I was looking around for some parts for an adjustable 1.5v-30vDC regulated 1-amp bench testing power-supply and I came up with a some easy & low cost parts.

I wanted to buy an Lm317 IC regulator and a few other required parts. ( although most of us already have the extra parts laying around)

But I found ALL the parts in a nice little kit including a printed circuit board to build it on for only $13.
Then... I also lucked-out and found a "neat" 2 amp transformer for it for only $2.88.

The entire thing went together in minuets and had I bought all the parts separately I don't think it would be much cheaper.

The nice thing about the transformer is that it is a 25v CT.

12.5 - 0 - 12.5

The 25 volts AC output from the transformer kicks up at the input filter enough to get about 30vdc out nicely if needed. And with a proper heat sink the LM317 should run cool.

Now since the Lm317 is a "linear" regulator it normally has to dissipate all the extra wattage for the excess voltage not used should you decide to adjust the supply way down to about 5 volts or less.

So In order to use a reasonable sized heat-sink, I realized you can put a switch on the center tap so that when you want to keep the output below 15 volts you can switch in only one 12.5v AC winding and if the output is going to be over 15 volts you can switch it so that both windings are used to total 25vac.

This should significantly reduce any wasted heat dissipation and strain on the Lm317 at the lower voltages.

Here's the cheap parts:
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=129-035

http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=320-216

This might be a nice DC supply for supplying DC tube filaments.

I assume that with a heat sink and the lower AC voltage, it could supply more than 1A?
Doug

:I was looking around for some parts for an adjustable 1.5v-30vDC regulated 1-amp bench testing power-supply and I came up with a some easy & low cost parts.
:
:I wanted to buy an Lm317 IC regulator and a few other required parts. ( although most of us already have the extra parts laying around)
:
:But I found ALL the parts in a nice little kit including a printed circuit board to build it on for only $13.
:Then... I also lucked-out and found a "neat" 2 amp transformer for it for only $2.88.
:
:The entire thing went together in minuets and had I bought all the parts separately I don't think it would be much cheaper.
:
:The nice thing about the transformer is that it is a 25v CT.
:
:12.5 - 0 - 12.5
:
:The 25 volts AC output from the transformer kicks up at the input filter enough to get about 30vdc out nicely if needed. And with a proper heat sink the LM317 should run cool.
:
:Now since the Lm317 is a "linear" regulator it normally has to dissipate all the extra wattage for the excess voltage not used should you decide to adjust the supply way down to about 5 volts or less.
:
:So In order to use a reasonable sized heat-sink, I realized you can put a switch on the center tap so that when you want to keep the output below 15 volts you can switch in only one 12.5v AC winding and if the output is going to be over 15 volts you can switch it so that both windings are used to total 25vac.
:
:This should significantly reduce any wasted heat dissipation and strain on the Lm317 at the lower voltages.
:
:Here's the cheap parts:
:http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=129-035
:
:http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=320-216
:

Doug:
...Here's an excerpt:
"There are two important things to keep in mind when working with an LM317T:

First, the internal circuitry of the LM317T can only handle 1.5 A of current as an absolute maximum. More on this below.

Secondly, and this is IMPORTANT, the LM317T is a linear regulator. This means that it dissipates the excess voltage as heat, with more heat produced as the voltage is set lower and lower. You MUST have a heatsink mounted on the regulator that is capable of dissipating the heat produced.

If you need more than 1.5A of current, and have a heatsink large enough to handle the wattage
....here's some at a low cost:

http://www.futurlec.com/HeatTO220.shtml

or...
(old stereos and amplifiers are great for finding massive heat-sinks, btw), you can use an LM350, which is rated up to 3 A and is identical to the LM317 in all other respects. The LM338 is also compatible, and is rated up to 5 A."

: you can use an LM350, which is rated up to 3 A and is identical to the LM317 in all other respects. The LM338 is also compatible, and is rated up to 5 A."

Keep your circuit using the LM317 and simply add a series pass transistor. This will up the power by tenfold (if the transformer can cope with the power). Using a motorola 250W transistor for a few dollars, 2 resistors and a nice heatsink.An MJ11015 would be a good start, that is what I use.

My idea to use a 12V batt and an LM317 (heatsinked) driving a 2N3055 (heatsinked) to derive a 6V (10A)source for older radios worked great unloaded, but the rules changed with a 6A load. Wanted a continuously variable 6-12V source derived from hardware on hand, a 12V batt and charger. Found a LT1083 variable regulator (7.5A) in my junk box that can be easily paralled for 15A. Film at 11

marv

:: you can use an LM350, which is rated up to 3 A and is identical to the LM317 in all other respects. The LM338 is also compatible, and is rated up to 5 A."
:
:Keep your circuit using the LM317 and simply add a series pass transistor. This will up the power by tenfold (if the transformer can cope with the power). Using a motorola 250W transistor for a few dollars, 2 resistors and a nice heatsink.An MJ11015 would be a good start, that is what I use.

