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Voltage rating of LM317

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.
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).

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.
 
Dave said:
:
: rebel wrote:
: > On 9 Nov 2006 06:32:23 -0800, [email protected] wrote:
: >
: > >
: > >So the options are : multiple series diodes, a different
: > >transformer, switched-mode regulation, or a heavy heatsink.
: > >Comments please ?
: >

I haven't read all replies so excuse me if this is redundant
or irrelevant, but have you considered using two 317's
to split the voltage drop between them?
:
Thanks for the input. No, I hadn't considered that option.
It's certainly a possibility. Since the input can drop to
as low as 24V, maybe a little less, the first regulator
will have to be set for an output of no more than 20V.

For stability, NatSem recommends a minimum of 1uF Tantalum
or 25uF Al at the output. This again necessitates ensuring
a slow input rise time or other protection scheme for the
first regulator.

The voltage divider transistor I mentioned in the previous
post takes a similar approach.
 
E

ehsjr

Jan 1, 1970
0
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
 
ehsjr said:
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.

I appreciate all the feedback and enlightening discussion
that expanded from my original question. Please remember
that I - and the equipment - have to work under primitive
conditions. Surges of +20% do happen, though not an everyday
occurence. (We use 230V mains here).

I'm including a mains window circuit and VDRs at strategic
points, but I want everything to survive a momentary spike
in mid-operation before the protection circuit has time to
kick in. OTOH, sustained dips of more than -20% are common,
and the equipment has to keep on working.

Yes, a linear regulator is not the best choice from a
technical point of view, and I may ultimately go for a
switched-mode supply. But I was exploring the feasibility
of using the much simpler linear version.
You misunderstand. The zener goes across the 317.
I do understand that the zener goes across the 317. When
the output is momentarilry shorted to ground, the only
thing that limits the current is the series resistor.
I = (Vin - zener V)/R. What I didn't think of was that a
part of that current will be shunted by the regulator.
 
T

Terry Given

Jan 1, 1970
0
I appreciate all the feedback and enlightening discussion
that expanded from my original question. Please remember
that I - and the equipment - have to work under primitive
conditions. Surges of +20% do happen, though not an everyday
occurence. (We use 230V mains here).

I'm including a mains window circuit and VDRs at strategic
points, but I want everything to survive a momentary spike
in mid-operation before the protection circuit has time to
kick in. OTOH, sustained dips of more than -20% are common,
and the equipment has to keep on working.

Yes, a linear regulator is not the best choice from a
technical point of view, and I may ultimately go for a
switched-mode supply. But I was exploring the feasibility
of using the much simpler linear version.


I do understand that the zener goes across the 317. When
the output is momentarilry shorted to ground, the only
thing that limits the current is the series resistor.
I = (Vin - zener V)/R. What I didn't think of was that a
part of that current will be shunted by the regulator.

another method is a transistor/zener pre-regulator (its in some LM317
datasheets, or at least used to be). that also works when the output is
shorted (OK the shunt zener can too, but it gets trickier).


from there you can easily make the pre-regulator drop a constant voltage
across the LM317, or, say, one half of the differential voltage.

the disadvantage of the transistor is that it has no thermal protection.

using a pair of regulators is therefore a better choice. a LM317 (or
78xx) can easily be used to drop a fixed voltage across the output
regulator, and can also be used to share the voltage drop power
dissipation equally, which has many thermal advantages.

the single LM317HV solution has a lot less parts, but will have higher
peak temperatures, may well be more expensive, and is probably harder to
get.

regardless, I'd add in the various reverse-protection diodes, and ensure
it can cope with a continuous output short-circuit.


a historic solution: you could drive the unit with a
Constant-Voltage-Transformer (Also Known As (AKA) a Ferro-Resonnat
Transformer)? assuming you can find somebody who still knows to wind you
one....

Cheers
Terry
 
Terry said:
another method is a transistor/zener pre-regulator (its in some LM317
datasheets, or at least used to be). that also works when the output is
shorted (OK the shunt zener can too, but it gets trickier).


from there you can easily make the pre-regulator drop a constant voltage
across the LM317, or, say, one half of the differential voltage.

the disadvantage of the transistor is that it has no thermal protection.

using a pair of regulators is therefore a better choice. a LM317 (or
78xx) can easily be used to drop a fixed voltage across the output
regulator, and can also be used to share the voltage drop power
dissipation equally, which has many thermal advantages.

the single LM317HV solution has a lot less parts, but will have higher
peak temperatures, may well be more expensive, and is probably harder to
get.

regardless, I'd add in the various reverse-protection diodes, and ensure
it can cope with a continuous output short-circuit.


a historic solution: you could drive the unit with a
Constant-Voltage-Transformer (Also Known As (AKA) a Ferro-Resonnat
Transformer)? assuming you can find somebody who still knows to wind you
one....
The transistor-zener pre-regulator idea provides a solution
similar to the transistor voltage divider I mentioned earlier.

The use of two regulators in tandem to distribute power
dissipation is an important point. In fact, I overestimated
the thermal capability of an LM317 (plain or HV). I assumed
that a TO-220 package would be able to handle the 10-15W
wasted power quite well with a heatsink.

But on closer inspection of the specs, I found that the j-c
thermal resistance is fairly high at 4 deg C/W. Making it
dissipate up to 15W with a reasonably-sized heatsink would
push the junction temp closer to the max rating than I'd
like.

An alternative would be to let a parallel transistor take
most of the load current. Another would be to divide the
load into two or more sections, and provide separate
regulators for each section. And then there's always the
switch-mode option.

