me, even for as a simple task as an electronic switch. I'll consider using it in any future upgrades of the project I'm working on, thanks for suggestion

P.S. Why is this question is in analog IC and layout section ?

Because I thought that was the better section for it, sorry if it's wrong.

So since BD135 looks fine for the task I'm done here. Thank you all for your time!

 

[godfreyl]

Re: Low Ib current NPN transitor for PIC, Ic=1A

For future reference, there's quite a nice transistor selection guide here: Small signal transistors

There's links at the top to "VAS" and "driver" transistors (medium power), and "output" transistors (high power).

BTW, the odd terminology is just because the lists were put together with audio power amplifier designers in mind. e.g. "VAS transistors" = transistors suitable for the Voltage Amplification Stage.

 

[T3STY]

That's a very nice and useful link! Thank you very much, godfreyl!

 

[khangzing]

what if my collector current is 50-75mA and which types of NPN should suitable for me?

 

[pplus]

what if my collector current is 50-75mA and which types of NPN should suitable for me?

Practically anyone with β(hfe)>3 :razz:

 

[khangzing]

hfe>3 mean more than 3 should be ok? what if this type:

then just control the Ib to get desire Ic?

**broken link removed**

 

[pplus]

Yes.

\[Ic=\beta*Ib\]

\[Ib=\frac{Uout-0.6}{Rb}\]

\[Rb=\frac{Uout-0.6}{Ib}\]

in that case

\[Rb\leq\frac{(Uout-0.6)*\beta}{Ic}\]

\[Rb\leq\frac{(5-0.6)*40}{0.075}=2.3K\]

 

[khangzing]

0.6 is the voltage drop? it is about 0.6 to 0.9 right? so choose the minimum 1? when i look at the datasheet, some mentioned min hfe 40 max 240, how to know and how to control the hfe?

and why the formula after "in this case" is divided by Ic but not Ib?

 

[T3STY]

Check my post in your previous thread:
https://www.edaboard.com/threads/290407/#post1247205

 

[pplus]

0.6 is the voltage drop? it is about 0.6 to 0.9 right? so choose the minimum 1?

Yes.See at images. This current-voltage characteristics of the diode

\[Ib=\frac{Ic }{\beta }=\frac{0.075 }{40}=1.9(mA)\]

The voltage drop of 0.9V implies a much larger current.

when i look at the datasheet, some mentioned min hfe 40 max 240, how to know and how to control the hfe?

and why the formula after "in this case" is divided by Ic but not Ib?

The hfe transistors have a large variation even within the series (40-240) so you need to orient to the worst case (40) and desired collector current.