LM317 tracking pre-regulator and current regulator

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yagoolar

yagoolar

Member
Joined 2004
#1
Based on tracking pre-regulator circuit form National and this link I expect to achieve 1A regulated current. But in reality ripple voltage is around 0.2V RMS.
The voltage across second reg is around 2V and does not change if one or two red LEDs are connected between Vout2 and Vadj1.
Sims in LTspice are ok.
 
#2
Two setups were tested - same result.
Below there are LT1086, just for comparison - LM317T were used for both
 

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#8
The drop across reg2 should be 1.25V + the voltage drop across the LEDs. So it should definitely change 1 LED vs 2 LEDs. Unless something is wrong.
This topic hits close to home, because the below circuit is next in my circuit build queue:
 

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#11
yagoolar said:
Two setups were tested - same result.
Click to expand...

I recently built one of these tracking preregs and it works fine.
Looking at your circuits, the values look a bit odd. Here are some specific comments.

1) The 1000uf cap at the output of the 1st regulator isn't needed although it shouldn't cause any problems.

2) The resistors around the 2nd regulator look odd. What voltage are you trying to regulate? Compare your circuit to the referenced circuit. They don't look the same.

3) The value of R6 (top circuit) or R2 (bottom circuit) = 6.3ohms is much too low. Typical values should be 100 - 240 ohms. Similar comment for R7 in the top circuit.

--Gary
 
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