Re: Voltage rating of LM317

pjdd@xxxxxxxxxxxxxx wrote:
ehsjr wrote:

pjdd@xxxxxxxxxxxxxx wrote:

ehsjr wrote:

So the options are : multiple series diodes, a different
transformer, switched-mode regulation, or a heavy heatsink.
Comments please ?

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.


The reason for not including the zener option is simple -
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).


It is time that you do make some detailed calculations.
You started out asking a question with an unreasonable
change in the DC at Vin to 45 volts. First of all,
that won't kill the 317 based on Vin-Vout. Nor will the
concept of the uncharged cap on Vout be a concern as
has been shown and particularly with the resistor in there.
But a line voltage variation that would yield Vin of 45
volts would be way more than normal worst case design,
and would strongly suggest a different approach. Your
calculations will show that. You'd need the line voltage
to rise from 120 to ~146 to get 45 volts at Vin. That's
over a 21 % variation in line voltage, and if your equipment
must function in that kind of environment, a basic linear
regulator is not your best choice.


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.

You misunderstand. The zener goes across the 317.


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.



Fine. Use the HV unit.

Ed
.

Relevant Pages

  • Re: Estimating transformer voltage for B&K CS117 preamp
    ... 25% regulation transformer. ... Top voltage limit will be dictated by margins & cap rating. ... regulator whilst it is still cold eg before it goes into thermal foldback? ... you can chase a short down, by measuring at each of the ICs' supply pins, ...
    (sci.electronics.repair)
  • Re: How best to drop ~180V?
    ... I have a tube power supply that is putting out 520VDC. ... massive resistive voltage divider ... I'll suggest you use a HV regulator with a feedback loop. ... The irfpc40 is rated for 150W max with a 25C heatsink, ...
    (sci.electronics.design)
  • Re: Whats the transformer voltage at different currents ?
    ... I needed 24vdc to drive the coil of a relay. ... >regulator should be right. ... >voltage after rectification and filter when you are not drawing the full 0.8 ... using it usually runs about 30%, which means that when the transformer ...
    (sci.electronics.design)
  • Re: Voltage rating of LM317
    ... multiple series diodes, ... transformer, switched-mode regulation, or a heavy heatsink. ... resistor (for an input of 15V to the 12V regulator). ...
    (sci.electronics.design)
  • Re: Whats the transformer voltage at different currents ?
    ... I needed 24vdc to drive the coil of a relay. ... >>regulator should be right. ... >>voltage after rectification and filter when you are not drawing the full 0.8 ... >using it usually runs about 30%, which means that when the transformer ...
    (sci.electronics.design)
Quantcast