Can't understand pot resistance, amps, etc.

[Dog]

Hi! I am trying to build a 7-color box (Red, Green, Blue, Yellow, Cyan, Magenta, and White) with a 200mw 650nm module, 100mw 405nm module, and a 50mw 532nm module with a 6000mAH 3.7v 18650 for each. I was afraid that the output ratio was off, but someone suggested I used potentiometers to adjust if necessary. After some research, I realized that if I gave a pot to each module, I could effectively have an 'unlimited' color system like this. I thought I had everything figured out until I looked more into the specs of the pots. 2 more hours of research was no help, so I thought I could have the smart people on here give me simple explanations for a few things I find confusing.

1) What exactly are Amps? What do they measure? Are more Amps better?

2) What do the specs of pots mean? I found a useful Ohm's law video to help determine the type of pot I should get, but I decided on a 200ohm pot (after plugging 200mw module/.2w module and 3.7v battery in watts=volts*amps to get the amps and plugging that in for volts=amps*ohms to get the resistance, in ohms). However, almost all the pots I could find online were in the thousands (had a K after it) when I only needed a 200 ohm pot. What effects would occur if chose a 5K ohm one instead of 200 ohm one?

3) Some pots have a power marking. (like 1w) What does this mean? Was the pot built with some 1w power thing in it? Or is it a recommended amount of power that the pot should adjust resistance for? If I needed a pot for a certain output, and there is none available for exactly that output, should I round up or down? What would be the effects of rounding up or down?

4) Can I use 3 pots with the same specs for each: 200mw red, 100mw violet, and 50mw green modules? Or do I need a lower/higher ohms pot for higher/lower output?

5) Would 3 of these work? If not, why? If so, what would make it better?

I would prefer knowing this information for future purposes, but I would still appreciate if you rather give me 3 links to pots you recommend for each module instead. (or 1 link if it will work for all 3)

 

[RedCowboy]

A pot/potentiometer is a variable resistor.

A resistor is like a restriction in a water pipe, a pot is like a valve,
Voltage is like water pressure and amperage is like the volume of the water flow.

Wattage is volts times amps. Joules is power over time, but never mind that now.
A 100 watt light bulb running on 120 volts draws 0.8333 amps. A 100w incandescent light bulb has internal resistance and needs no resistor.

For instance is you have a 4 volt battery and you connect a 1 ohm resistor between the polls, how many amps will flow? If you have near zero internal resistance in the battery you can flow 4 amps, this is how you can test the internal resistance of our batteries.

If you used a 10 ohm resistor then 0.4 amps would be your max flow.



So if you have a m-140 laser diode and you want to limit it to 5 volts at 1.8 amps and your battery source is two 3.7V lipo batteries in series, i.e, 7.4 volts, then what size resistor would you use to limit flow to no more than 1.8 amp
R = E over I
The Diode will only draw 5 volts at 1.8 amps so your voltage drop is 2.4 volts, so your resistor would be 2.4 divided by 1.8 so your resistor would be 1.33 ohms

But this is inefficient, you will be wasting in heat all the power the diode is not using.
Plus your battery will sag and discharge, so you really need a driver, a linear is simple, such as the lm317 just replace the resistor with a pot that covers the range you will need.

Now a switching driver is more efficient and you can buy those ready made, a dc to dc limiting converter you can buy on ebay, they can buck and boost 4 to 36 volts input with 2 to 28 volts out or what ever the specs say, I have some with digital readouts that cost just a few dollars, BUT..... switch the output to your laser diode and break the circuit before turning off/on the dc to dc or you can get a spike.


http://www.ebay.com/sch/i.html?_fro...2.A0.H0.Xdc+to+dc.TRS0&_nkw=dc+to+dc&_sacat=0


You could use flexmod drivers, or buy from ebay what you need.

http://www.ebay.com/sch/i.html?_fro...+driver.TRS0&_nkw=laser+diode+driver&_sacat=0


Building a linear is cheap, just not as efficient.
Here's the 317

 

[Dog]

Thanks for the informative reply, but I'm still confused on a few things.

Are more amps better or worse?
Is it possible to use a potentiometer on a module if it already has a driver? (without removing it) If so, what ohm pot should I get for each module (200mw, 100mw, and 50mw)?
What effects would I get if I used a higher/lower ohm pot instead of the recommended? (I know the amps would go up/down, but what's bad about that?)
Is there a better way I can adjust output of each laser while it's on without removing its driver (without pots)? If so, how do I do it?

I only want to spend ~$10 on the capability of adjusting the output of the modules. Would buying 3 of these and hooking them up like this (pretend the fan is a laser module) work? Would doing that allow me to adjust the output while the laser is on?

 

[RedCowboy]

Too much current " amps " will burn up your laser diode, they are not like light bulbs that have internal resistance, laser diodes will draw more power than they can stand and burn up quickly.

Putting a pot between your drivers output and your diode will be changing your loads resistance. Not recommended.

Without a driver using just a pot is wasteful and pots can only take just so much current, a rheostat would be better because you are turning all the power the diode is not getting into heat.

It's a very wasteful way to do it, but I would not do it that way if you already have a driver, your driver may have a ttl connection so you can pulse control, but I doubt you want to tackle that.

Why not build the linear regulator with a 317?

The 317 is good for 2 amps or more heatsinked. You can get rid of the capacitor and diode, but you need to make sure you limit your output to the diodes max at the end of your pots adjustment.

Look at this circuit, it's not a schematic, I drew it in laymans so you can hopefully see what I am saying.

