power supply

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Kraglaan

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I am building a power supply for a train set. The PS has 3 O/P, 1 12V ±3A and 2 variable 12V ±1.5A suplies. Now the problem I am getting is that I am not able to supply the required current, as soon as I connect the required load the O/P voltage drops from 12V to 6V. I tried connecting a power transistor to supply the extra current but when it is turned on I only get the I/P voltage to the regulator at the O/P? I would like to know how I can get the extra current if there is a way. The regulator that I am sing is LM317, smoothing cap form rectifier is 3300uF O/P smoothing cap is 1000uF. For the static 12V R1 = 82hm: R2 = 820hm: and for the adjustable: R1 = 500hm: R2 = 5khm: would appreciate any advice as how to resolve this.

This is sort of what i want to use, it's just the basic layout of the circuit.
**broken link removed**

These are the circuits i used as refrence to try and increase the current.


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Last edited:

Hello Kraglaan
What you've typed doesn't really make much sense. Please post a schematic to clear things up. Make sure to include transformer ratings.
 
R1 should not be more than 120 ohms for an LM317 and not more than 240 ohms for an LM117 or the output voltage will rise without a load. It can be less if you want.
 

Well you could buffer it, but you would lose the over current protection. The LM317 can only supply 1.5A.

On the other hand you would get better low speed torque if you PWM it instead.

D.
 
What sort of load are you using?

The LM317 can't supply 3A (unless you're very lucky) the current rating will go down as the input-output differential increases.
 
"Thanks to ImageShack for Free Image Hosting."
Why upload to ImageShack? Then we must wait for them to wake up. Upload your schematics here with your posts.
 
They're not easy to find, but there are plenty of 'clip board' web sites out there that will temporarily cache files of almost any type for a short time (it's usually a que it's there till it gets pushed out)
 
If the power supply is capable of 6 amps (12V@3A + 2X12V@1.5A) then connecting the outputs thru diodes will fix the problem. But, if the supply is only capable of 3 amps (12V@3A or 2X12V@1.5A), you are stuck and need a better supply. Adding a transistor cannot increase the current.
 
Diodes aren't good idea because of the associated voltage drop; adding a booster transistor is normally the preferred option.
 
LM317 can provide a max of 1 amp...no question of getting 3amp.

3 amp regulator has to be made from scratch.

also for proper functioning of 317regulator..Vin-Vout should at least be 3volts..
 
jeevee said:
LM317 can provide a max of 1 amp.
Click to expand...
No. It is guaranteed to provide 1.5A if the Vin-Vout is 15V or less.

3 amp regulator has to be made from scratch.
Click to expand...
No. An LM350 provides 3A and an LM358 provides 5A with a restriction on Vin-Vout.
 
Thks for the help so far.

The thing is that i tried to use the booster transistor from the pdf file

but instead of using the lm195 I tried to use a FET now i am not sure if the result should be the same or diffrent but i thought that they should function in the same way. After the FET did not work I tried to use a diffrent power transistor similar to that of the lm195 and still the input was just taken straight through to the O/P so I don't get how the booster configuration can only supply more current and not act like a dirrect sort from I/P to O/P.

Can you explain to me why I can't use more than 120hm: for R1? Also would diodes not work if you bridge them accros the LM317 and change the resistor values accordingly?
 

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Kraglaan said:
but instead of using the lm195 I tried to use a FET now i am not sure if the result should be the same or diffrent but i thought that they should function in the same way.
Click to expand...
A Mosfet is completely different to a transistor. It needs about 5V to 10V from its gate to its source to conduct. A transistor needs only 0.7V from its base to its emitter.

I don't get how the booster configuration can only supply more current and not act like a dirrect sort from I/P to O/P.
Click to expand...
When more than 30mA flows through the 22 ohm resistor into the LM317, the PNP transistor turns on and turns on the LM195 transistors. When enough current flows in the LM195 transistors then less current flows in the LM317 and it turns off a little, enough to regulate the output current.

Can you explain to me why I can't use more than 120hm: for R1?
Click to expand...
The operating current of the LM317 circuit flows to its output pin. It max is 10mA for an LM317. If R1 is 120 ohm then it is a load on the output of the LM317. Its current is 1.25V/120 ohms= 10.4mA. If the operating current is 10mA and you use a resistor value higher than 120 ohms for R1 then the output voltage will rise when the external load current is less than a few mA.

The max operating current of the more expensive LM117 is half at only 5mA so a 240 ohm resistor can be used.

Also would diodes not work if you bridge them accros the LM317 and change the resistor values accordingly?
Click to expand...
The resistor values determine the output voltage. Diodes connected in reverse protect the IC from capacitor discharge damage if the input or output is shorted to ground.
 
"as soon as I connect the required load the O/P voltage drops from 12V to 6V. "
You can't get blood from a stone: if the supply can only put out 3 amps, no amount of booster transistors will increase it.
 
Its not the supply that experiances a drop in voltage but the regulator the booster transistor supplies the extra current from the supply. The input to the regulator stays unchanged only the regulator output drops in voltage with no booster transistors connected, the process is explained in audiogurus last post.
 
audioguru said:
A Mosfet is completely different to a transistor. It needs about 5V to 10V from its gate to its source to conduct. A transistor needs only 0.7V from its base to its emitter.
Click to expand...
That varies from FET to FET. Older common FETs often have a threshold of 3V, are saturated at 10V, and breakdown of 20Vgs.

There are FETs that are rated as saturated power switches at 2.5Vgs with a threshold of 0.5V and a breakdown of 8Vgs.

D.
 
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