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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 22nd Sep 2019, 11:40 am   #21
Jolly 7
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Default Re: Maximum input voltage for LM317T ?

Quote:
If you do need to use your existing transformer, what about using the 16V output into a voltage-doubler? Personally, I rather like voltage-doublers in these sorts of low-power applicatiions because you can arrange for the doubler-capacitors to have values that mean the output-voltage falls-off rapidly once you reach the design maximum-current output which means that under overload the regulator never actually goes into current-limit, and the whole thing can sit there in any degree of 'overload' without any parts being overstressed.

Thanks for the suggestion. I have made a voltage doubler using two 1N4007 diodes and two 100uF 50V capacitors. But the AC voltage is not constant on my multimeter, it is 'swinging'. How do I convert this efficiently to DC ? Is it OK to use a full bridge rectifier and a smoothing capacitor ?

Last edited by Jolly 7; 22nd Sep 2019 at 11:41 am. Reason: Additional details
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Old 22nd Sep 2019, 12:04 pm   #22
paulsherwin
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Default Re: Maximum input voltage for LM317T ?

100uF seems very low for a voltage doubler.

Unless I've missed it somewhere, you haven't said what the current requirement is. This greatly affects your options.
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Old 22nd Sep 2019, 12:16 pm   #23
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Default Re: Maximum input voltage for LM317T ?

The no-load current of my BLDC motor is stated as 0.14A at 12V or 0.17A at 24V. The AC voltage doubler circuits using 100 uf caps are widely available on the Net, but I will be grateful if a better circuit can be suggested. The motor is not the only device I intend to run on my variable regulated power supply, but is probably the one that will be using the maximum current as per above.
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Old 22nd Sep 2019, 5:50 pm   #24
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Default Re: Maximum input voltage for LM317T ?

Did I miss something somewhere?
I understand there are 4 individual windings?
You can expand on Heralds idea, if you series connect the 6 volt and 16 volt windings you have 22 volts AC. That will get you about 31 or 32 volts off load DC. Well within the 37 volt max. (You could try the 10 and 16 volt windings, that gets you to 37 volts DC...)

All you need to do is make sure the two windings you choose are in phase - easy just test the results with your meter on AC. If you pair the 6 an 16 volt ones make sure you get 22 volts (correct) not 10-12 volts (wrong)... ;-)

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Old 22nd Sep 2019, 8:30 pm   #25
Jolly 7
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Default Re: Maximum input voltage for LM317T ?

@Alan : Thank you for making this clearer. I now have 23V AC by connecting the two 6V and 16V AC windings as advised. I have a selection of bridge rectifiers, but was wondering what value of smoothing capacitor should I use. I have seen 1000uF being used in old wallwarts, I have a couple of caps but was wondering is this a universal value to cover every possible device that might be connected to the power supply ? Should I also select a 50V rated one as a minimum ?
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Old 22nd Sep 2019, 8:37 pm   #26
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Default Re: Maximum input voltage for LM317T ?

1000uF is a pretty typical value. You don't need to worry too much about hum if you are only driving a motor, so 470uF or even less may be acceptable here. There is no harm in using 2000uF or even more.

50V would be sensible, though you might get away with 40V.
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Old 22nd Sep 2019, 8:39 pm   #27
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Default Re: Maximum input voltage for LM317T ?

Here's a picture of the transformer tappings.
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Old 22nd Sep 2019, 9:03 pm   #28
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Default Re: Maximum input voltage for LM317T ?

Bearing in mind that your motor will draw higher currents under load are you confident that the transformer windings are capable of handling those currents? See various earlier posts regarding transformer specs.

Also, I don't understand the difficulty you encountered with the voltage doubler circuit mentioned in one of your posts. A correctly wired doubler with two diodes and two capacitors should output full wave rectified DC.

Alan
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Old 23rd Sep 2019, 10:13 pm   #29
Jolly 7
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Default Re: Maximum input voltage for LM317T ?

@ajgriff: I don't have the current specs for the transformer and I don't exactly remember which VCR I pulled it out of. Till my LM317 regulator arrives on the slow boat from China or I build one myself, I wont really know for sure. However, I was able to feed a 7806 IC from the rectified 37V DC and got 6V with enough current to run a 6V motor at full torque.
I am not sure why my voltage doubler didn't work, will need to investigate at a later date. Thanks though for the interesting idea.

Last edited by Jolly 7; 23rd Sep 2019 at 10:26 pm.
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Old 24th Sep 2019, 3:07 pm   #30
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Default Re: Maximum input voltage for LM317T ?

Then there is the Texas Instruments TL783 which is an LM317-like device but capable of handling up to 125V.
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