Hi,
Would someone Kindly explain why circuits such as this switch mode power supply use 2 zener diodes in series with the base of the Darlington Pair?
I think I just about understand what 1 zener diode would do in this application but for the life of me I can't work out why there are 2 or why one is rated at 12v & the other at 13v, is this just clamping the base voltage at 1v?
Many thanks,
Mick.
Edit:can't seem to post the image so here is a link : 24V / 2A DC Power Supply - Power Supply Circuits
LarryD
February 27, 2018, 9:49pm
2
Two zeners in series makes an effective zener equal to both ratings.
12 + 13 = 25
larryd:
Two zeners in series makes an effective zener equal to both ratings.
Thank you,
So effectively what is that doing in this circuit?
LarryD
February 27, 2018, 9:54pm
4
12 + 13 = 25
They regulate the output voltage to ~24.3 volts.
Thank you very much,
I understand what they are doing now.
A LM317T would do a much better job.
Allan
MarkT
February 28, 2018, 2:14pm
7
Typically a zener series string is either to get a specific voltage by combining standard E12 values,
allanhurst:
Allan
Would you share why it would please?
Much better regulation. It has a lower output impedance.
Very low noise . Zeners are noisy.
More accurate - 2% , as against zener's 5-10%
Easily adjustable with a preset to get the exact required voltage.
There may be more......
Allan
Using 2 low cost common parts versus one less common or more costly one is often the reason you can see design decisions like this as well.