Adjusted to wide open your pot should allow the M-140 no more than 1.8amps, it's safe limit, maybe a little less from your wire resistance, adjusted the other way it will dim down to below threshold/go off.
p.s. I wrote grounded for the 317 tab, I meant heat sinked, but not grounded, use a simple silicon isolator that will pass the heat load, they often come with the 317 when you buy them, if not get some if your diode is not case neutral or you are using 3 different diodes. Now if your 317's heatsink is not touching any other metal then you can skip the isolator.
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Now if you are using a 200mw red a 100mw 405 ( VERY static sensitive diode ) and a 50 mw 532 then you can use a pot, but without a limiting resistor you can turn it up to zero resistance and blow your diodes, you need to figure for and use limiting resistors in series with your pot. Resistor = voltage drop divided by max current for each diode.

 

[Dog]

I would make a linear regulator if I hadn't already bought the modules with the drivers. Would it still function if I connect it to the wires of a module (the driver) instead of directly to the diode? And will the same design work for all 3 modules?

 

[RedCowboy]

I would make a linear regulator if I hadn't already bought the modules with the drivers. Would it still function if I connect it to the wires of a module (the driver) instead of directly to the diode? And will the same design work for all 3 modules?

The driver will try it's best to maintain it's pre set constant current, if it's a buck/boost driver then you won't do anything but turn it on or off with a pot on the supply side, on the output side you change the diodes resistance and will likely kill it trying to adjust it, or at best it just goes from dim to off.

If you have a soldered in resistor or pot you could insert your pot in series with it to turn the regulation down, but you need to put your pot in series with the factory resistance so you never overdrive it, this means un soldering one side of the drivers control setting resistor or pot and inserting your pot in series with it. If your pot is a multi turn that specs from zero to 200 ohms then your control may be just a part of a turn at one end, the rest will just be below threshold.

 

[Dog]

So, without configuring the driver, it's nearly impossible to adjust the resistance of the diode by connecting a pot to the driver? With one of my cheap $3 Ebay green pointers, I put in 2x 1.2v rechargeable AAA batteries and the green dot was hardly visible (sometimes, it didn't even work), but when I popped in new, non-rechargeable 1.5v AAA batteries, it became VERY bright. Could it have been the voltage? Could it have been a defective driver (that doesn't try to maintain preset current)? Would I be able to add a pot to the power supply to adjust the INPUT voltage? The driver wouldn't just STOP the current because there isn't enough, would it? Why wouldn't this work?

 

[RedCowboy]

If you want to try putting a pot on the battery supply side then go for it, you may get some adjustability, but it likely wont be incremental, but it shouldn't hurt anything, it will simulate batteries running down.
Now understand your pot is not made to handle a lot of current, but as small as your diodes are you can probably get away with it, give it a shot at your own risk, I don't think it would hurt anything.

http://www.ebay.com/itm/200-Ohm-359...382703?hash=item3cfe3bffaf&afsrc=1&rmvSB=true

The pots you listed will handle 2 watts and are 200 ohms, so your adjustability will only be at one far end, most of it's range will just be too much resistance for the driver to work.

 

[Dog]

Would a 200ohm pot work? Which amount would be best?

 

[RedCowboy]

Would a 200ohm pot work? Which amount would be best?

At the 200 ohm end your driver wont get enough power to work, turned all the way to the other end of it's 10 turns it should be zero ohms so your driver will get full battery, as you turn it towards the 200 ohm end you will see some dimming before it cant lase, but your range of brightness adjustment will likely be very narrow, it's not the right way to do it, but because your diodes are so small they may work with 10 ohms in the supply so 200 will shut it off.

The thing is the driver will try to maintain current, as you limit it's input current you will see it dim at some point, but it's going to be a narrow area along your turns that have any effect. Fine tuning where the driver winks out depends on the diode and the driver, as well as your supply voltage.

It's just not the right way to do it, your trying to make something bend that pushes back harder as you push harder until it cant anymore, and you may get some adjustability at that fail to compensate point, if you knew exactly where that was you could get a pot to span that range, but it depends on factors you will have to discover. But it's not the right way and your results can vary, if you have a pot in hand you can try it, just don't burn more than 2 amps in that blue pot.


If your driver has a TTL you can use 0-5 volts to pulse it, but you would need to build a pulse width modulator.....so, maybe you want to get some lasers that have adjustable drivers, or get adjustable drivers for your lasers, or build adjustable linear drivers.
Or try playing with starving the input current and see if you can make it work to your satisfaction.


http://www.ebay.com/itm/Full-color-...4254b35&pid=100034&rk=4&rkt=8&sd=252235510452


http://www.ebay.com/itm/200mW-White...8649c01&pid=100005&rk=1&rkt=6&sd=371273659883

 

[Dog]

I guess it wouldn't hurt to get 3 pots for $10, even if it has little function. You said there would be a narrow range of resistance this would actually function for. Instead of 200 ohms, I'd rather get a pot of that range value so more, if not all, of the turns would effect the output. Would, like you said, 10 ohm ones be better? And would each output have a different pot? If so, what values would you recommend?

 

[VisibleGreen]

How inefficient is the LM317?

 

[RedCowboy]

Not as efficient as a switching/buck/boost driver, but better than a series resistor. The 317 is good for 2.25 amps if heat sinked, but LM 1084's are good for 5 amps.

Efficiency also depends on how much voltage you need to drop, the closer to the voltage you need the less it has to waste, but dropout is like 2 volts over what you need, but there is a low dropout model also, so as your battery wears down it gets more efficient LOL


I prefer buck drivers, this was just a simple cheap way to go.