Here you go Marv:
http://www.circuit-projects.com/cimg/LM317_high_current_regulated_power_supply.gif

:Here you go Marv:
:http://www.circuit-projects.com/cimg/LM317_high_current_regulated_power_supply.gif
:
High Current Regulated Supply
The high current regulator below uses an additional winding or a separate transformer to supply power for the LM317 regulator so that the pass transistors can operate closer to saturation and improve efficiency. For good efficiency the voltage at the collectors of the two parallel 2N3055 pass transistors should be close to the output voltage. The LM317 requires a couple extra volts on the input side, plus the emitter/base drop of the 3055s, plus whatever is lost across the (0.1 ohm) equalizing resistors (1volt at 10 amps), so a separate transformer and rectifier/filter circuit is used that is a few volts higher than the output voltage. The LM317 will provide over 1 amp of current to drive the bases of the pass transistors and assuming a gain of 10 the combination should deliver 15 amps or more. The LM317 always operates with a voltage difference of 1.2 between the output terminal and adjustment terminal and requires a minimum load of 10mA, so a 75 ohm resistor was chosen which will draw (1.2/75 = 16mA). This same current flows through the emitter resistor of the 2N3904 which produces about a 1 volt drop across the 62 ohm resistor and 1.7 volts at the base. The output voltage is set with the voltage divider (1K/560) so that 1.7 volts is applied to the 3904 base when the output is 5 volts. For 13 volt operation, the 1K resistor could be adjusted to around 3.6K. The regulator has no output short circuit protection so the output probably should be fused.

More info here:
http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm

The LM317T output current can be increased by using an additional power transistor to share a portion of the total current. The amount of current sharing is established with a resistor placed in series with the 317 input and a resistor placed in series with the emitter of the pass transistor. In the figure below, the pass transistor will start conducting when the LM317 current reaches about 1 amp, due to the voltage drop across the 0.7 ohm resistor. Current limiting occurs at about 2 amps for the LM317 which will drop about 1.4 volts across the 0.7 ohm resistor and produce a 700 millivolt drop across the 0.3 ohm emitter resistor. Thus the total current is limited to about 2+ (.7/.3) = 4.3 amps. The input voltage will need to be about 5.5 volts greater than the output at full load and heat dissipation at full load would be about 23 watts, so a fairly large heat sink may be needed for both the regulator and pass transistor. The filter capacitor size can be approximated from C=IT/E where I is the current, T is the half cycle time (8.33 mS at 60 Hertz), and E is the fall in voltage that will occur during one half cycle. To keep the ripple voltage below 1 volt at 4.3 amps, a 36,000 uF or greater filter capacitor is needed. The power transformer should be large enough so that the peak input voltage to the regulator remains 5.5 volts above the output at full load, or 17.5 volts for a 12 volt output. This allows for a 3 volt drop across the regulator, plus a 1.5 volt drop across the series resistor (0.7 ohm), and 1 volt of ripple produced by the filter capacitor. A larger filter capacitor will reduce the input requirements, but not much.

:The LM317T output current can be increased by using an additional power transistor to share a portion of the total current. The amount of current sharing is established with a resistor placed in series with the 317 input and a resistor placed in series with the emitter of the pass transistor. In the figure below, the pass transistor will start conducting when the LM317 current reaches about 1 amp, due to the voltage drop across the 0.7 ohm resistor. Current limiting occurs at about 2 amps for the LM317 which will drop about 1.4 volts across the 0.7 ohm resistor and produce a 700 millivolt drop across the 0.3 ohm emitter resistor. Thus the total current is limited to about 2+ (.7/.3) = 4.3 amps. The input voltage will need to be about 5.5 volts greater than the output at full load and heat dissipation at full load would be about 23 watts, so a fairly large heat sink may be needed for both the regulator and pass transistor. The filter capacitor size can be approximated from C=IT/E where I is the current, T is the half cycle time (8.33 mS at 60 Hertz), and E is the fall in voltage that will occur during one half cycle. To keep the ripple voltage below 1 volt at 4.3 amps, a 36,000 uF or greater filter capacitor is needed. The power transformer should be large enough so that the peak input voltage to the regulator remains 5.5 volts above the output at full load, or 17.5 volts for a 12 volt output. This allows for a 3 volt drop across the regulator, plus a 1.5 volt drop across the series resistor (0.7 ohm), and 1 volt of ripple produced by the filter capacitor. A larger filter capacitor will reduce the input requirements, but not much.
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