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Old 5th September 2011, 06:15 PM   #11
Mooly is offline Mooly  United Kingdom
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It works like this...

The zener diode always develops around 5.6 volts across it when fed with a current of a few tenths of a milliamp up to a few 10's of milliamps. The power rating of the zener determines the upper current limit. The "resistance" feeding the zener determines this current. It can be a resisor or a CRD.

So far so good.

What we do now is apply that fixed 5.6 volts to the base of transistor. The transistor base voltage is now fixed (at 5.6 volts). The collector goes to the unregulated positive supply (9 to 50 volts). The emitter is constant at around 5 volts (Vbase minus the base emitter volt drop... the B-E junction behaves like a diode dropping around 0.6 to 0.7 volts).

The transistor allow for the current fed "into" the base to be amplified and thus the transistor can supply a reasonable current into the load.

If you were to build this for real then it should include a small cap across the zener (say a 0.1uf) and a small electrolytic cap across the 5 volt rail.

The CRD is not not like a diode at all... they just call them that.

Have to see if I can find any links on simple discrete voltage regulators.
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Old 5th September 2011, 06:19 PM   #12
Mooly is offline Mooly  United Kingdom
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I think tbh something like this would best,


It's a classic text and very comprehensive. You need to understand all the basics first really.
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Old 6th September 2011, 05:11 AM   #13
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Thanks for all the help.
Here's to a working project soon (i hope)
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