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2N3055 resistance question

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Ash Small

Fri May 04 2012, 05:43PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Having now moved house, and traded both my car and girlfriend for newer, higher performance models, I now seem to have the time to get back to some experimenting, etc.

I've been re-reading an old thread of mine from last August regarding 2N3055s:

and decided to see if the 2N3055 is turning on completely.

I assume I need to connect 12V, via a resistor, to the base, connect the emitter to the negative terminal, and measure the resistance between the collector and emitter.

Is this the correct method, and what should I be looking for?

(I can't find any figures/graphs relating to collector-emitter resistance)

Ben Solon

Fri May 04 2012, 05:58PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

Maybe a voltage divider formed by the transistor and the series resistor? Measure the voltage at the qr junction.

Ash Small

Fri May 04 2012, 06:02PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

ben123324 wrote ...

Maybe a voltage divider formed by the transistor and the series resistor? Measure the voltage at the qr junction.

I'm not sure I quite follow. Any chance of a simple schematic, Ben?

Proud Mary

Fri May 04 2012, 06:15PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The base bias is provided by the voltage divider consisting of the two resistors in series on the left.

Ash Small

Fri May 04 2012, 06:41PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

OK, I think I see what you are suggesting. If I use a 'scope connected across C-E I can see what's happening while the circuit is running?

klugesmith

Fri May 04 2012, 07:48PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Ash, it sounds like you are trying to fit a MOSFET model to a BJT.
That's like forcing a square peg into a round hole!

BJT's (like 2N3055) are "turned on" according to the amount of base CURRENT, I_B.
The value of V_BE doesn't change very much. You sure don't need more than
1 volt to turn on a 2N3055 "all the way".

And when used as a switch, the path between C and E is never very resistor-like.
What you call "turned on completely" is a BJT operating region called saturation.
(Very different from saturation in a MOSFET).
Traditionally it requires base current of, say, 1/10 of the maximum collector current.
Then the C-E path is sort of like a diode. No reverse conduction.
No forward conduction without a minimum forward voltage (typically around 0.2 volts).
The forward voltage V_CE_sat increases only slowly with exponentially increasing collector current.

I will leave it as an exercise to look up traditional BJT circuit models.
-Rich

Ash Small

Fri May 04 2012, 08:05PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks for pointing that out Rich.

It sounds like I'll have to measure current from C to E directly, but that's not straightforward when the transistor is switching at around 25kHz.

Mattski

Fri May 04 2012, 08:13PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

BJT's are typically described in terms of the CE saturation voltage Vce,sat, because this describes how they work better than an on-resistance. When you increase current through a fully-on FET switch you expect the voltage drop to increase linearly because it is described reasonably well as a resistor, but with a BJT the voltage drop increases at a slower rate (up to a point), which is why saturation voltage is used. By increasing base current you decrease this voltage drop but only up to a certain point, and the datasheet provides plots of this.

As ben said, put some kind of load resistor between your power supply and the collector, turn your transistor on with some base current, and measure the collector voltage and current. If you increase the base current but can't reduce the voltage drop any further then the transistor is fully turned on.

Edit: kluge beat me to it :)

Ben Solon

Fri May 04 2012, 08:42PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

heres just a quick schematic showing what we are talking about. bjt's have a voltage drop across them, and the only way to tell if they are fully on is to measure if the current through them.

Ash Small

Fri May 04 2012, 09:05PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks. I think I have it now.

To measure whether the BJT is turning fully on at 25kHz I connect the 'scope across a series resistor and measure V/R, which gives me a plot of the current flowing.

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