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[SOLVED] AC Rectified input voltage drops connecting to LM317T

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lddang

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I am trying to convert my magnetic inductance voltage from L1 and C4. The AC input is about 25Vpp at 6.78Mhz. After going thought the full bridge rectifier, the voltage is about 15V with around 200mVripple. I am feeding this 15V into the LM317T, as seen on the schematic, to get 8V output but I cannot get it to work. After connecting everything, the LM317 output a very low voltage (as small as in the mV) also the AC signal drops to about 10Vpp. I am connecting on a breadboard with the LM317 GND connect to the rectifier's GND.

RX.jpg
 

R

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It may be a defective IC (LM317), an output short circuit or wrong pin connections (double check the LM317 pinout).
 

lddang

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Did all of that. Verified the LM using a PSU and it works as intended...but when connecting with the rectified circuit that's when nothing is working. The AC input drops and the regulator output is in miliVolts.
 

Audioguru

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Your AC input cannot supply enough current.
Which diodes? Are they fast enough? Why do they cause a voltage drop of 10V instead of 2V?
The LM317 does not have a ground, all grounds on your schematic should be connected together.
 
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lddang

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I am using 1N649 for the rectifying. It is rated 1V @400mA.
 
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I am connecting on a breadboard with the LM317 GND connect to the rectifier's GND.

If you tie the LM317T "GND" pin (actually, the ADJ pin) to ground you should only get 1.25V at its output.

Your posted schematic is correct so check it twice, as there's no ADJ pin connection to ground. As I mentioned before, check the LM317T pinout configuration as it's different than LM78XX regulator series, by example.
 

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1N649 is a slow recovery silicon rectifier diode, not suited for MHz frequencies. Better use 1N4148 or a low power schottky diode.
 
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lddang

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1N649 is a slow recovery silicon rectifier diode, not suited for MHz frequencies. Better use 1N4148 or a low power schottky diode.


I switched to 1N6338 that I have available and it worked.
The 1n6338 spec Vf 1.1V@200mA, trr=20ns. I am still trying to understand why it worked...so if you guys have insight please share.
 

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