Using a potentiometer in conjunction with a badboy

[gmarshall139]

I want to build another light, regulated, with a potentiometer for brightness control. Thought about a lux III with a bb1000. I want to go from off to full bright.

I know I can just get a buckpuck already set up but like most things i'd like to set it up myself if possible. Plus I'd rather drive it off 3 volts instead of the 5 required by the buckpuck. I was reading the buckpuck specs and it uses a 5k linear taper pot to achieve just this.

How will I wire it and what size pot will I need? Check this link out: http://rocky.digikey.com/WebLib/CTS/Web%20Data/026%20Series.pdf

These are the only pot's listed in the digikey catalog rated at over 2 watts.

Thanks,

Greg

 

[greg_in_canada]

Are you trying to put the pot in series with the LED?
That's the only way you'd need a high wattage pot.
If it is for the control circuit then any 1/4 or 1/8th
watt pot should do.

Greg

 

[gadget_lover]

Re: Using a potentiometer in conjunction with a ba

A pot in parallel with the sense resitors may do the trick for you.

Daniel

 

[dat2zip]

Re: Using a potentiometer in conjunction with a ba

You can not parallel a pot with the sense resistor. Since the sense resistor is lower than 1 ohm adding a parallel pot is rarely that accurate. You would need a 1 ohm pot that can be turned down to 0.050 ohms with accuracy and handle 1 Amp of current. Most carbon pots wiper contact resistance is usually a 0.1 ohms or greater.

Adding the pot in series with the LED won't affect the outcome since the driver is constant current. It will still drive the same current no matter what resistance you put in series with the LED.

You can add a series resistance between the battery and the converter board. Just be aware that to get full power the pot must handle up to 2A of current and needs to have zero ohms of resistance in this mode.

There are a lot of better solutions than this and one might want to look at a step down configuration using a linear regulator wired as constant current source. The LM317 constant current source is a common circuit and can be found here on the forum or a google search.

 

[gadget_lover]

Re: Using a potentiometer in conjunction with a ba

I don't dispute what wayne says in any way, but.....

If you have a badboy set for 100ma, and add a 0 - 10 ohm pot accross the sense resistor, would the end result be a variable from 100ma to max? It would not give consistamt results, but it should alter the output. I figured the wiper and wiring resistance would keep it from going too high, and if it went open you'd still have the original resistor.

It's late, and I'm just thinking out-loud.

Daniel

 

[dat2zip]

Re: Using a potentiometer in conjunction with a ba

Daniel,


You will probably start seing the parallel resistance of the pot and the sense resistor affect the brightness somewhere around the pot resistance of 10X the sense resistor. In this case, that would be 5 ohms so, yes, that would be true.

But, if there is no limiting resistor in series with the pot and the trim pot went to a true zero, it would send the Badboy into maximum output current which could be greater than 1A.

Adding a external pot requires some careful thinking. The pot must carry current through it as well. Thus, the pot needs to be able to handle the power in Watts as used.

A much cleaner solution if we are still talking Badboy is to lift the Iadj pin and apply a 0-2.5V to this pin using a low power trim pot to this pin. This will adjust the output from near zero to the maximum set by the sense resistor. The Iadj pin is high impendance and this form of control does not require a high power potentiometer. This pin is also DC and will not affect the loop stability.

One can fake this control by assuming the Vf output is nearly constant over the full control range and use Vout to the trim pot to derive the 0 - 2.5V control.

 

[greg_in_canada]

Re: Using a potentiometer in conjunction with a ba

Thanks data2zip,

That answers a few of my questions from this
( link ) thread.

Is a bb1000 a badboy that puts out 1000 mA? I
don't see it on your web page. Is it just sold out?

Thanks - Greg

 

[dat2zip]

Re: Using a potentiometer in conjunction with a ba

You need to order a BB blank and specify the two resistors as 0.1 and 0.1. It's an option in the BB blank when you order.

Wayne

 

[greg_in_canada]

Re: Using a potentiometer in conjunction with a ba

Thanks Wayne,

I see. So is this equivalent to overdriving a LED
or is the BB rated to put out 1 Amp?

Greg

 

[dat2zip]

Re: Using a potentiometer in conjunction with a ba

To get the full 1A out of the BB you would need to have the battery when loaded to not drop below 2.8V. Most batteries internal resistance drops the battery voltage when loaded. The 123 with a 1A load drops from 3V to 2.5V.

Alkaline batteries I think drop even more.

In this case, it's probably more efficient just to use a 3C configuration and a small series resistance.

The BB1000 is not very efficient.

For a 123, the highest output configuration is a BB600 or so. Any higher with 2AA or a single 123 and you would be not get any more output to the LED.

In a step up configuration you need to carefully balance the battery, converter and LED.

Wayne

 

[greg_in_canada]

Re: Using a potentiometer in conjunction with a ba

I bought a 3D mag for this project. I was going to
start with a direct drive approach and later get
a converter.

But I could get a BB now and run with a dummy cell
(with alkalines), or add a series resistor to limit
the current with fresh batteries.

So the BB isn't damaged by Vin higher than Vdiode
(as long as the current is limited by resistance
somewhere)?

I haven't damaged the mag yet, so I could trade it in
on a 2D if that was a better approach. But it sounds
like 2D alkalines would drop to a low voltage pretty
quick.

Thanks for the great info - Greg

 

[dat2zip]

Re: Using a potentiometer in conjunction with a ba

A 3D mag.

I would test it, but, you might be lucky with that configuration and the battery voltage sag might be enough to keep the Badboy happy.

I think others have done this. You just need to be careful since the Vf of the LED, battery and converter all need to be taken into consideration.

Wayne

 

[greg_in_canada]

Re: Using a potentiometer in conjunction with a ba

Thanks for all the advice Wayne. I'll measure my Vf
when I get my heatsink then decide how to go.

Greg

 

[MegaMe]

Re: Using a potentiometer in conjunction with a ba

i dont understand why cant you just use a mad max+ with a potentiometer??? as mad max'c and be adjustable in the first place cant they?

(sorry im a newbie /ubbthreads/images/graemlins/smile.gif )