A little question about an Adjustable Supply

[GetRich]

Hi,
I'd like to build this small power supply:
http://www.ladyada.net/library/equipt/diypsupp.html
Thats probably bit of a newbie question, but I want the adjustable range to be 0-15V.
what changes are necessary? (I guess I need larger caps..?)
Thanks from advance!

[stinkbutt]

Can't be done. The voltage regulator isn't capable of spitting out voltages higher than Vin, so unless you can find an 18V wall wart with the right barrel plug, (which I suppose might exist,) it couldn't work. And I dunno if the regulators rated for 18V, or the rest of the components for that matter.

[westfw]

laptop power supplies are frequently in the 18V range. They're expensive "new", but frequently show up in dumpsters or thrift stores once they are divorced from the original laptop for one reason or another. They also tend to be capable of 3 to 4 amps of output, but the lm317 should protect things on the low-voltage side...

[mitpatterson]

In regards to waht stinkbut said, the LM317 is rated to 40V(per mousers product page linked in the tutorial). But he is right about the can't be done with the 9V batter, would need a adapter at least rated to 18V or so(as the drop out seems to be around 3V or so, as it list max input as 40, and out put as 37)

[stinkbutt]

OK, perhaps "can't be done" is a bit too strong.

If you insist on doing it, this is what you'll need to do.

1. Find an 18V wall-wart with the 1.2" barrel connector. I found this on amazon, maybe it'll work for you, maybe if you buy it you'll have set fire to four bucks. I have no idea if the barrel connector's the right size, or the right shape. Or perhaps this'll work.

2. Get a bigger heat sink for your regulator. I'm not exactly sure if you're using the LM317 or the MIC2941AWT from the breadboard power supply kit, but they're both TO-220 packages so any of these heat sinks ought to fit. The heat sink in the kit is in that list, which is why I'm suggesting this company.

Going into how heat sinks work is a bit far afield of your question, but I'll put things very simply: You have 9V going in and a certain amount of heat coming out of the regulator. When you increase that to 18V you're going to get somewhere between twice as much and four times as much heat going out. So you're going to need a heat sink that is at least twice as good at dissipating heat. Your heat sink has a rating of 23.2 degrees C/W, so you'll need something with a rating of half that. Yes, less is better in the world of heat sinks.

3. Double-check the parts list on the power supply kit. Check the datasheets to make sure they can handle 18V in. The 1N5818 diode, for example, is rated for 30V reverse voltage, which would probably be OK, but the 1N5817 is only rated for 20V; uncomfortably close to the 18V input voltage. You'll have to verify which one was used in your kit, as it seems they're being treated as fungible by Adafruit. (Certainly a fair assumption for a 9V regulator.) Check those numbers for all the parts and replace where appropriate.

4. Finally, you may need to increase a couple of the resistors. R1 and R2 are current-limiting resistors for LED's. Double the voltage on those things and you double the current. You might need to increase them a bit.

Once you do all that, I suppose you can give it a try, assembling the regulator and giving it 18V, and hoping it operates the same way. That's all I can think of for now.

[GetRich]

I'm not that much of a noob, of course it would be connected to 12 X AA batteries
According to the product detail on mouser, it's input voltage MAX is 40V and output voltage is 1.2V to 37V, so there shouldn't be a problem with my voltages.
Now, for the changes in the circuit [I would like to know if I'm correct]:
1. As I said, there will be 18V input.
2. That should be sufficient.
3. The diode in the part list is 1N4001 [rated for 50v] which is more then enough. The caps should be changed to something rated 20V+
4. I think you're wrong. R1 and R2 are the adjusting resistors. 3k pot and 240 ohm resistor should be fine [I think..]

Thanks a lot for your help!

[ImaginaryAxis]

Going into how heat sinks work is a bit far afield of your question, but I'll put things very simply: You have 9V going in and a certain amount of heat coming out of the regulator. When you increase that to 18V you're going to get somewhere between twice as much and four times as much heat going out.

Simply increasing the input voltage does not automatically require a heat sink. To first order, it depends on the product of the differential voltage across the LMx17 and the output current it delivers.

[zener]

Simply increasing the input voltage does not automatically require a heat sink.

It is reasonable to assume that he might want say 5V at 100mA (or more), even at that you are talking 1.3W, heatsink.

[ImaginaryAxis]

Which is why I said it depends on the product of the voltage across the device and the output current.

[stinkbutt]

I'm not that much of a noob, of course it would be connected to 12 X AA batteries
According to the product detail on mouser, it's input voltage MAX is 40V and output voltage is 1.2V to 37V, so there shouldn't be a problem with my voltages.
Now, for the changes in the circuit [I would like to know if I'm correct]:
1. As I said, there will be 18V input.
2. That should be sufficient.
3. The diode in the part list is 1N4001 [rated for 50v] which is more then enough. The caps should be changed to something rated 20V+
4. I think you're wrong. R1 and R2 are the adjusting resistors. 3k pot and 240 ohm resistor should be fine [I think..]

Thanks a lot for your help!

Excuse me, I was referring to the Adjustable Breadboard Power Supply kit from adafruit, while you're looking to put together the LM317 Supply from here.

1-3 you've got covered, it appears.

As for the current limiting resistors, yeah, they're called R1 and R2 in the Breadboard Power Supply, while they're the voltage adjustment resistors in the DIY power supply. The current limiting resistor is the optional one at the end with no number, and if you're going to spit out nearly 18V of output, you'll want to have a larger current limiter.

[stinkbutt]

Which is why I said it depends on the product of the voltage across the device and the output current.

Thank you for pointing out that power is equal to voltage times current. Have you informed Stockholm?

[GetRich]

A 2k current limiter is enough?

[stinkbutt]

Sure. 2K is the most you'd need to increase it to.

[GetRich]

Thanks a lot stinkbutt! I really appreciate your help.

[ImaginaryAxis]

Which is why I said it depends on the product of the voltage across the device and the output current.

Thank you for pointing out that power is equal to voltage times current. Have you informed Stockholm?

Strange, you didn't seem to know how to calculate power with a constant current source.

I kid, I kid.