What made me reluctant to abandon the idea of a linear
regulator based on the LM317 is the elegant simplicity.
(And yes, protection diodes are a must, of course). But
if I'm going to have to use multiple power devices and a
more elaborate circuit to distribute the voltage or
current, then I might as well go for a switch-mode design.
It will probably be best to S-M down to about 15V, and
provide further regulation with a 7812.

The CVT option is technically interesting, and I could do
the designing and winding myself, but I'd rather wind a
switch-mode transformer (done it a few times) and go for
the switch-mode option.
 
T

Tim Auton

Jan 1, 1970
0
Terry said:
ehsjr wrote:
[email protected] wrote:
ehsjr wrote:

another method is a transistor/zener pre-regulator (its in some LM317
datasheets, or at least used to be). that also works when the output is
shorted (OK the shunt zener can too, but it gets trickier).

from there you can easily make the pre-regulator drop a constant voltage
across the LM317, or, say, one half of the differential voltage.

the disadvantage of the transistor is that it has no thermal protection.

using a pair of regulators is therefore a better choice. a LM317 (or
78xx) can easily be used to drop a fixed voltage across the output
regulator, and can also be used to share the voltage drop power
dissipation equally, which has many thermal advantages. [snip]
The transistor-zener pre-regulator idea provides a solution
similar to the transistor voltage divider I mentioned earlier.

The use of two regulators in tandem to distribute power
dissipation is an important point. In fact, I overestimated
the thermal capability of an LM317 (plain or HV). I assumed
that a TO-220 package would be able to handle the 10-15W
wasted power quite well with a heatsink.

I think using something in series to drop the voltage and dissipate some
power is a useful idea if sourcing a 317HV is difficult, but as you
don't need regulation as such from the first device I think a voltagee
regulator with heatsink would be overkill. A power resistor of 10-20
ohms would drop enough voltage at high currents - including the
switch-on transient when the 317 will be 'wide open' - and at low
currents the heat problem is minimal and the output cap has charged so
the voltage differential is no longer a problem either. A 10-15 W power
resistor would be cheaper, smaller and require less assembly than a
second regulator with heatsink. You might still need a heatsink on the
317, but nothing very large as the resistor could handle most of the
power dissipation.


Tim
 
T

Terry Given

Jan 1, 1970
0
The transistor-zener pre-regulator idea provides a solution
similar to the transistor voltage divider I mentioned earlier.

yep. thermally it is better to organise the power to be evenly split,
assuming equal Rthetajc.

if you use a transistor with a much lower Rthetajc, it should carry the
bulk of the dissipation, which a bjt-zener circuit does nicely by
returning the zener to the output (thus maintaining a constant voltage
across the regulator)

The use of two regulators in tandem to distribute power
dissipation is an important point. In fact, I overestimated
the thermal capability of an LM317 (plain or HV). I assumed
that a TO-220 package would be able to handle the 10-15W
wasted power quite well with a heatsink.

But on closer inspection of the specs, I found that the j-c
thermal resistance is fairly high at 4 deg C/W. Making it
dissipate up to 15W with a reasonably-sized heatsink would
push the junction temp closer to the max rating than I'd
like.

thats pretty high. so I went and checked a whole bunch of regulators,
and its about right. yuk. I was expecting about 1K/W....

so I went looking at some TO-220 transistors, only to discover none of
my datasheets even list Rthetajc! how un-useful. so I looked at a few
FETs, and found 0.5K/W for TO-220. thats a bit better!
An alternative would be to let a parallel transistor take
most of the load current. Another would be to divide the
load into two or more sections, and provide separate
regulators for each section. And then there's always the
switch-mode option.

What made me reluctant to abandon the idea of a linear
regulator based on the LM317 is the elegant simplicity.
(And yes, protection diodes are a must, of course). But
if I'm going to have to use multiple power devices and a
more elaborate circuit to distribute the voltage or
current, then I might as well go for a switch-mode design.
It will probably be best to S-M down to about 15V, and
provide further regulation with a 7812.

the smps will be a lot more complex, and generate truckloads of noise
(unless you are a smps expert, in which case its piss-easy).

The CVT option is technically interesting, and I could do
the designing and winding myself, but I'd rather wind a
switch-mode transformer (done it a few times) and go for
the switch-mode option.

Cheers
Terry
 
Terry said:
the smps will be a lot more complex, and generate truckloads of noise
(unless you are a smps expert, in which case its piss-easy).
Not an expert, but I think I can manage. It won't be the
first time. Yes, noise always has to be considered. Output
noise can be suppressed, but shielding EMI radiation could
be a problem with the materials I have access to.
 
T

Terry Given

Jan 1, 1970
0
Not an expert, but I think I can manage. It won't be the
first time. Yes, noise always has to be considered. Output
noise can be suppressed, but shielding EMI radiation could
be a problem with the materials I have access to.

if you know how to do a good layout (and if not, read linear techs AN47,
and/or M.J. Nave's "book power line filter design for SMPS"). make sure
the inductance of any high dI/dt loop is as small as possible. the PCB
contribution should be made negligible c.f. the magnetics, which should
be minimised.

DONT use bobbin-core inductors (the shielded ones are bad; the
un-shielded ones are essentially unusable if you care about EMI). gap
only the centre-leg of the core, or use an appropriate koolmu/mpp
toroid. and of course pay close attention to cap ESR, ripple current &
lifetime.

Cheers
Terry
 
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