LM350T variable voltage regulator

[Mike Taylor]

12 V, 3 amp transformer
Bridge using 4 three amp diodes
LM350T
One Ohm per ft nichrome wire for testing
Aluminum heatsink and fan
22,000 mfd capacitor on out side of bridge.

Problem
The output voltage is fine at 800 mA. As I continue decreasing the load resistance the output voltage starts dropping but the output current stays relatively the same. It's acting more like a constant current regulator.

Desired out voltage is 6.3V.

I need 2 amp output at 6.3V. Any idea what is wrong?

Mike

 

[bigone5500]

Welcome to the forum, Mike!

I did a quick search and came up with this.
http://www.circuitstoday.com/3a-adjustable-regulator-using-lm350

Maybe you can modify it for your needs?

Can you post a list of components you have used (part #s)? Maybe a schematic?

Thanks.

 

[AnalogKid]

Any idea what is wrong?

Not without your actual schematic. Too many variables. OBTW, you will be moving over 15 W in the device. Heatsink?

ak

 

[Mike Taylor]

Welcome to the forum, Mike!

I did a quick search and came up with this.
http://www.circuitstoday.com/3a-adjustable-regulator-using-lm350

Maybe you can modify it for your needs?

Can you post a list of components you have used (part #s)? Maybe a schematic?

Thanks.

That is the circuit I'm using. I have not added the caps yet. Parts list at top of first msg.

I think I got a bag of mislabeled LM417's. Tried 4 out of the bag with same results. Some cheap stuff from China. I've made many of these and never encountered this before.

Not without your actual schematic. Too many variables. OBTW, you will be moving over 15 W in the device. Heatsink?

ak

 

[Mike Taylor]

That is the circuit I'm using. I have not added the caps yet. Parts list at top of first msg.

I think I got a bag of mislabeled LM417's. Tried 4 out of the bag with same results. Some cheap stuff from China. I've made many of these and never encountered this before.

I am aware of heat sink requirements.

 

[73's de Edd]

.



Sir Mike . . . . .

We confirm the components that you utilized, along with the somewhat uber- healthy selection of the 22K Ufd's of initial raw DC filtering.

In using our Right Honorable and Esteemed "bigone5500"'s reference schematic, but, with our yet not FULLY knowing YOUR circuitry utilized:

The utilization of secondary C2 and C3 electrolytics are of definite importance.
With manufacturers harping on utilization tantalum types for C3, but I have never found that "cut in stone", with the possiblity of squirrelly operation at some voltage and current nodes.

Also, the selection of the value of R1, as it is directly related to the current limiting and foldback aspect of the 350's internal regulatory circuitry.

Get it too low of a value, and some overiding constant current mode tendencies will start showing up .

Your transformers voltage and current capability spec certainly seems sound, since you are dropping from a hefty raw 12 ++++++ volts all the way down to a 6 VDC level.

But, for further analysis, it would be nice to have feedback of the RAW DC voltage coming into the 350 at sample points of 100 ma-500 ma and your mentioned 800 ma and then at 1 amp . . . and 2 and 3 amps current draw, if the latter 3 levels are now being attainable.

The mention of power dissipation at the full voltage and current condition, and stress on the 350, can be scaled down by the shunting of the input to output terminals of the 350 with a fixed power resistor.

Its ohmmic value is being selected so that half of the power passage current will be will divided beween it and the 350.
With the 350 then saying:
" Wheeeeew . . . .thanks !

The only last thing in mind, might be the quality of the power transformer.

If I had a quality old school Thordarson or Triad power transformer in that circuit . . . . I could expect the FULL rectified raw DC voltage not to drop from that value down to a 12 VDC @ 3A draw. (With it never exceeding a >20 % difference.)

When I evaluate a Radio Shack or other "over the waters" transformers, they come up far short of that spec along with their running hotter.

Standing by for more info from you . . . .




73's de Edd


.

 

[73's de Edd]

.



Sir Mike . . . . .

Hmmmmmmm, I just now caught your short, add on snippet . . . I had previously overlooked.

A LM417 . . . . ? ? ?

I certainly am familiar with the :

LM117
of Aerospace and Mil spec qualifications and its sapphire substrate and liberal use of gold and platinum in its construction.

LM217
a lesser "117 wannabe" but well within its commercial and industrial specs.

LM317
The general purpose unit that you and I can afford, and find it suiting the purpose, and serving us well.

BUT now, the . . . . .
LM417
With no known available data, it indeed must be a dysfunctional unit of such origin as you just mentioned.

If I ever find any 350's at flea markets-Hamventions, etc. they are being in the metal to-3 cases and I snap them right up !

Cascade two of them and you can therewith, make one FINE variable voltage regulated AND variable current limited linear power supply.






73's de Edd


.

 

[davenn]

That is the circuit I'm using. I have not added the caps yet. Parts list at top of first msg.

what ?? none of the caps ??

of not, how do you expect the circuit to work correctly

again as has been asked several times .... show us YOUR circuit diagram AND a couple of photos of your construction

the ability of people to give you good advice depends on the quality of information you supply


Dave

 

[Mike Taylor]

.



Sir Mike . . . . .

Hmmmmmmm, I just now caught your short, add on snippet . . . I had previously overlooked.

A LM417 . . . . ? ? ?

I certainly am familiar with the :

LM117
of Aerospace and Mil spec qualifications and its sapphire substrate and liberal use of gold and platinum in its construction.

