Author Topic: transistor confusion  (Read 515 times)

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Offline jghostzTopic starter

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transistor confusion
« on: July 26, 2012, 07:45:58 PM »
I am completely new to robotics and am working on my first one.  When I post this I already know people all over are going to want to slap me for this but here goes.  I am building an h-bridge for my robot which is going on the frame to an old rc car.  When I was messing around with the transistors i found that no matter what i had hooked up through the collector the output on the emitter is just slightly lower than my base even if nothing is hooked up to the collector.  is this normal and if so then what is the purpose of them as i could just hook my base to my emitter and get slightly stronger results.  I am sure I am doing something wrong and after searching and searching I can't seem to find an answer to this question.  I am primarily messing around with npn transistors (2n4401, mps 2907a, mps 2222a)
« Last Edit: July 26, 2012, 07:51:12 PM by jghostz »

Offline Soeren

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Re: transistor confusion
« Reply #1 on: July 26, 2012, 08:38:16 PM »
Hi,

When I was messing around with the transistors i found that no matter what i had hooked up through the collector the output on the emitter is just slightly lower than my base even if nothing is hooked up to the collector.  is this normal and if so then what is the purpose of them as i could just hook my base to my emitter and get slightly stronger results.  I am sure I am doing something wrong and after searching and searching I can't seem to find an answer to this question.  I am primarily messing around with npn transistors (2n4401, mps 2907a, mps 2222a)
Yes, that's how they should behave, a voltage drop base-emitter of around 0.65V.
It's the high side transistors that you have problems like this with, when you try to use NPN devices in that place - swap them for PNP's and it works. If you drive the bridge from eg. 5V logic and with a higher motor voltage, drive the PNP's with NPN's.

A bipolar transistor used as you do (mainly is a current amplifier) will allow a larger current collector-emitter than the controlling current (base-emitter) - with the transistors you mention, the current amplification is around 100..200 times, so to control 100mA, you only need 0.5mA to 1mA.
Just remember... The N means 'Negative and the P means positive, so it's easy... 2 P's and 1 N means it goes to the positive side and v.v.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline jghostzTopic starter

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Re: transistor confusion
« Reply #2 on: July 26, 2012, 11:39:52 PM »
Thank you for that dumb slap myself in the head moment where all that reading I did just clicked and made sense, and thank you for the tip about controlling the pnp's with the npn's.

 


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