LM317 regulator question - Page 1
Forum section: electronics / projects

author: wilfred
author_rank: Super Contributor
timestamp: January 20, 2012, 12:18:04 pm
content: /.

author: Armin_Balija
author_rank: Regular Contributor
timestamp: January 20, 2012, 12:33:33 pm
content: From what I can gather yes, since you're going to be using a higher voltage transformer, because of the inefficiency of linear power supplies and regulators your LM317 is going to be wasting a lot of excessive power. Overheating might be an issue depending on the output you want and it might even cause an automatic thermal shutdown if the LM317 has one integrated into it (haven't checked the datasheet I just know some IC's do have this feature.) Either way, I tend to stay close as close to the dropout voltage as I can of my regulators so I'm not wasting any power and creating excess heat. Of course you know that putting things like capacitors and other components near your regulator can be bad if your regulator is producing excess heat. It can diminish the lifetime of the components.Anyway, I'm sure there's a solution for this, It's 7:30 AM here so I've given as much as I could without falling asleep at my desk. Hope I answered somewhat of what you were looking for. Good luck!

author: IanB
author_rank: Super Contributor
timestamp: January 20, 2012, 03:25:11 pm
content: If your transformer is centre-tapped, i.e. it has a 14-0-14 secondary, then you can wire it as a 14 V transformer instead of an 18 V transformer. This will reduce the voltage drop over the regulator (but of course will reduce the maximum regulated voltage you can provide). You can even wire up both options as alternatives with a switch to select between them giving a high voltage range and a low voltage range. Another trick to reduce the burden on the regulator is to put a big power resistor in series with it to drop some of the voltage.The power dissipation requirements depend on the load current. If you tried to drop 30 V at 1 A you would have 30 W of cooling to provide. However, if you only need 250 mA that reduces to 7.5 W, a huge difference.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: January 20, 2012, 06:18:30 pm
content: The absolute maximum voltage into an LM317 is 40 volts. A 28VAC transformer will be pushing this pretty hard - sqrt(2) * 28 = 39.6 volts.The power dissipation is a bigger issue. Power = (voltage drop across the regulator) * current. Say you want a 5 volt output at 500 mA.Power = (40 - 5) * 0.5 = 17.5 watts! That's not going to work for long without a massive heat sink.

author: IanB
author_rank: Super Contributor
timestamp: January 20, 2012, 06:47:14 pm
content: Quote from: Jon Chandler on January 20, 2012, 06:18:30 pmRemember that this is a differential voltage, not an absolute voltage.If we suppose our 28 V transformer produces 30% higher when unloaded it would be 38 V. Rectifying and smoothing that would produce 38 x 1.4 = 53 V.Now if we subtract 40 V from 53 V we get a minimum regulated output voltage of 13 V before we risk killing the IC.So it does indeed mean we can't have an adjustable output voltage from 0 V to 15 V. It will be more like 14 V to 30 V.Also it must be borne in mind that the LM317 can't go down to 0 V. The lowest output voltage is 1.25 V, so even with a lower voltage transformer it will take a special circuit configuration to go all the way down to zero.

author: ahes
author_rank: Contributor
timestamp: January 20, 2012, 07:06:47 pm
content: what about using a switch mode regulator before your linear regulator?you could use a buck converter i.e if you want you output to go to 12V you can design your buck converter to produce 14 volts that's two volts higher drop on you IC's junctionanother option instead of using a regular bridge for rectification I think that you can use controlled converter " thyristors instead of diodes" adjust your firing circuit for what ever average voltage you need. I haven't tried that so I'm not sure.

author: ejeffrey
author_rank: Super Contributor
timestamp: January 20, 2012, 07:11:06 pm
content: Technically an LM317 can be used 'floating' with a high input voltage as long as the input-output voltage doesn't exceed the limit. However, you have to be careful against transient/fault conditions. For instance, if the output is shorted to ground you can get the full output voltage across the regulator.

author: IanB
author_rank: Super Contributor
timestamp: January 20, 2012, 07:13:43 pm
content: Quote from: ejeffrey on January 20, 2012, 07:11:06 pmThat's true. I didn't consider that.

author: Zero999
author_rank: Super Contributor
timestamp: January 20, 2012, 10:23:21 pm
content: Quote from: IanB on January 20, 2012, 07:13:43 pmThat's not always a problem and there are way round it, e.g. adding a chunky crowbar set at 39V across the LM317 and a fuse before the input.