LM217
a lesser "117 wannabe" but well within its commercial and industrial specs.

LM317
The general purpose unit that you and I can afford, and find it suiting the purpose, and serving us well.

BUT now, the . . . . .
LM417
With no known available data, it indeed must be a dysfunctional unit of such origin as you just mentioned.

If I ever find any 350's at flea markets-Hamventions, etc. they are being in the metal to-3 cases and I snap them right up !

Cascade two of them and you can therewith, make one FINE variable voltage regulated AND variable current limited linear power supply.






73's de Edd


.

Y bad.

.



Sir Mike . . . . .

We confirm the components that you utilized, along with the somewhat uber- healthy selection of the 22K Ufd's of initial raw DC filtering.

In using our Right Honorable and Esteemed "bigone5500"'s reference schematic, but, with our yet not FULLY knowing YOUR circuitry utilized:

The utilization of secondary C2 and C3 electrolytics are of definite importance.
With manufacturers harping on utilization tantalum types for C3, but I have never found that "cut in stone", with the possiblity of squirrelly operation at some voltage and current nodes.

Also, the selection of the value of R1, as it is directly related to the current limiting and foldback aspect of the 350's internal regulatory circuitry.

Get it too low of a value, and some overiding constant current mode tendencies will start showing up .

Your transformers voltage and current capability spec certainly seems sound, since you are dropping from a hefty raw 12 ++++++ volts all the way down to a 6 VDC level.

But, for further analysis, it would be nice to have feedback of the RAW DC voltage coming into the 350 at sample points of 100 ma-500 ma and your mentioned 800 ma and then at 1 amp . . . and 2 and 3 amps current draw, if the latter 3 levels are now being attainable.

The mention of power dissipation at the full voltage and current condition, and stress on the 350, can be scaled down by the shunting of the input to output terminals of the 350 with a fixed power resistor.

Its ohmmic value is being selected so that half of the power passage current will be will divided beween it and the 350.
With the 350 then saying:
" Wheeeeew . . . .thanks !

The only last thing in mind, might be the quality of the power transformer.

If I had a quality old school Thordarson or Triad power transformer in that circuit . . . . I could expect the FULL rectified raw DC voltage not to drop from that value down to a 12 VDC @ 3A draw. (With it never exceeding a >20 % difference.)

When I evaluate a Radio Shack or other "over the waters" transformers, they come up far short of that spec along with their running hotter.

Standing by for more info from you . . . .




73's de Edd


.

my bad. I have fat fingers. Meant to say LM317.

I have examined by circuit closely. The voltage regulator is acting just like a 317 leaving me to believe that my new package of 10 from China were just miss-labeled. 22,000 mfd was all I could find in my bin. I aim to replace it with a 4700 mfd. no biggie. I just ordered 10 TI LM350T. Mike

 

[rickselectricalprojects]

i know that with a lm317 you have to put a 0.1 uf cap on the input of the regulator (the 0.1uf cap isn't the filter capacitor though) and a 1 uf capacitor on the output of the regulator

 

[davenn]

yup that's correct and the same goes for pretty much all linear regulators

 

[eetech00]

As "dark side" said, you need the capacitors..
double check the pinouts of the LM317, maybe connections are transposed....

Are you sure R1 is 120 ohms? Is it maybe something much less?
Is there actually a connection from "adj" pin thru the pot to gnd?
If this connection is missing it will behave like a current limiter..

 

[Luis Costa]

but does this actually divide voltage while providing an amp-limited voltage? i've been trying that but the fact that the formula Vout=1.25.(1+R2/R1)+Iadj.R2 is scientifically proven true i'm prolly wrong... Can somone explain how to get and output of 6v/200mA from a 12v source using LM317?
Thanks in advance guys.

 

[Luis Costa]

meaning what... well, if it was about a usual voltage divider, and assuming of course the components could handle the power variation the first resistor would limit current right? Doesnt seem to happen with regulators, Anways,... doesnt source voltage come into the equation? is it the Iadj.R2?

 

[davenn]

but does this actually divide voltage while providing an amp-limited voltage?

no, in the circuit above its shows as voltage regulator the current supplied is set by the load up to the max the chip can supply ( 1.5A ( less at the higher voltage range of the chip))

if you want a constant current supply then the chip is wired differently ... see the datasheet
http://www.ti.com/lit/ds/symlink/lm117.pdf

Can somone explain how to get and output of 6v/200mA from a 12v source using LM317?

again see the datasheet

 

[davenn]

meaning what... well, if it was about a usual voltage divider, and assuming of course the components could handle the power variation the first resistor would limit current right?

No, for a 2 resistor voltage divider, any current that the load requires is also going to have to run through first resistor ( call it R1). The current and voltage across R1 produce power, which is dissipated in the form of heat. If that power exceeds the rating of the resistor (usually between ⅛W and 1W), the heat begins to become a major problem, potentially melting the poor resistor.

DONT use 2 resistor voltage dividers to supply any significant current ... anything over 1A (preferably less)

? Doesnt seem to happen with regulators, Anways,... doesnt source voltage come into the equation? is it the Iadj.R2?

the source voltage needs to be at least 1.25V higher than the required output voltage

again, read the datasheet to understand the theory of operation of the LM317 or other linear regulators


Dave

 

[Luis Costa]

thanks dave and guys