P
[email protected]
- Jan 1, 1970
- 0
The reason for not including the zener option is simple -ehsjr said:What do you have against the zener? Is it the same issue
with the HV 317 - parts availability? Or for that matter,
why choose a heavy heatsink over a resistor? I'm not
trying to push one idea over another, but am interseted
to hear why you do not include those in the options.
I forgot to, when I made the list . Then there's the
matter of availability of a high-power zener diode too.
Also consider practical design parameters. Worst-case
input on the low side will be when line voltage is low
and the transformer-rectifier is fully loaded. Without
making detailed calculations, I estimate the DC input to
be around 24V then, which allows a max drop of 9V by the
resistor (for an input of 15V to the 12V regulator). This
works out to 15 ohms with a dissipation of 5.4W at 0.6A
(a 10W resistor will do).
Now take the other extreme case where the DC input rises
to 45V at light load and abnormal line voltage. If I use
a 39V zener, the resistor will briefly pump 0.4A into it
at startup. I wonder if that's advisable even with, say
a 5W zener diode. And I'm not sure I can get a 39V 5W
zener.
OTOH, consider the case where the regulator is fully
loaded to 0.6A, but the other load is off, AND line
voltage is high. Estimate the regulator input at about
36V. The regulator dissipation under that worst-case
condition is 14.4W - quite high, but still manageable
with a 317HV (the K package will be preferable). A 15
ohm series resistor or 12 diodes (say cheap 1N5402s) in
series will reduce that to about 10W.
Another option would be a 9V 10W zener in series instead
of in parallel with the regulator. There's the
availability issue again and it will have to be
heatsinked too.
I could lower the input with a transistor voltage divider
- say a TIP122. But that will need a heatsink too and the
dissipation will simply be distributed between regulator
and transistor without being reduced. The main advantage
will be that a plain LM317 can be used.
I think that's a clumsy approach that can be avoided by
using a 317HV and a good heatsink, or a separate
transformer of lower voltage. I sometimes use a center-
tapped 9V transformer which is easily available, and use
it as an 18V unit at reduced current rating.