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What should be the maximum current and voltage for an LM317?

I have a transformer, which has the rating, primary: 220V Secondary:42V, 7A. Can I use this to make a variable power supply using an LM317 adjustable voltage regulator? I mean, can I put the rectified voltage from this transformer as input of an LM317? If not, what should I do with this transformer to design a power supply using the LM317?

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  • \$\begingroup\$ Welcome to EE.SE! Your question is a bit broad and you will get better answers if you make it more specific. For example, what have you learned from the datasheet and exactly what part of it is confusing? What do you know about rectifiers and transformers? \$\endgroup\$
    – Joe Hass
    Dec 24, 2013 at 21:47

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LM317 has no max input because there is no ground connected directly to the regulator, there is only a max allowed IN/OUT voltage difference which is 40v, if you can keep restriction and the voltage difference doesn't go over that then the regulator will be fine.

An alternative is LM317HV which is the high voltage version, it has a max allowed IN/OUT voltage difference of 60v

Current is really not an issue, having a source that can provide more current is not a problem but you will only be able to get about 1.5A from the regulator assuming that you can provide sufficient cooling.

Another idea is to use a pre-regulator, that can be a transistor with a zener to reduce the voltage supplied to the LM317, something like

enter image description here

I assume you are aware that in order to feed the regulator you need to rectify the AC output of your transformer to DC using a rectifier bridge and a capacitor.


I'm tempted to suggest the use of a switching regulator that is very efficient compared to a linear regulator like LM317.
You can easily find a ready board like the following (in ebay) using a LM2596 that can provide up to 3A with minimum power consumption which means less wasted power and less heat. The only problem is that you have to use the pre-regulator I have described or another method to lower the voltage at about 40v (or lower) in order to feed the switching regulator because there is a max input limit of 45v.

enter image description here


Regarding the pre-regultor solution, you can check this application note High Voltage Adjustable Power Supplies - Texas Instruments.
It uses a tracking pre-regulator that keeps the IN/OUT difference of LM317 constant at 5v and although the specific design is for high input voltage and low output current it can give you ideas of how to proceed with your project.

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  • \$\begingroup\$ If one were to use e.g. the 40-volt part to try to convert 60V to 30V, wouldn't one risk frying it if the output current exceeded the device's capabilities even briefly? \$\endgroup\$
    – supercat
    Dec 25, 2013 at 4:12
  • \$\begingroup\$ @supercat When the output protection kicks in then the output will drop to very low levels so the IN/OUT voltage difference with rise above the specified max and will damage the regulator. It seems that using a high voltage input practically renders the current protection useless because if needed will destroy the device UNLESS there is a proper pre-regulator circuit that will drop the input voltage too. I has attached a link of such a solution in the reply. \$\endgroup\$
    – alexan_e
    Dec 25, 2013 at 10:02
  • \$\begingroup\$ I think the solution might be better if the anode of the zener connected to the output of the 317, so the supply voltage feeding the 317 would be limited to some number of volts above its output. That would allow the 317 to be used to produce e.g. a 100-volt rail from a 120-volt supply [if the overcurrent trips, the input to the 317 would sag]. The transistor wouldn't dissipate much heat except when the 317 was overloaded, but would have to be sized to survive such a condition. \$\endgroup\$
    – supercat
    Dec 25, 2013 at 19:49
  • \$\begingroup\$ @supercat (unless I'm mistaken) what you describe is a tracking pre-regulator, a similar topology is shown in the application note I have attached at the end of my reply \$\endgroup\$
    – alexan_e
    Dec 25, 2013 at 19:59
  • \$\begingroup\$ I misread your comment slightly--I thought you'd been saying the displayed schematic was the solution; I hadn't realized the solution was "only" a link. \$\endgroup\$
    – supercat
    Dec 25, 2013 at 20:06
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Page one of the datasheet explains a lot. Here is a link to Fairchild's part

40 volts max input/output differential, 2.2 amps max output current. Yes you can use it to make a variable power supply with the LM317 but it is a waste of a good transformer becuase you can not use its full power output capabilities. You will also not be able to go to very low voltages because of the input/output voltage differential.

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  • \$\begingroup\$ You mention 40V max input/output, yet the transformer is 42V (hence up to 59V DC on the elco). This requires some explanation! \$\endgroup\$ Dec 26, 2013 at 17:25

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