# Minimum load current of LM317 Resistor

#### ericgibbs

Joined Jan 29, 2010
18,851
hi mike,
I would say they are within an acceptable range, OK...

BD250F 0.352V/0.22R = 1.6A
TIP36CW 0.355V/0.22R =1.61A
TIP36CW 0.349V/0.22R = 1.58A
TIP36CW 0.354V/0.22R = 1.6A

Total 6.39Amps

Another check I would make for any power supply is to check the heating of each power transistor, thermometer or 'touch'...
Using maximum load on the Vout.

E

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#### ArakelTheDragon

Joined Nov 18, 2016
1,362
The voltage drops are the following:
BD250F 0.352V
TIP36CW 0.355V
TIP36CW 0.349V
TIP36CW 0.354V

The principle is the same for both the base and 0.22R. Theoretically replacing the transistor might work But you need to check at different loads, load voltage and current, as bipolar transistors have a different beta on different loads.

#### mike69

Joined Dec 22, 2012
83
What is the difference that should be between the output voltage without load and with about 6A load at 19.74V ?
I saw that if I set the output voltage at 19.74V and after connecting a load of about 6A, the voltage drops to 19.70V. Is that normal ?
The voltages were measured at the marked locations from the attached drawing.

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#### ArakelTheDragon

Joined Nov 18, 2016
1,362
What is the difference that should be between the output voltage without load and with about 6A load at 19.74V ?
I saw that if I set the output voltage at 19.74V and after connecting a load of about 6A, the voltage drops to 19.70V. Is that normal ?
The voltages were measured at the marked locations from the attached drawing.
its normal, but 0.04V do no matter. If its a bigger voltage drop, than its a problem. What was the load, a battery?

#### ericgibbs

Joined Jan 29, 2010
18,851
hi mike,
I would say for that basic LM317 with bypass transistors thats quite good.
E

#### mike69

Joined Dec 22, 2012
83
Regarding the schematic from post #14, it is possible for this circuit to work normally due to LM317AHVT malfunction or if the LM317AHVT is defective ?
And it is possible for this circuit to operate abnormally if I change the defective LM317AHVT with a new and good one ?

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#### ericgibbs

Joined Jan 29, 2010
18,851
hi mike,
How is it working abnormally, what are the symptoms.?
E

#### mike69

Joined Dec 22, 2012
83
Hi Eric,
I tested the circuit in the last 2 days and I found that it is working normally.
I used a single LM317AHVT, so I am wondering if the LM was defective could it operate normally ?

PS: let's say that there was a short circuit between the input and the output of the LM317AHVT and it is not working correctly anymore.

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#### ericgibbs

Joined Jan 29, 2010
18,851
hi,
If the LM317 was not working correctly you would not be able to regulate/set the output voltage.
also if there was an input/output short.
E

#### mike69

Joined Dec 22, 2012
83
So if the output voltage (measured with multimeter) was stable during load and stable without load and if the voltage could be varied between min (1.25V) and max (about 28V) then the LM317 should be good ?

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#### ericgibbs

Joined Jan 29, 2010
18,851
Typically it is usual to carry out a more comprehensive test, eg: if you tried a range of output voltages and load currents.
Compare them with the expected calculated results.
Keep a record for future reference.
E

#### mike69

Joined Dec 22, 2012
83
Yes, I tested with 26.0V (without load) and with 7.26A load and the voltage during load was 26.0V.
And I tested with 19.96V (without load) and with 6.26A load and the voltage during load was 19.95V.
I hope it is ok.

#### ericgibbs

Joined Jan 29, 2010
18,851
If that was my PSU I would be satisfied with that result.
Do you have a scope.?
E

#### mike69

Joined Dec 22, 2012
83
Yes, I have a scope, but I only know to do simple tests with the scope and I am not experimented using the scope.

Also, I have done some tests with this power supply, but using another LM317AHVT. The tests were done a few months ago and I did not found any problems. The screenshots (from the scope) from a few months ago were seen by some specialists and they said that the power supply is working good.

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#### mike69

Joined Dec 22, 2012
83
It is acceptable if the difference between the current that flows through the transistors is as follows ?
0.345 V/0.22 R = 1.56 A
0.352 V/0.22 R = 1.6 A
0.349 V/0.22 R = 1.58 A
0.354 V/0.22 R = 1.6 A

#### ericgibbs

Joined Jan 29, 2010
18,851
hi,
Those values are OK.
E

#### mike69

Joined Dec 22, 2012
83
I am encountering the following problem: sometimes, the voltage at the output of the attached schematic drops from 19.87V to 17.87V and sometimes it drops from 26.1V to 24.3V. This happens in both situations: with load and without load (the load is about 1.5A). I checked the input voltage and was ok in both situations with normal voltage at the output and with the lower voltage at the output.
I soldered the wires that came from the pot to the PCB and the voltage still dropped. Initially I was using a screw connector.
I replaced the 2k/2w pot with a new one and the problem disappeared.
I took the initial pot and I connected it to another power supply, built using the same (attached) schematic and the voltage was good, no drops.
I reconnected the initial pot back to the first power supply and it worked good.
I did not managed to put the scope probe on the output of the power supply while the voltage was lower. I managed only to check with the scope while on the output was a normal voltage.

Why the voltage dropped ?
Are there any problems that I cannot see ?

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#### ericgibbs

Joined Jan 29, 2010
18,851
hi mike,
Its hard to say what the problem was, sometimes you can get a 'noisy' pot that can cause a jump in output at certain points on its rotation.
E
By 'noisy' the wiper to track in the pot maybe making a loose/poor contact

#### mike69

Joined Dec 22, 2012
83
I measured the voltages on the output while the output voltage was 17.87V instead of 19.87V and I found that the voltage on the 100R resistor was 1.25V while the voltage on the potentiometer was lower (I think 16.xx V) in comparation with the moment when the output was 19.87V.
I used a multiturn potentiometer.

How can I check to see what component is causing those problems ?

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