LM317 input voltage

[[email protected]]

I have a circuit where I need about 300ma at 10v regulated and a raw
voltage of about 19v that I can feed to the input of the LM317T.

I know for the regulator to do it's job, you need around 3v or so
higher at it's input but I always wanted to know....

Other than the heat generated from the voltage drop, is there any
reason NOT to feed an excessive voltage to the input, as long as the
317 can take it and everything is seemingly alright?

Or is it better design practice to put in a series resistor and drop
the input voltage to a lower level?

Is there a maximum limit on input voltage or is just a factor of what
the 317 can dissipate in heat?



I have the 19v because I just happened to have a transformer that is
"more than I need"......don't want to buy another one.

 

[mpm]

I have a circuit where I need about 300ma at 10v regulated and a raw
voltage of about 19v that I can feed to the input of the LM317T.

I know for the regulator to do it's job, you need around 3v or so
higher at it's input but I always wanted to know....

Other than the heat generated from the voltage drop, is there any
reason NOT to feed an excessive voltage to the input, as long as the
317 can take it and everything is seemingly alright?

Or is it better design practice to put in a series resistor and drop
the input voltage to a lower level?

Is there a maximum limit on input voltage or is just a factor of what
the 317 can dissipate in heat?

I have the 19v because I just happened to have a transformer that is
"more than I need"......don't want to buy another one.

Depending on what you're building, you might consider a switching
supply to save on total device power consumption. I'm thinking:
EnergyStar-compliant type stuff here. LM317's are cheap, though. So
if your product design doesn't benefit from EnergyStar or similar (in
some cases regulatory requirement), then a linear design might prove
the best option.

-mpm

 

[Apostrophe Police]

.

I know for the regulator to do it's job, you need around 3v or so
higher at it's input but I always wanted to know....

THERE IS NO APOSTROPHE IN THE POSSESSIVE ITS!!!!!

Or any possessive pronoun, as a matter of fact.

Thanks,
Rich Grise, self-appointed Chief,
Internet Apostrophe Police.

 

[Jamie]

I have a circuit where I need about 300ma at 10v regulated and a raw
voltage of about 19v that I can feed to the input of the LM317T.

I know for the regulator to do it's job, you need around 3v or so
higher at it's input but I always wanted to know....

Other than the heat generated from the voltage drop, is there any
reason NOT to feed an excessive voltage to the input, as long as the
317 can take it and everything is seemingly alright?

Or is it better design practice to put in a series resistor and drop
the input voltage to a lower level?

Is there a maximum limit on input voltage or is just a factor of what
the 317 can dissipate in heat?



I have the 19v because I just happened to have a transformer that is
"more than I need"......don't want to buy another one.

when you say transformer, do you mean 19 volts before rectification?

If so, beware that after the conversion to DC, you'll have a little
less than ~ 19*1.414 DC.
this puts you in the 27 DC range.

So, the W = (27-10)=17*0.300 = 5 watts.

That maybe ok with out a heat sink. It's been a while since
i've checked that Reg..

Also, it maybe a good idea to place a back flow diode on the
input and output in the event that the input should drop below
the output.. This helps save the reg.

Those figures may not be exact but they are close enough..


http://webpages.charter.net/jamie_5"

 

[Eeyore]

I have a circuit where I need about 300ma at 10v regulated and a raw
voltage of about 19v that I can feed to the input of the LM317T.

That means it'll need a heatsink if it's the TO-220 device I assume you
mean.

I know for the regulator to do it's job, you need around 3v or so
higher at it's input but I always wanted to know....

Other than the heat generated from the voltage drop, is there any
reason NOT to feed an excessive voltage to the input, as long as the
317 can take it and everything is seemingly alright?

Excessive = > Vin (max)

Or is it better design practice to put in a series resistor and drop
the input voltage to a lower level?

Well, resistors tend not to need heatsinking and keeping the LM317 temp
lower will increase lifetime.

