Electronics > Beginners
Zener regulator circuit, and power ratings.
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sureshot:
Thank you for the replys. Yes i did think this is way above nominal. I got the hfe wrong for the transistor i was going to use, its minimum hfe is 15 it was the TIP35C, but i can find npn darlington transistors. And i did think about the power dissipation of the zener diode, and it all looked to much in my working out figures. 3 Amps output current would be plenty, if i use a darlington transistor won't i have two forward voltage drops ? Say 1.4 Volts. So if my zener diode is rated at 5 watts, i shouldn't take it to its maximum power rating ? Plan a bit less so its not maxed out. Is that right ? Thank you for the explanations, it takes a few attemps to get my head around the numbers.
Simon123:
Yes, darlington means 2 voltage drops.
Yes, you shouldn't take zener to its max, give it little space.

And stop worrying about how you need 5W zener, from start of your design. Calculate as I showed you in the example above, and then you will know what power rating you need.
sureshot:
I vaguely remember when i put a 14 Volt zener diode with a TIP142 transistor, with no load on the output the zener diode got way hotter than when i put the circuit under load. I guess it was passing the resistors power.
Simon123:
Yes, it gets hotter, because current instead of flowing into base of transistor flows into the zener. And if it got really hot, i guess you didn't use good resistor value.
sureshot:

--- Quote from: Simon123 on July 25, 2018, 11:06:47 am ---Yes, darlington means 2 voltage drops.
Yes, you shouldn't take zener to its max, give it little space.

And stop worrying about how you need 5W zener, from start of your design. Calculate as I showed you in the example above, and then you will know what power rating you need.

--- End quote ---
Yes i will, its starting to sink in now. Thanks for the help with this, that's to everyone that's put up with me.
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