# Potentiometer Question

I’m working on a circuit that does not include the arduino, but have some questions about potentiometers and thought this might be a good place to ask. The circuit is going include around 100 LEDs and a potentiometer for dimming them. I have some questions about the potentiometer though. The circuit is going to run on 6 AA @ about 9v. The 100 LEDs will pull around 8-9 watts of power from the power source. What potentiometer would work best for this setup? Will I need a transistor and/or other components to make it work properly? Thanks for the help.

-duemilanove

You are possibly talking about in excess of 1 AMP of current through a Potentiometer. When current like that is involved, you are talking about a WIRE-WOUND Potentioemeters (sometimes called rheostats…) Even though a rheostat is technically a 2 terminal device… it has sort of become the defacto name for the wirewound variety of a potentiometer too.

These use a resistance WIRE instead of a carbonized coating is the way to create the higher current variable resistance element. A normal “audio” POT would literally vaporize the carbon material in this application.

Here’s a problem: Looking at WIRE-WOUND devices, the current you are talking exceeds all but the rarest of units available and therefore… it would be a costly device… I’m guessing on the order of \$30-\$50 or even more.

At that price, you could easily afford to build an active driver circuit that could employ a very simple PWM solution for intensity control.

The simplest circuit would employ say a LM555, some passive devices and a power transistor as the active device. A quick Google search using the following "LED PWM 555 " should give you plenty of reading material.

Thanks for you help so far! I appreciate it. I have come across a couple more questions. I found this site An LED Array PWM Dimmer with the 555 It has some pretty useful information, and even a video about the LED PWM 555 dimmer. It states that the main circuit can be run from 4-15 volts. Is that statement true? The 9v led array given on that site states a max of 66 LEDs. Could I just connect the array of the 100 LEDs that I will have, to the main circuit shown on the website? I will probably need a transistor that handles more current flow. All of the LED calculators I have tried give me a little under 1 amp of current used by the whole array.

Thanks for the help
-duemilanove

yes the 555 can handle 15 volts (max) but not enough current for your application, so yes you would need a transistor able to handle the load to do the dirty work (which also means the voltage on the 555 would not matter much)

Can you show us the math behind 8-9 watts?
I’m thinking you will have 33 columns fof 3 LEDs+current Resistor each, up to 20mA per column, that 0.66AMP x 9V = only 6W.
(maybe you can get 4 in a column - the voltage drop across each LED will be 2V+, so 3 for sure will work, 4 maybe)
If you have superbright LEDs from a place like superbrightleds.com you can use way less current & still get a blinding amount of light.
RL5-R5015 Red LED, \$0.18 each for 100. 5,000mcd!
So 0.66Amp at 0 resistance on the pot, with current dropping as the pot resitance increases.
0.66Amp (660 mA) is not going to run too long on 1500-2000mAH batteries.
If the 2N2222 can really do 800mA, that’d be the way to go - but it might get kinda warm at 660mA - I’d suggest a package style that would accept a heat sink also.
Or go for the high brightness LEDs and less current per column.

Great! Thanks for all your help! It looks like this 555 PWM circuit will work very well for the application. One more thing. Would this transistor be good? http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=PN2222AFS-ND

Can you show us the math behind 8-9 watts?

I did not really do any math for it. I used a few online led calculators. (Here is one: http://led.linear1.org/led.wiz) I typed 9v power, and the LEDs I’m using run at 3.2v @ 20mA with 96 LEDs. The online led calculator calculated around 9 watts total with resistors.

I would say no to that transister- its only rated to 500mA MAX.
You want something that can handle at least twice the expected load.
I think you want something more like this:

It has a gain of least 20, so at full current (my previous 660mA calculation) you will need 33mA or less going into the base to turn it full on. That 33 mA will be coming thru Ra, and to turn off the transistor the 555 will turn on its output and sink the current to ground (see a 555 datasheet, such as http://www.national.com/ds/LM/LM555.pdf)

And get a heatsink also.
Have the transistor “below” the LEDs - that is, +9V connected to the 33 columns of current limit resistors in series with the 3 LEDs in series to the collecter of the transmitter, with the emitter to ground.

I would say no to that transister- its only rated to 500mA MAX.

yea some of them are max 600 (depending on maker) either way a 2222 pushing an amp even with non constant current is a recipe for smoke

It has a gain of least 20, so at full current (my previous 660mA calculation) you will need 33mA or less going into the base to turn it full on. That 33 mA will be coming thru Ra, and to turn off the transistor the 555 will turn on its output and sink the current to ground (see a 555 datasheet, such as http://www.national.com/ds/LM/LM555.pdf)

agreed a 555 will do it no problem, without looking at a datasheet I want to say one can handle a couple hundred ma keeping voltage in consideration (200ma at 5v is a different wattage dissipation than 200ma at 15 volt, though its a very stout little chip, with 9 volts it should be ok, again without looking at the datasheet or running the numbers …)

lol anyway yes PWM is the most practical answer to your problem, it will give you the effect you want without having to get a big fat nasty expensive wire-wound pot

( I actually have a Korean war era big fat nasty wire-wound potentiometer in my collection, bought it in a thrift store with 3 other coffee jars of stuff obviously from a old, probably recently upgraded to the sky ham for 25 cents for all 3 jars, and its gold to me, military grade construction and precision even for this era, its like geek money and feels like s*x while turning it ;D … its nice. 1.5 inches wide, 2 inches tall, 1 inch thick, and potted in “sculpted” rubber)

So in theory the diode drop of 0.7V and with 33mA set by R1, the 555 should only be seeing P=IV of (0.7*.033)= 23mW, which would be within its capacity for the discharge pin. It is not defined as robustly as the output pin.