Is there a maximum limit on input voltage or is just a factor of what
the 317 can dissipate in heat?
Both.


I have the 19v because I just happened to have a transformer that is
"more than I need"......don't want to buy another one.

That's fine but don't forget the heatsink. You'll be dissipating 2.7W and
a TO-220 device in free air can't handle that.

Graham

 

[mkr5000]

Apostrophe police --
If I had dementia, you would say "great post".



So, really the 317T can take up to 40v input and as long as it's heat
sinked right, you can set it for whatever
output voltage you need. period.

In my case, 19v at input -- 10 volts out.

I'm drawing 300ma worst case but it is heat sinked very well.

I'd use a resistor but didn't allow a space for it on the PCB.

 

[[email protected]]

Lose some of that heat by choosing a filter capacitor to _maximize_
ripple but trough still staying above drop-out.

That method worries me, because it depends on the stability of the
capacitor under high ripple current conditions. I like adding the
series resistor upstream of the rectifier, or even upstream of the
transformer. Especially with cheap transformers that barely have
enough iron to avoid saturation, the last option can really cool off
the transformer as well as the regulator.

 

[mkr5000]

Ok John -- I'll take your advice...it bothers me to because I always
hate excessive heat in any device.

You've enlightened me to an interesting word as well...."upstream".

Often times, this resistor being an example, I should take more time
to think about the "best place" to install the component.

I would have immediately put it in front of the regulator...yet
putting it in front of the rectifiers I suppose makes all the
components happier.

It's really just a simple 555 switching circuit that the supply is
for.

Can you clarify the filter cap stability thing for me? Are you saying
that excessive voltage on the regulator input could make the cap work
harder than it needs to or something?

 

[Eeyore]

Can you clarify the filter cap stability thing for me? Are you saying
that excessive voltage on the regulator input could make the cap work
harder than it needs to or something?

Which cap ? The reservoir cap ? No, the ripple current will be (almost)
identical regardless of the rectified AC input.

Graham

 

[Eeyore]

Another thing you could do is to put the resistor between the rectifier and
the filter capacitor. This will give very low ripple voltage, and low peak
diode and transformer currents at the expense of really poor regulation.
Pick a resistor low enough in value to meet minimum headroom requirements at
maximum load. You can not calculate the resistor value from a DC analysis;
it will end up at around 10 % - 20% of what it would be if it was after the
filter capacitor.

When the overvoltage is considerable you can use a BIG , say 5W, zener.

Incidentally, ever worked out hoe to make a 78/79 series regukator into a
constant current souce ?

Cuts the EMI from the load like a scimitar.

Graham

 

[whit3rd]

[using an input resistor to reduce three-terminal regulator heat]

Lose some of that heat by choosing a filter capacitor to _maximize_
ripple but trough still staying above drop-out.

It's oddly hard to make that work. When I tried, it always turned
out that the smaller filter capacitors weren't rated for enough
ripple current. The ripple current 'rating' is hidden in the
data sheet somewhere, I found that the cheapo filter
capacitors only allowed 1A per thousand uF of capacity.

Maybe the low-ESR modern units are better.

 

[JosephKK]

Which cap ? The reservoir cap ? No, the ripple current will be (almost)
identical regardless of the rectified AC input.

Graham

No the issue was precisely about reducing the size of the reservoir
cap by maybe 80 % (1/5 the cap you would normally use) with about 1/3
the ripple current and about 5 to 10 % reduced power for the
regulator.

 

[JosephKK]

glass... "really bad for the glass" ;-)

...Jim Thompson

Just glass? I had you figured for a nitride kind of guy.

 

[JosephKK]

"really bad for the glass" is a line from a movie. Anyone know the
name of the movie?

Actually haven't done any chips with nitride caps... mostly "poly".

...Jim Thompson

Would that be from a James Bombed movie?

Oh well, nitride is kind of a passe` process now.