Electronics > Beginners
LM317AHVT and Heatsink
MrAl:
--- Quote from: mike_mike on March 05, 2019, 06:05:39 am ---Hello,
The load that I used was a 1R resistor at the beginning of the tests. I also used as load a 40W/24V light bulb. In both cases the output current was 0.45A.
--- End quote ---
Hi,
Ok well the minimum output from an LM317 without a negative power supply is about 1.25 volts, and with a 1 ohm load, that means 1.25 amps, but because the load is so low it could be competing with other resistances in the circuit so you see 0.45 amps. It could also be something wrong with the circuit itself like wiring error or something.
Check it with other loads to make sure it works as you think it should without overload.
Also, there is usually some input offset voltage on common op amps and that would mean an error in the measured current. You should also try changing the 0.1 ohm resistor. Try 0.2 ohms and see what you get. It should agree with the 0.1 ohm resistor except be about 1/2 the current. So 0.45 should drop to 0.225 unless there is a very bad overload and then it may be 0.45 again.
mike_mike:
Hello, I have got some trouble regarding the attached schematic. When I unplug the transformer (24V/100VA) from the wall socket (230V) then the output voltage from the power supply is rising for a short amount of time. Can this problem be simply solved or it needs a more complicated solution ?
I will also do more experiments in the next days to see more exactly what is happening.
iMo:
A diode from output to input?
mike_mike:
I drew a more detailed schematic. Please have a look.
I did not understood completely the idea behind the place were I should install the diode. Could you please be more specific ?
MarkF:
Diodes D4 and D5 are there if you connect something stupid to the outputs.
They are basically protection in the case you connect something that has a voltage on it.
- D4 is in case you connect something with a higher voltage to the output then is being output.
- D5 is in case you connect a reverse polarity voltage to the output.
Capacitor C3 (1000uF) on the output is extremely high. The LM317 response to transients on the load will be slowed down because of it. I would not put anything higher than 100uF. It may be causing your voltage rise when you unplug the AC.
Also, you typically do not see diodes (D1 and D2) across R2. Although, they do limit the amount of bypass current.
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