Electronics > Beginners
Zener regulator circuit, and power ratings.
sureshot:
Your probably getting bored with all my questions, but thank you very much for the help given in my posting. Much appreciated.
So I'm looking at zener diodes again and regulator circuits..
I've built the basic zener circuit and that worked fine. I'm wandering if there's a way to increase the output power with a series pass transistor ? I did give this a go with a 2N3055 npn transistor, but noticed the zener diode getting very hot. High power zener diodes seem hard to source. Mine are 5 Watt 13 volt diodes.
I'm thinking along the lines of how linear regulators can be used with little current drawn by them, but a power transistor is doing most of the work. Only in the zener circuit the device dissipates a lot of heat, and the zener power rating has to be chosen for the intended output current. By that i mean the more current you draw the higher wattage the zener needs to be.
But with a linear voltage regulator the power that dissipates stays the same, as in more series pass transistors equals more current. But i can't find a way to do this with a zener diode. Meaning the zener diode just contributes to a very small amount of the total output power, and the series pass transistors do the overall majority of the work load. So in a word, keep the zener diode in sensible limits whilst the series pass transistor does most of the work load with out burning up the zener diode as current increases. I know its possible up to about 1 Amp, but is there a way to do it that achieves up to say 3 Amps or maybe even 5 Amps ? I have searched a fare bit for the answer but have found little on this question.
Thanks for reading, and any help or ideas appreciated.
HB9EVI:
The question, how much current you can drain, depends from the voltage drop over the series pass transistor, you know Udrop*Idrain=Ploss.
If your zener is getting too hot, you're likely running a too high bias current through it. In a series pass with zener, you have to calculate the necessary base current plus the bias current for the zener. Don't use a 2N3055, there are much better transistors available today; of course you still keep seeing those ones in linear psu, but imo it's a no-go.
sureshot:
Thank you for your reply. So if I lower the bias current base resistor, I can draw more current from the zener regulator circuit ? And would a pnp transistor configuration for a series pass transistor be a better idea in a zener regulator circuit. What would be the theoretical maximum current that could be drawn from a 5 watt 13 volt zener diode and say a TIP2955 transistor ? Or maybe a TIP147 transistor. Thanks again for the help.
sureshot:
This is the circuit below I built, but I had to drop that 680 ohm resistor to get anything close to 12 volts output. This circuit claim 3 Amps output current, from memory I loaded it with a 20 watt halogen lamp. That was close to 1.6 Amps and the zener diode was quite hot. I can't remember the value resistor I used, I think it might have been a 100 ohm 2 watt resistor.
HB9EVI:
what transformer are you using (rating, voltage)?
that circuit has several disadvantages regarding the regulation behaviour; the bias through the zener heavily depends on the load on the circuit, thus the zener voltage shifts. You would be better off using an opamp and place the zener on the regulated side - like that you have a constant bias through the zener and the transistor bias also matches better the load.
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