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wuemura

How to get better current from LM317?

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I never use the LM317 and this was the first time, i have a NET-MD and old Walkman(cassette) that i want to use in my car, the NET-MD uses 3.0V and the walkman 1.5V, they need about 200ma to work.

The thing that i've noticed is that the output of the regulator oscilates too much, at first i calculated Vout to be 3.05VDC but the voltage drop at use was 2.98VDC, so i recalculate the values to give a 3.20VDC (3.23VDC).

The regulator is a LM317T(1.5A) and dont know why it can't handle this devices, the walkman works fine with radio, but do not work with cassette, when the motor start to run the REG led start to blink.

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Hi Wellington,
Your cassette motor uses a very high current to get started.
The LM350 supplies 3A and the LM338 supplies 5A.
There is a way to connect a PNP booster transistor to an LM317 but then you lose the current-limiter feature. A second transistor can be added for current-limiting.

The voltage drop with load should be typically only 3mV at 1.5A. It is a max of 15mV. It is more because it is affected by your pcb wiring layout as explained in the datasheet.

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Hi audioguru,

The walkman that i use is the old SONY WM-FX822, here there is a nice picture of it:
http://www.geocities.jp/sony_transistor_radio/walkman/_index.html

It works with one AA battery and gives a nice 8hrs non-stop playback (cassette) and a few days with radio. It uses only about 10ma to operate, about 200ma to work (cassette) and the worst case when you do a reverse is about 368ma.

The LM317T is rated to handle 1.5A right?

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It was Ldanielrosa.
Mine some times made the led REG flash like a HDD everytime the MD lenses was moving.

I just remove the choque, add jumper and its working fine, the LM317 just get a bit hot, it will get burning hot when is chargin the MD player batteries.

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Hi Wellington,
I don't see how the choke caused oscillation unless it has a high resistance.
The LM317 can dissipate a little less than 2W without a heatsink. If it gets too hot then it shuts down until it cools, then starts working again. 2W from a 13.8V car battery to 3.0V is a current of only 185mA.

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Hi audioguru, it was a high resistence choke and was the cause of the problem.
The LM317 only get burning hot when i set the NET-MD/Walkman to charge the battery because of the current to do it. I did not build this to charge nothing but is working, with a small heatsink it will not shutdown, the circuit can handle it fine, i can full charge the bateries without problems.

Just playing MD or cassette it doesn't even get hot.

Thank you.

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