zener trouble

Erik Durling · 2004-10-28T13:03:21+00:00

We're a couple of students doing an electronics project. We are toconstruct a battery indicator circuit. We had a finished circuit onpaper and we had simulated it with Multisim 7. But when we tested thecircuit in real life, it didn't work at all like we had planned (oraccording to simulation). The circuit was based upon the idea ofzenerdiodes having a constant voltage drop, and not conducting if thecircuit isn't able to supply the needed voltage across it. But thezener-diodes doesn't stop conducting at all, and the voltage drop isfar from constant.This is the main circuit: The zener diodes used in real life are: BZX55 and not BZC55, but thatought not change anything (right?).We did a DC Transfer Analysis in the simulator and got three nicecurves showing the voltage at the three collectors (compared toground) as the voltage from the source goes from 0-4.5 volts, andeverything seems right there: So, the problem we're having with our real circuit is that the LEDsnever turn off (the transistors are always saturated). The zenerdiodes doesn't stop conducting when the voltage across them goes undertheir specified value.What's our mistake/misconception?Do the zeners only operate correctly at a certain current?

zener trouble

Oct 28, 2004 23 Replies

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CBarn24050 21 years ago

Hi, if you look at the datasheet you will see why it doesn't work. Why you would even consider simulating such a circiut is beyond me.

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Watson A.Name - "Watt Sun, th 21 years ago

Some people just don't bother with the sims, and do just the experiments. ;-)

Move the 10k resistors from the cathodes to the anodes of the zeners.

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John Larkin 21 years ago

And some people just do the math, lay out boards, and go into production.

John

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rayjking 21 years ago

The circuit will work if you add a resistor from base to emitter of all of the bipolars to allow the zener to reach its test current. usually the test current is 5-10ma. This is also needs consideration when making the divider. LM317 and LM339's are popular because of these drawbacks.

Ray

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