23 mw is hard to argue on something of such an old design, although I am not an electronics guru it sounds more than reasonable

Ah, but I Am an EE and I have been reading the datasheet (and going to bed after I hit post next too).

Spec sheet says the commercial package is good for a watt of dissipation

Power Dissipation (Note 3)
LM555CM, LM555CN 1180 mW MAX

and that pin 7 will go down to 0.18v
Pin 7 Sat (Note 7)
Output Low VCC = 15V, I7 = 15mA ->180 mV,
and at higher current:
Note 7: No protection against excessive pin 7 current is necessary providing the package dissipation rating will not be exceeded.

so at 33mA we should not be adding much to the heat being dissipated as he is apparantely running at 9V and not 15.
If he ran the 555 at just 4.5V or 6V instead of the full 9, that would help it run cooler also.

Okay, I have come across some more simple questions:

I would say no to that transister- its only rated to 500mA MAX.

Thanks, I found this one http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-2573-5-ND It is the MJE3055T, one of the transistors on that data sheet you linked to. So that transistor should work fine?

Have the transistor “below” the LEDs - that is, +9V connected to the 33 columns of current limit resistors in series with the 3 LEDs in series to the collecter of the transmitter, with the emitter to ground.

I got a little lost here. Can this be explained a little more? I should have clarified this before, but I’m using 96 LEDs. 48 columns of 2 LEDs in each column. (of course, with resistors)

And get a heatsink also.

If he ran the 555 at just 4.5V or 6V instead of the full 9, that would help it run cooler also.

So you are saying the 555 will run warm, and I will need a heatsink for the 555 chip? Or did you mean the transistor needs a heatsink because it will run warm?

I don’t know if I will run it from 4.5v to 6v. The LEDs will probably be used for short periods of time (at the most ten minutes) and being adjusted by the potentiometer quite frequently when it is on. (In most situations not at the highest level) All of the parts are going to have plenty of breathing room also.

Thanks for the help so far
-duemilanove

Yes, the MJE3055T NPN transistor should handle the current.

Where you are running from 9V, I would recommend less columns with more LEDs in each. Unless you are using some ridicuously inefficient LEDs, they shouldn’t be dropping more than 2.4-2.5V each, so you should be able to get at least 3 per column, and possibly 4 (try it, see how many will light up) so you can get more life from your batteries.
From 9V, connect your current limit resistor, then the LEDs in series to the “top” of the transistor (on the schematic), which is the Collector. Then the 555 drives the Base high and turns on the transistor to connect the LEDs to the Emitter, which is grounded.

Heatsink is for the transistor.

The 555 will for sure run on 6V. 4.5V might be iffy as the batteries get low, or when there is high current draw.

Where you are running from 9V, I would recommend less columns with more LEDs in each. Unless you are using some ridicuously inefficient LEDs, they shouldn’t be dropping more than 2.4-2.5V each, so you should be able to get at least 3 per column, and possibly 4 (try it, see how many will light up) so you can get more life from your batteries.

Thanks for the suggestion. I don’t have the LEDs yet, but I will try this when I get them.

From 9V, connect your current limit resistor, then the LEDs in series to the “top” of the transistor (on the schematic), which is the Collector. Then the 555 drives the Base high and turns on the transistor to connect the LEDs to the Emitter, which is grounded.

So something like this schematic is what you were thinking?

Heatsink is for the transistor.

Would a little heat sink like this work good?
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=WA-T247-101E-ND

Schematic: yes, exactly. R & LEDs in series between 9V & transistor.

Heatsink: that should work okay. If you find the part getting really hot (like burn your finger hot), then find a heatsink with a lower Thermal Resistance, likely one that needs to be screwed on instead, such as
http://media.digikey.com/pdf/Data%20Sheets/Aavid%20Thermal%20Technologies%20PDFs/504222B00000.pdf

MOSFET transistor like this one should work also.
http://www.irf.com/product-info/datasheets/data/irf3707pbf.pdf
I have some, haven’t tried them out yet. I used IRF630s in my prototype (had 2 on hand in my sobs Radioshack Electronics lab kit of all places).
Ended up using some NPN transistors in cans that I had a little drawer full of from college days. But I am only drawing maybe 16mA total thru 4 colums with 5 LEDs each across 12V. Was thinking I’d be running 80-100mA, but the 5,000mdd LEDs were so bright I toned it way down. Still needed 20 to define the display size I wanted.

Well, Thanks all for your help! I really appreciate the time you gave.

-duemilanove

You’re welcome. Be sure to write back and let us know how it worked out.

Hi,

Can anyone help me to understand how to read the value of potentiometer on the LCD? I managed to get time and Hello world displayed on my 16x2 LCD, but would like to read the pot value too.

Yashar

Can anyone help me to understand how to read the value of potentiometer on the LCD? I managed to get time and Hello world displayed on my 16x2 LCD, but would like to read the pot value too.

Read the data-sheet of your display. That’s the way how this kind of questions are answered and this is a perfectly good example to start learning how to do this. If you find no mention of a potentiometer, it might be there to control the backlight level and not available for reading. But again, nobody will know, except you if you start reading the data-sheet.

Korman