LM317 Power Supply

Thread Starter

RodneyB

Joined Apr 28, 2012
697
I have built this power supply and its only going to 11.6 volts. I have a 24 Volt transformer. I wanted to set it to 16 Volts. I cant see where I have gone wrong
 

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bertus

Joined Apr 5, 2008
20,041
Hello,

What will be the load of the powersupply?
C2 is rather small and R1 should be 120 Ohms for a LM317.
For a LM117 the 240 Ohms would be OK.

Bertus
 

MikeML

Joined Oct 2, 2009
5,444
Wait until the battery is fully charged, and then measure the final voltage. If the battery is initially discharged (or is bad), then the design is current-limited, and the voltage will not come up until the battery stops drawing current. An SLA or flooded-cell 12V battery will never reach 16V.

C1 is in the wrong place. If used at all (not really necessary here), it should be between pin 2 and pin 1.
 
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MikeML

Joined Oct 2, 2009
5,444
Hello,

What will be the load of the powersupply?
C2 is rather small and R1 should be 120 Ohms for a LM317.
For a LM117 the 240 Ohms would be OK.

Bertus
Bertus, that is an issue only if the supply is unloaded. This one always has a minimum load (LED and battery) in parallel with the feedback sensing resistor.... R1 can be anything up to ~1K
 

Thread Starter

RodneyB

Joined Apr 28, 2012
697
Hello,

What will be the load of the powersupply?
C2 is rather small and R1 should be 120 Ohms for a LM317.
For a LM117 the 240 Ohms would be OK.

Bertus
The load is 70mA. I have not got C2 connected at the moment. I am not sure if connecting the 2 windings is correct. The model transformer I have is 042-5052.0
 

Thread Starter

RodneyB

Joined Apr 28, 2012
697
Wait until the battery is fully charged, and then measure the final voltage. If the battery is initially discharged (or is bad), then the design is current-limited, and the voltage will not come up until the battery stops drawing current. An SLA or flooded-cell 12V battery will never reach 16V.

C1 is in the wrong place. If used at all (not really necessary here), it should be between pin 2 and pin 1.
I don't have the battery connected at the moment. I was trying to set the output voltage first but it will not go over 11 volts.
 

MikeML

Joined Oct 2, 2009
5,444
If the transformer is wired right, the voltage at J1 should be about 35Vdc with some ripple. Are you sure the LM317 is wired right? Does it get hot?
 

MikeML

Joined Oct 2, 2009
5,444
Without C2 this circuit won't work properly.
C1 might be needed for stability only if the charger is operated without the battery. It needs to be moved so that it is directly across the 317's pins 2 and 1.
 
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gerty

Joined Aug 30, 2007
1,250
What is the AC voltage into the bridge?
What is the dc voltage out of the bridge?
Is the voltage adjustable adjustable ?
 

Thread Starter

RodneyB

Joined Apr 28, 2012
697
I have fitted a 1000uf capacitor for C1. The circuit is now working properly.

I am using transformer 042-5213.0 between pins 6 and 10 I have 21 Volts and I have bridged pins 7 and 9.

What should the correct size of capacitor C1 be and how do you calculate it.
 

AnalogKid

Joined Aug 1, 2013
8,100
Without C1 were you measuring the output voltage with a DMM? If so, it lied as all DMM's do. Without C1, there is full-wave rectified AC going through everything, not DC. This is not a problem if that's what you want and you know how to use it.

With C1 installed, what is the voltage across C1 and what is the voltage on LM317 pin 2?

ak

EDIT - I meant C2, the input bulk filter capacitor.
 
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MikeML

Joined Oct 2, 2009
5,444
...What should the correct size of capacitor C1 be and how do you calculate it.
To properly calculate C2, here is what you need to know:

The rms voltage of the secondary winding.
The effective winding resistance.
The forward voltage drop of the rectifiers under load
The maximum output voltage out of the 317.
The maximum load current out of the 317 at the highest output voltage (above).
The drop-out voltage of the 317.

Give me all of these, and I will show you how...
 
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Thread Starter

RodneyB

Joined Apr 28, 2012
697
Without C1 were you measuring the output voltage with a DMM? If so, it lied as all DMM's do. Without C1, there is full-wave rectified AC going through everything, not DC. This is not a problem if that's what you want and you know how to use it.

With C1 installed, what is the voltage across C1 and what is the voltage on LM317 pin 2?

ak
The Voltage across C1 is 32.8Volts and the voltage on pin 2 is 16.1 Volts
 

MikeML

Joined Oct 2, 2009
5,444
But C2 is a input bulk capacitor not C1, you have a typo.
Me and everybody else screwed this up. As posted in the original schematic, C2 is the (input) filter capacitor, and C1 is the LM317 (output) anti-oscillation capacitor.

Everything I said about the output capacitor should have referred to C1. I went and edited my previous posts... now my references to C1 and C2 agree with the schematic.
 

Silvio

Joined Jul 2, 2015
14
Is it possible that you got the multi turn pot wired the wrong way? If its a bourns pot the wiper is the pin on the back.
 

Thread Starter

RodneyB

Joined Apr 28, 2012
697
To properly calculate C2, here is what you need to know:

The rms voltage of the secondary winding.
The effective winding resistance.
The forward voltage drop of the rectifiers under load
The maximum output voltage out of the 317.
The maximum load current out of the 317 at the highest output voltage (above).
The drop-out voltage of the 317.

Give me all of these, and I will show you how...

The RMS voltage rating of the secondary winding is 30.8 Volts
The effective winding resistance is 28.9 ohms
The forward voltage drop of the rectifier under load is 1Amp
The maximum load current out of the LM317 is 70mA
The maximum load Voltage 16.00 Volts

I don't know what drop out voltage is?
 
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