Help with lm350 /lm317 as Constant Current Source

Thread Starter

Broncos

Joined Feb 2, 2012
33
I'm usig an lm350 and built a circuit pretty close to this
http://www.eleccircuit.com/wp-content/uploads/2007/12/lm317-current-regulator.png
Without the capacitor and going thru this switch (both the the power and resistor are fed thru the switch(different poles))(this switch)

I'm using a 2ohm resistor on the vout (expecting around 625mA's). I was getting around 1.5 amps. I measured the voltage between the Vout pin and ADJ pins. The voltage was 0.04 volts! I got the same results when using a 1 ohm resistor too.
what could the problem be?


I pulled the lm350 and tested it to work ok at lower currents with a .1 capacitor[vin to gnd] got (1.20 volts across Vout and ADJ) Could I need a capacitor on the higher current one, and where?
I'm using a 7.5 volt battery pack.
 

Thread Starter

Broncos

Joined Feb 2, 2012
33
I tried a capacitor from Vin to ground, same results. Draws 1.52Amps...
.047 volts across the resistor, and 0.00 volts across another resistor I tested - still 1.52 amps
 

Ron H

Joined Apr 14, 2005
7,014
Whats the minimum voltage from the battery required?
The datasheet guarantees that it will work if (Vin-Vout)>3V. The attached graph is of typical performance.
Keep in mind that, when used as a current regulator, you have to add the 1.25V reference voltage. This means that the guaranteed minimum Vin is the voltage across the load, plus 4.25V. If you are fortunate enough to get a typical part, that number is about 3.25V.
 

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Thread Starter

Broncos

Joined Feb 2, 2012
33
Now when I connect power, with the vout pin floating, the adj pin still delivers 1.5 amps! (7.5 Vin , 3.3v across load)
Bad chip? I'm using that pinout with my [lm350t P+]?
 
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