Author Topic: power supply help please  (Read 920 times)

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Offline glinjikTopic starter

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power supply help please
« on: December 07, 2019, 07:48:12 pm »
hi guys im having a bit of bother with a power supply can anyone help
this is what im building all seems good volt reg works current works my problem i used  two 2n3o55 instead of tip3055 and e13005-1 instead of bd139 my problem is i get normal volts 17.7 on collector of 2n3055 but only 10v on emmiter any ideas? lm317t shows 17v input only 10.1 output
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Offline xavier60

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Re: power supply help please
« Reply #1 on: December 08, 2019, 12:34:31 am »
A properly functioning LM317T should produce an output voltage With Respect To ground that cause the R1,R2 divider to apply 1.25V the Adjust pin WRT the Output pin. If the layout is very poor, oscillations can alter expected voltages.
I can't see how the current setting section can ever work properly.
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Online Ian.M

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Re: power supply help please
« Reply #2 on: December 08, 2019, 01:35:31 am »
Yes, that design is ludicrously bad.  There is no compensation for the varying output transistor Vbe drop with load current, nor for the emitter resistor drop,  using Ib as a proxy for Ie for the current limit is a dumb idea in the first place due to the large variation of hFE with Ic, and the current limit circuit doesn't - its just an overloaded Vbe multiplier.

Boosting its output with an emitter follower is just plain dumb - see Figure 23. High-Current Adjustable Regulator Circuit in the Texas Instruments LM317 datasheet for the right way to go about it.

However, adding variable current limiting to a LM317 that works right down to 0V out isn't easy, so its probably best to throw it out and start from a regulator that's easier to apply limiting to, or has it built in or drop back to the classic two opamp CC/CV control loop solution.
« Last Edit: December 08, 2019, 01:37:45 am by Ian.M »
 

Offline David Hess

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Re: power supply help please
« Reply #3 on: December 08, 2019, 01:51:29 am »
If it worked, that sort of current limiting is called beta current limiting.  It relies on limiting the current into the base to control the maximum collector current.  But since beta (hfe) is not well controlled and varies significantly with temperature, it is really only suitable for short circuit protection and often not even that.

If you can do without the current limiting, consider starting with a circuit like that shown below.
 
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