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Old 17th Sep 2019, 6:37 pm   #1
Jolly 7
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Default Maximum input voltage for LM317T ?

I want to use an old mains transformer to supply a preassembled adjustable LM317T regulator and get 24V DC. However, the transformer is outputting 47V DC (34V AC pre-rectification), but the regulator cannot take more than 40V DC. How can I step down 47V to 40V DC or less to prevent damage to the LM317T ? Any ideas please ? Not sure if a simple resistor might do the trick ?
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Old 17th Sep 2019, 6:53 pm   #2
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Default Re: Maximum input voltage for LM317T ?

Isn’t the 40v actually the input output differential ? If so, you could be ok but I would be uneasy about using it in this mode. Can you drop the input (taps on the transformer maybe) ? A resistor would not be advisable unless the load current is well defined but a Vbe multiplier using a single power transistor could be used to lose a reasonably constant 10v say.

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Old 17th Sep 2019, 7:12 pm   #3
Jolly 7
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Default Re: Maximum input voltage for LM317T ?

Thanks Ken. The other outputs from this transformer are 6, 10 and 16 V AC, so maybe I could try getting 20V DC out of the 16V AC tap. A 24V DC output may not be absolutely essential for my requirements but would be nice to have.
The LM317T datasheet does mention the 40V differential, as you say, but the module I have ordered specifies a 40V max input.
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Old 17th Sep 2019, 7:16 pm   #4
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Default Re: Maximum input voltage for LM317T ?

Wouldn't a string of silicon diodes do the job? 0.7V per diode and they're cheap enough.

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Old 17th Sep 2019, 7:38 pm   #5
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Default Re: Maximum input voltage for LM317T ?

I think the 40V has to be the maximu voltage between Vin and Vout. The LM317 has no connection to the overall -ve supply rail, it works by controlling the output voltage so that the 'Adj' pin is a known voltage (which I would have to look up) below the output pin (Vout), which given the usual potential divider between Vout, Adj, and Ground gives the right voltage at Vout.

The LM317 can't 'know' what the voltage between VIn and Ground is given that it has no connection to ground.

BUT...

The problem comes if the output is short-circuited to ground. Normally an LM317 will shut down (I forget the limit current) if this happens. But if you have 47V on Vin then there will be more than 40V across the chip if the output is grounded. Which will probably damage the LM317. So I wouldn't use it like that
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Old 17th Sep 2019, 7:42 pm   #6
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Default Re: Maximum input voltage for LM317T ?

Depending on the transformer design, you may be able to unwind some secondary turns if you're worried about excess in/out voltage differential or power dissipation. Make a note of both off-load secondary voltage and the primary input voltage at the time, remove say 5 or 10 turns and note the new secondary voltage and what the primary input voltage now is (it's not at all unusual for the mains voltage to wander around a bit and you don't want to introduce an un-necessary error into your calculation). A bit of straightforward maths will give an idea of how many turns to unwind to get to the desired output. It's not an efficient way of using a transformer if you want to make a big change to output voltage but it can make a transformer that you've got anyway more useful for say a 20% reduction. Of course, quite a few transformers may be too thoroughly varnished/taped/otherwise inaccessable for getting at the secondary anyway but it's a measure I've resorted to from time to time.
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Old 17th Sep 2019, 7:53 pm   #7
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Default Re: Maximum input voltage for LM317T ?

There are details in the references for a tracking pre-regulator for reducing the input. I am not sure of the fine details as I have only thought, not used it. It uses a similar 317 for dropping the voltage to the main regulator. This would share the power dissipation. You could then use zeners across the regulators if there are possible problems? (this may be allowed for in the recommended circuitry anyway).
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Old 17th Sep 2019, 7:57 pm   #8
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Default Re: Maximum input voltage for LM317T ?

More transformer details are needed.
What are the secondaries? Is it a single winding with taps 0,6,10,16 &34V AC?
Or several windings? Some combination of taps and "buck" windings (subtracting Voltage due to anti-phase connection) should get what you want.
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Old 17th Sep 2019, 7:58 pm   #9
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Default Re: Maximum input voltage for LM317T ?

Ifv the tyransformer outputs are 0-6-10-16-34 from one secondary, you can get 24Vac between the 10 and 34 volt taps which would be a satisfactory solution giving about 31V unsmoothed dc.


Just check that 7V*required amps doesn't give any overdissipation problems for the 317 and its heatsinking (if any).
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Old 17th Sep 2019, 8:09 pm   #10
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Default Re: Maximum input voltage for LM317T ?

Quote:
Originally Posted by Jon_G4MDC View Post
More transformer details are needed.
What are the secondaries? Is it a single winding with taps 0,6,10,16 &34V AC?
Or several windings? Some combination of taps and "buck" windings (subtracting Voltage due to anti-phase connection) should get what you want.
This is an old VCR transformer I dont have the specs for. However, using my multimeter, I could ascertain that there are four coils in the secondary that are not interconnected. The four voltages mentioned earlier correspond to each of these windings.
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Old 17th Sep 2019, 9:16 pm   #11
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Default Re: Maximum input voltage for LM317T ?

