LM317T not working properly?

I am using an LM317T, trying to use it to create a constant 5v output. I've wired it from the diagram shown here. I am using a pot trimmer to control R2. As far as I can tell, it's wired properly (checked a few times).

What's happening is; if the input voltage is above 5v, it will limit the voltage to 5 volts. But if the input voltage goes below 5v, so does the limit voltage. It seems that it is creating a cap instead of a constant. It's capping the voltage to 5v, but when I go below 5v, it mirrors whatever the voltage is.

Is there a common mistake that I might be making? I've made this circuit before with success, but I think I was using an LM317 instead of the LM317T. Would this make a difference?

2 Answers

It's a linear regulator. It can only decrease voltage, cap it, as you've said. It is working appropriately as you've described it.

If you want to be able to go up or down from the input voltage you need a buck/boost topology.

• Ahh, thank you. I believe the application I did before only had to limit to 2.5v. Can't I achieve pretty much the same thing with just a pot trimmer and resistors? Is there something much more special with this LM317? Jun 9, 2014 at 21:29
• No, you can not do the same thing with resistors, not if you intend to draw any current at all. Unless you mean you're going to sit there at the pot trimmer and constantly adjust the voltage depending on load. An activity called regulating. The LM317 is a regulator. It attempts to keep output voltage the same regardless of the load. Jun 9, 2014 at 21:37
• Great, thank you for the information! There's a lot to learn still... Jun 9, 2014 at 21:40
• It's attainable. I was right where you are six years ago. We all were, at some point. Except Olin, I think he's been alive forever. Jun 9, 2014 at 21:47
• @Samuel - Nah. I'm older than Olin and I haven't been alive forever. Jun 10, 2014 at 6:07

I've often thought that these three-terminal linear regulators (LM317, LM78xx, etc.) should be called "voltage limiters" rather than "voltage regulators", as they can only reduce the supplied voltage, not increase it.

All this type of regulator have a "dropout voltage" or "minimum headroom" spec - the input voltage must be some minimum amount above the desired output voltage for the regulator to produce the desired output voltage.

If the dropout voltage is 2 volts (common for the 78xx family, I think) , then you require at least 7 volts into the regulator to get 5 volts out. If you only supply 6 volts, the output voltage will only be 4 volts, keeping the 2 volt headroom.

There are "low dropout (LDO)" regulators that will operate with 0.5 volts headroom (perhaps some even lower).

• great detail Peter! I'll need to look into LDO regulators for sure. Do they make these in a micro size as well? Jun 9, 2014 at 23:48
• I'm sure that LDOs come in various packages - I think there's a small surface-mount one on my Raspberry Pi. Jun 10, 2014 at 0:23