Please help with transistor issue...

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Hi,

I picked up three 7 segment numeric LED's for my first AVR tests where I am going to attempt to multiplex them.

All 7 segments have a 100 ohm resistor which at 5V gives each one about 26ma. The package says 30ma max. They have a common cathode. So I have 7 places on my breadboard that go to a 100 ohm resistor, then to each of the three segments. If I tie all the cathodes to ground and apply postive voltage to each of the 7 segments I see all three digits same segment light up as expected.

Now I am trying to implement the transistor control of the cathode. I have a NPN transistor I am connecting the collector to the LED cathode, emitter to ground, and I am trying to use the base to turn it on and off.

With the cathode shorted to ground and all segments on, it runs about 200ma. The transistor says 200 hfe so does that mean 200x gain ? If I take .200 and divide it by this gain I get .001 (1ma) and I read something about adding 30% to it, so 1.3ma. My voltage is 5V so 5/.0013 = 3846 ohms. Would this be the correct resistor to connect the base to +5V ?

I have to think I've done something wrong because a 100 ohm resistor yields about 170ma instead of 200ma and a 460 ohm resistor yields about 56ma. It looked like about 1/4th of the current was flowing into the base. Would that be 4 hfe?

I must have something wrong, please correct me!!

Thanks,

Alan

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hhm.. this has been a while ago... n my transtor bias calculation is rusty.

what you essentially need is to operate the transistor in Fully ON or OFF region and not in the Bias region. hfe figure is realy applicable to the bias region i believe. so its no good for you.

To operate the transistor in the fully ON or OFF region essentially you need a small enough bias resistor but not too small. I would say at 5V operation to turn a typical NPN transistor (eg BC548) you need no less than 1-4K ohm resistor.

any such resistors should make the transistor fully ON.

The other alternative and more power efficient method for you would be to use a MOSFET. they are cheap and easy to hook up. and perfect for you situation. transistors are 19th century item. We live in 21st century now. :)

Mosfets either turn ON or OFF. there is no middle ground for them (ie: bias region).

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It has been awhile! Try 20th century for transistors! You can bias MOSETS in the linear region.

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Hi,

Package is some NPN resistors "switching type" from radio shack I had laying around. Does it seem normal that it would take 1/4th the current on the base that is flowing through the transistor? Surely I have something wrong...

Thanks,

Alan

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lol dont get too upset :P
uum... u can bias a mosfet? how effectively and for wat reason? it defeats the purpose of a mosfet ...

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Hi,

Nevermind, stupid mistake. I had the collector and emitter backwards. Packaging said "bottom view" and I was thinking "top view"... HFE measures 4 backwards and 200 forwards. Will test tomorrow.

Thanks,

Alan

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Hi,

Nevermind, stupid mistake. I had the collector and emitter backwards. Packaging said "bottom view" and I was thinking "top view"... HFE measures 4 backwards and 200 forwards. Will test tomorrow.

Thanks,

Alan

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Quote:
uum... u can bias a mosfet? how effectively and for wat reason? it defeats the purpose of a mosfet ...

Stereo amplifiers, amplifiers for just about digital modulation mode that you can think of and even SSB.

Sure MOSFETs make great switches, but they are also quite useful for linear applications.

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davef wrote:
Quote:
uum... u can bias a mosfet? how effectively and for wat reason? it defeats the purpose of a mosfet ...

Stereo amplifiers, amplifiers for just about digital modulation mode that you can think of and even SSB.

Sure MOSFETs make great switches, but they are also quite useful for linear applications.

digital amplifiers? we talking about D-Type? they dont bias mosfets! they turn then on n off using PWM only!!!

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There are Class AB Mosfet Amps as well..

Here is an example...
http://sound.westhost.com/project101.htm

Michael

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wat on earth? first time seeing this...
whats the point of them being mosfets then?? they should be called transistors!

how does this work? it seems to me its got some differential BJT circuits in the inputs with some feedback system. then push pull method in output.

so if its being operated in triode region. then wats the benefit of not using BJT?

Last Edited: Fri. Jul 24, 2009 - 05:50 AM
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There is some info on their operation here...
[url]
http://sound.westhost.com/amp-ba...

Michael

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MOSFET = metal–oxide–semiconductor field-effect TRANSISTOR
http://en.wikipedia.org/wiki/MOSFET

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lol come on i know wat a mosfet is! its just i never seen them be used for amps like that. anyway i guess we learn things everyday :)

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Hey! Please do NOT cloud the issue with facts! :)

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Alan,

When you re-test your circuit use a new transistor(s), if you have them available. You may have zapped the others placing them backwards.

When it works you can try the old ones in the circuit to see if they still work.

To add further to what was said above:

The resistor in series with the LED segments is to limit their current. The transistor acts as a full on or full off switch. It does not function to limit the current in this mode.

Note that the voltage across the transistor will be higher for an NPN than for a typical Mosfet, and this voltage would be part of the calculation for the series resistor.

A typical NPN for your use might be a 2N2222, while a similar Mosfet would be a 2N7000.

JC