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zener B+ drop

PostPosted: Tue Jun 03, 2008 1:16 am
by susomora
Hi guys ,there goes my last question : which side do my zener cathodes should be pointing towards in order to get the desired B+ , the transformer or the ground ?
Cheers one more time.

PostPosted: Tue Jun 03, 2008 12:38 pm
by mesherm
The cathode (banded end) of a zener diode connects to the + or positive voltage. Zeners are meant to be reverse biased. They also require a resistor in series to limit the current depending on the wattage of the zener.

PostPosted: Tue Jun 03, 2008 2:13 pm
by susomora
i thought that all that is needed is connecting the zener between the HT CT and ground bypassing it with a 0.1 uF cap to avoid noise from the zener ....

PostPosted: Tue Jun 03, 2008 4:06 pm
by mesherm
It depends on how much current the zener will draw. Divide the zener wattage rating by the zener voltage and that will give you the maximum current the zener can draw although you should derate it by 50% for safety. If your using a 20 watt zener at 50 volts then your max current should be 200ma to be safe. Now if your power supply can only deliver 150ma max then you don't need a current limiting resistor and can just use the zener directly. If you intend to drop 50 volts off of your B+ it will work. Connect the cathode to the B+ after the last filter cap and the zener anode will be your new B+ connection 50 volts less than before.

PostPosted: Sun Jun 08, 2008 8:05 pm
I think just use a zener diode to regulate the voltage is not that practical, with the current limited resistor, you only get a very little current enough for voltage reference, not enough to drive anything.

The best way suggested to do is by adding a series passing transistor, thus could avoid excess heat and lost of power. I think it's difficult to find a 20W zener with the desired voltage. Yellow_Light_Colorz_PDT_12 Yellow_Light_Colorz_PDT_12 Yellow_Light_Colorz_PDT_12