A string of diodes will of course drop 0.7v each but sometimes a power transistor simulating this string can be more convenient for heatsinking, changes etc. For example , connecting a resistor value R across BE and 9R across CB will drop 10 times Vbe across CE for a wide current range. Known as a Vbe multiplier and widely used at low currents, particularly in ICs but can be useful in power circuits too.
In this case though, using a lower transformer tap is greatly preferred!
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Old 17th Sep 2019, 9:18 pm   #12
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Default Re: Maximum input voltage for LM317T ?

OK then it becomes a little complicated.
Can you see the wire gauge of them - perhaps where they come to the connection tags?

If they are all wound with the same gauge wire then more/less they can all handle the same current. These could be added or subtracted to get other Voltages.

If you have some which are wound in thick wire while others are only very fine wire then boost/buck connections are not a good thing to do.

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Old 17th Sep 2019, 9:29 pm   #13
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Default Re: Maximum input voltage for LM317T ?

My spec-sheet for then LM117/317 says 37V maximum input.

To my thinking, rather than worrying about the maximum input, look at the dissipation in the regulator. What will the maximum current (and hence the likely dissipation) be>? And what happens (how hot do things get) if you short the output so the regulator goes into current-limit?

Rather than frobbing-about with series-diodes/resistors, I'd be looking at the options for a more-suitably-rated transformer. 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.

Last edited by G6Tanuki; 17th Sep 2019 at 9:51 pm.
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Old 17th Sep 2019, 9:43 pm   #14
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Default Re: Maximum input voltage for LM317T ?

LM317HVs also exist, which are good for 60V input to output differential. Though be aware that the heatsinking will need to be very good to run them at that voltage with any very significant current
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Old 17th Sep 2019, 11:04 pm   #15
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Default Re: Maximum input voltage for LM317T ?

I would first test the VA rating of each winding.
A rough guide can be got from measuring the DC resistance of the winding and working out the voltage drop under load.
If this is more than about 5% the total losses will overheat the transformer once you count the magnetic losses as well.
I have a feeling you might be looking at a low power winding that once powered a VF display.
The 16 volt winding is likely to be the one that powered most of the electronics in an old VCR.
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Old 17th Sep 2019, 11:05 pm   #16
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Default Re: Maximum input voltage for LM317T ?

I think Jolly has bought a board, not just an LM317T alone, so there may be capacitor voltage ratings imposing limits on voltages to ground in addiition to the LM317 rating for input-output difference.

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Old 17th Sep 2019, 11:22 pm   #17
Jolly 7
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Default Re: Maximum input voltage for LM317T ?

Yes, this is the one I bought. It's on its way from Shenzhen. Having miserably failed in my attempts to build a variable voltage regulator from a kit (covered in an earlier thread), I thought of going down this route. I may order more of these if it works fine.
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Old 18th Sep 2019, 12:08 am   #18
Jolly 7
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Default Re: Maximum input voltage for LM317T ?

Quote:
Originally Posted by Jon_G4MDC View Post
OK then it becomes a little complicated.
Can you see the wire gauge of them - perhaps where they come to the connection tags?

If they are all wound with the same gauge wire then more/less they can all handle the same current. These could be added or subtracted to get other Voltages.

If you have some which are wound in thick wire while others are only very fine wire then boost/buck connections are not a good thing to do.
The wire gauge appears to be identical for all four secondary windings. Ignore the yellow wires though, I soldered them on to the pins. Picture attached.
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Old 18th Sep 2019, 6:51 am   #19
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Default Re: Maximum input voltage for LM317T ?

That heatsink is a bit wimpy to dissipate that amount of power and isn't the Vmax of the LM317T around 40v? (37v as stated above) The LM317AHV is 48v. I've found around a 5v difference is about right for 317's apart from putting a big dropping resistor off board you'd be better finding a different tfmr. what sort of current are you after? Might have something.

Andy.
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Old 18th Sep 2019, 11:10 am   #20
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Default Re: Maximum input voltage for LM317T ?

Did you recover the transformer from a scrap VCR yourself? If so do you know the make and model number? It might be possible to track down a circuit diagram which can give valuable clues about the transformer's specifications.

I have a transformer from a scrapped eighties Philips VCR where the voltage and current ratings are marked on the schematic. As a consequence it's a very useful transformer for experimental purposes. Even if this level of detail is not included in the equipment manufacturer's documentation things like secondary fuses and circuit components can help to establish possible performance limits.

Re-purposing 'unknown' transformers can be a bit of a challenge without knowing something about the original circuit design requirements.

With any project like this I always think it's best to start with clear objectives for the finished item and work from there when sourcing the components needed to achieve those aims. In other words don't put the cart before the horse.

Alan
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