Hey everyone,
I've been building an adjustable power supply with the LM317T. The datasheet
http://www.st.com/web/en/resource/technical/document/datasheet/CD00000455.pdf states the adjust pin current is typically 50uA but maximally 100uA.
When I measure the current using my multimeter, I measure 4.17mA. This is a lot more than I was expecting.
I've attached a picture of my circuit.
If I understand multimeter burden voltage correctly, then this could not be the cause because the burden voltage could only lower the current and not increase it.
This leads me to believe my understanding of the LM317 is incorrect.
From what I understand, in the basic configuration, the output is 1.25V higher than the voltage of the adjust pin both referenced to ground.
R1 is placed from the output to the adjust pin in order to satisfy the minimum load requirement. This is because there is always a 1.25V differential across R1 giving you a reliable current. If you're driving the adjust pin with an op amp, you can take R1 out but then you need to guarantee the minimum load current in another way.
R2 is for.... well I don't actually know. I think this is where the problem lies.
What is the purpose of R2?Now back to the current measurement on the adjust pin. Since the LM317 is a linear regulator I am going to assume ohms law applies. As a result the current on the adjust pin should change depending on the voltage applied to it. This seems to compute since I'm applying 5V to the adjust pin with a resistance of 1.2K which gives me the measured 4.17mA. So why does the datasheet state 50uA - 100uA on the adjust pin?