A question about Potentiometers

[Israfil]

Ok...experimenting with implementing adjustable voltage via potentiometer and I'm hitting a little snag in that I can't seem to figure out which one to buy to implement it. I lack anything but the most basic knowledge of the workings of electricity. (it goes from here to here, positive/negative, etc)

Using standard 510 atty, my meter shows about 1.5 amps at 5 volts (ptn 04050 boosting 2 parallel 14500 batteries. The device I'm planning on installing this in is in serial configuration, same battery type running at optimal 8.2 volts, I assume the amperage would be the same with the same atomizer.

Tried an ohm's law calculator but I can't seem to figure out how to get the values I'd need. Don't have much cash so I want to get it right, and I've already burnt out one of radio shack's little volume adjustment pots. (it did say it could handle up to 50 volts...) I just want to be able to adjust it from around 3 volts up to around 6, to really tune the sweet spot, but if it goes higher or lower in total it doesn't matter to me.

Anyone know a model of pot i can pick up from the shack that'd work?

Edit:if i take an old slide pot off a dimmer enabled light switch....would that work/not asplode?
Killed one of my TI boosters, or damn near by running it without the 100uF caps...couldn't find one that isn't way too large to stick in my mods, that's what prompted the change.

EditEdit: If a pot wouldn't work at all solo, would a rheostat?

 

[misterD]

are you using the boobster? take a look at Nuck's fistpack tutorial and replace the sliding switch and resistors with a 200K pot rated at least 1/8 watt (the one im using is 1/2 watt). just get the caps listed in there too.
and you're not gonna be able to get the voltage rage you want cos the boobster cant go that low... unless you place an extra diode that you could turn on and off via an extra switch...
and to prevent the POT to go too low you have to add a resistor into the leg that goes to the adj pin. (Nuck recommends a 36K im using a 20K)

 

[Israfil]

Wasn't planning on using the booster, trying to reduce the parts that are likely to fail in the field. Was just gonna resist down the voltage of 2x 14500 batteries configured serial via the pot. Theoretically that'd put the top end of the pot around maximum voltage...although it wouldn't take too much more room to have a resistor wired in before the pot to make the maximum voltage out around 6 or 6.5.

I'm starting to think I'm just not able to run algebraic expressions in my head when they have to do with electricity...

 

[Nuck]

If you want to use a pot to directly modify the circuit resistance you will have to use a very large pot. Typically pots are used to modify the adjustment pin of whatever regulator/booster chip you are using rather than modifying the power directly.

 

[Israfil]

Nuck! <3 You sir are a legend...

Hmm....so a rheostat wouldn't be strong enough to modify it directly either? Spent so much time lately trying to figure out electricity's workings that I may soon be able to shoot lightning bolts from my ....... ^_^

 

[ThePuck]

You would need to use an adjustable voltage regulator to do this correctly without frying anything. Pots are only like a quarter watt, reostats a bit more but still most will not handle 1.5 amps.

 

[WillyB]

Well the 3A barrel diodes from RS drop the voltage by about .8V according to Ralph. So if you figure a way to progressively add 4 of them in series you should end up with a voltage range of about 7.4 - 4.2V.

 

[Israfil]

OOkay well I've found myself an LM317t adjustable voltage regulator...and a calculator I think I've figured out. According to this LM317 Voltage Calculator If I set my R1 with a fixed 260 ohm resistor (may have to deviate to account for availability) and get a 1k ohm pot for my R2 I should be able to adjust the voltage output from 1 up to 6 at full resistance. Not really sure how I can adjust the voltage to a smaller range...but this appears to be a starting point.

Any advice on it would, of course, be much appreciated.

Edit: another option appears to be to get a 1k ohm resist for the R2, and get a 250 pot and a 250 resistor. It looks like that would allow me to have a minimum output of 3.75 and a maximum of around 6 volts. This is assuming resistance is additive in this case...And something tells me that isn't right, but there it is.

Editedit holy crap...apparently in a series circuit Rt=r1+r2... maybe I'm finally starting to understand this eklektricity thing....

 

[duby]

Yeah, Resistors in series add.
In parallel its a simple formula you can churn out quick.

 

[Nuck]

OOkay well I've found myself an LM317t adjustable voltage regulator...and a calculator I think I've figured out. According to this LM317 Voltage Calculator If I set my R1 with a fixed 260 ohm resistor (may have to deviate to account for availability) and get a 1k ohm pot for my R2 I should be able to adjust the voltage output from 1 up to 6 at full resistance. Not really sure how I can adjust the voltage to a smaller range...but this appears to be a starting point.

Any advice on it would, of course, be much appreciated.

Edit: another option appears to be to get a 1k ohm resist for the R2, and get a 250 pot and a 250 resistor. It looks like that would allow me to have a minimum output of 3.75 and a maximum of around 6 volts. This is assuming resistance is additive in this case...And something tells me that isn't right, but there it is.

Editedit holy crap...apparently in a series circuit Rt=r1+r2... maybe I'm finally starting to understand this eklektricity thing....

The dropout voltage on those is quite high making them unsuited for the mods we typically build. The TI LDO chip does the same thing with a much smaller dropout. You can also use the same calculator to figure out the resistors (they are compatible).

 

[Israfil]

Dang...guess until I can afford either one of those or find a low dropout one around here I'll have to make do with this. I'll figure something out i guess... Thanks guys.

Edit, good news! Got it working with it...after 6 hours of tinkering. Some jerk at the factory thought it'd be fun to draw the diagram of the chip upside down. Got the real pinouts from google and it's working great. the pot is one of the long blue ones from radio shack, and adjustable via fingernail Have a ball with it.

 

[USAGeorge]

TI.com,,,they have a pdf for each dc/dc with ohm to output scale. After you decide what your source voltage is,,single 3.7,two bat series,,punch in the info
on their power management and range of output desired,amps and they display what works,,get one for variable voltage and insert pot where the resistor
would be placed for variable voltage unless you want a fixed output. Take the time to read the pdf discripts. It seems that a 200 ohm pot fits a battery mod.
On Pot choice,,some trimmers/pots are not meant to be used,that is turned constantly,get something decent and usr what they call a one turn pot,,,,,the pots
that take 10,20 turns do NOT belong on mods that we use......Functionality comes first.

 

[WillyB]

... On Pot choice,,some trimmers/pots are not meant to be used,that is turned constantly,get something decent and usr what they call a one turn pot,,,,,the pots
that take 10,20 turns do NOT belong on mods that we use......Functionality comes first.

HUH? The best VV designs I've seen here, bigblue's, and commercial The Buzz, use high quality multi-turn pots. With these precision pots it's quite simple to dial in the exact volts you want. The only single turn I've seen is on an under-built toy lighter thing.