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hi, we have been tasked with creating a dc regulator and 6v constant output @ 250mA from a 9V battery.

Input voltage between 9.5 - 7V, Max current capability 400mA, Variation at output +/- 200mV, Max ambient temperature 45 C

using a 5.6v zener diode and BD137 npn transistor, how do i work out the resistor values needed for the circuit to function correctly?

Thanks

I would split the circuit at the transistor base. Then you have two circuits, one with R1 R2 D1, and the other with Q1. Then analyse each circuit on its own, and finally see how they behave together. Remember to include zener tolerance not specified but usually +/- 5%. The BD137 hfe falls as low as 40, so its base current could be 10 mA. Or not. The headroom can be as little as 400 mV, so R1 is very small. Watch the zener dissipation, it says 0.5W but keep it well below that if you can. Have fun. Remember Rome wasn't built in a day. mike

ps - The BD137 will handle 1.25 W without a sink and is being asked to dissipate 1 W here. You should show this in your working and perhaps suggest adding a heatsink as good practice. Also suggest adding foldback protection to protect it.

pps - The input voltage implies a PP3 non-rechargeable dry battery but 250 mA is an abnormally high drain. Battery life will be about one hour. The PP3 is usually used for much lower current loads. Although this is 'only' an exercise it is worth recommending a redesign of the load circuit to run on a lower current if possible.

Thanks that helps alot! will try to tackle it today