Are 50uA through a potentiometer realistic? - Page 1
Forum section: electronics / beginners

author: Stefan
author_rank: Contributor
timestamp: January 20, 2014, 10:02:33 pm
content: Hi,I planning to put a linear regulator into a box with a potentiometer and some analog V / A meters I have around, you get the idea.I was thinking of using a LT3081 or similar since they can regulate down to zero volts and have resistor-settable current limits.Those devices have a SET-pin that sources 50uA and is internally connected to the opamp as the reference. For a 20V range a 400kOhm poti has to be placed between SET and GND.I'm a bit sceptical if that is really such a good idea. potentiometer aren't exactly precision devices and 50uA is tiny. I'm thinking about leakage and such. Also such high-ohm potis are hard to get.thanks,Stefan

author: codeboy2k
author_rank: Super Contributor
timestamp: January 21, 2014, 10:56:35 am
content: It's just using the 50uA across a set resistance to force a voltage at the set pin. This voltage is used as the reference voltage in the internal error amplifier.It really only cares about the voltage there, so you can use your potentiometer with 2 ends at GND and +20V (for example) and feed the set pin with the wiper, the output of the LT3081 will follow the wiper voltage.NOW... having said that, there is a problem with the LT3080 and LT3081 that has been widely discussed here on the eevblog (just search for it)... If the output is at 20V (and you have output capacitance holding it there), and you change the SET pin voltage to something that is more than 10V differential from the OUT pin, it will lockup. So if you have 20V out, and you move the pot down to 5V, it can lockup. There is a maximum of 10V allowed between SET and OUT, and this has always been a problem for anyone trying to use the LT3080 and LT3081 in a variable output voltage configuration. With a fixed resistor on the SET pin it's fine. When the forcing the SET voltage with a potentiometer, an OP-AMP or a DAC , it can lock up when down-setting.CAVEAT-EMPTOR.

author: Stefan
author_rank: Contributor
timestamp: January 21, 2014, 01:15:45 pm
content: Thanks! As I understand you agree the 50uA-over-poti not a good idea? Too bad.Of course your approch is the alternative. The question is where to get the +20V from. Especially if IN might be in ~3-25V range... I could use a, say, 2.0V reference and an rail-to-rail opamp in non-inverting amplifier configuration to make SET=10*wiper. Ahhh... this was supposed to be a single-chip-with-some-passives-attached project...Thanks for telling me about that lockup! I tired to search forum and google, but didn't find anything? Anyway, this would only happen if one would turn down the voltage quickly, so that the (minimal) load does not discharge the output cap fast enough, right?-Stefan

author: codeboy2k
author_rank: Super Contributor
timestamp: January 21, 2014, 01:56:07 pm
content: Search google for this : LT3080 LT3081 site:eevblog.comYes, you can use a 2V reference and an opamp to supply the SET pin with a voltage. I didn't suggest it because understood you wanted a simple approach (few parts) and I thought you would already have a 20 to 24V supply at the input anyways, so the pot and wiper approach would be fine. An opamp also requires it's own supply, but that can come from the input too.

author: Stefan
author_rank: Contributor
timestamp: January 21, 2014, 03:25:21 pm
content: I browsed through those threads. Seems that you have to treat those LT308x like raw eggs.You understood correctly. I think I just live without <1.25V for this mini project, rather than adding voltage-reference, opamp and whatever belly rubbing those LT devices need to stay alive.Thanks, I think you saved me some gray hairs...-Stefan

author: codeboy2k
author_rank: Super Contributor
timestamp: January 22, 2014, 01:04:48 am
content: Quote from: Stefan on January 21, 2014, 03:25:21 pmI have plenty of gray hairs.. you can have some of mine, and glad I saved you growing some new onesGood luck!

author: MrAureliusR
author_rank: Unknown
timestamp: January 22, 2014, 02:46:27 pm
content: Quote from: codeboy2k on January 22, 2014, 01:04:48 amSorry for the off-topic, but codeboy -- CONGRATS! You are one post away from a THOUSAND!! Amazing!

author: codeboy2k
author_rank: Super Contributor
timestamp: January 23, 2014, 06:08:00 pm
content: Quote from: MrAureliusR on January 22, 2014, 02:46:27 pmOh, cool ! then I'll post in this thread today to make it 1000 and let it hover there for a while while I am busy and can't post anymoreCheers!

author: tszaboo
author_rank: Super Contributor
timestamp: January 23, 2014, 06:18:59 pm
content: Quote from: codeboy2k on January 21, 2014, 10:56:35 amwouldn't a zener prevent this? Sure, it would limit the speed you can turn the knob on the pot, but at least it would still work.