ultra low value pot.

[mercrazy]

i need to change resistance on a LED driver to vary current. i need a pot value that goes as low as 0.25 ohms. i can't find a pot with that low value. how can i do this so the LED brightness is adjustable? guess i could put a 0 to 10 ohm pot downstream of the driver but i would like to keep it more efficient than that. thanks.

 

[SemiMan]

i need to change resistance on a LED driver to vary current. i need a pot value that goes as low as 0.25 ohms. i can't find a pot with that low value. how can i do this so the LED brightness is adjustable? guess i could put a 0 to 10 ohm pot downstream of the driver but i would like to keep it more efficient than that. thanks.

You don't .. with a pot at least. You modify the circuit to allow adjustment or PWM in another circuit location other than the feedback resistor.

 

[Steve K]

a quick search on digi-key pulled up a Vishay brand potentiometer that is available down to 2 ohms. It's a multi-turn pot, so maybe that's good enough? You didn't mention how much current will be passing through the pot, so I can't say whether it is suitable for your application.
http://www.vishaypg.com/docs/63055/1202.pdf

Generally, though, a better practice is to adjust the reference voltage instead of the resistance of what I assume is the current sense resistor.

 

[mercrazy]

both responses are above my pay grade.
i would like to run LEDs at 450mA down to 0. yes, i was intending to vary the current sense resistor to change current. how do i vary reference voltage for ZLED7020? or any other solution for this chip? thanks

 

[markr6]

I can't believe noone said "the stuff growing on the side of the interstate" yet.

Sorry, I had to

 

[Steve K]

both responses are above my pay grade.
i would like to run LEDs at 450mA down to 0. yes, i was intending to vary the current sense resistor to change current. how do i vary reference voltage for ZLED7020? or any other solution for this chip? thanks

for the benefit of other readers, the datasheet for this converter is here:
http://www.mouser.com/catalog/specsheets/ZLED7020_Data_Sheet_rev1.2_01.pdf

paragraph 2.2 does provide the equation for calculating the sense resistor, so the 0.25 ohm value is roughly correct for the desired 0.45A output.

I would wire a pot in series with a 0.25 ohm sense resistor... this avoids accidentally dialing the pot below 0.25 ohms (if it is possible).
If you want to dial it down to 0 amps, then you'll need a pot that goes up to infinity (or something close), which is a bit problematic. A logarithmic pot might be better than a linear pot. These are usually used in audio applications, so that will really limit the values available, as well as the styles.

Since the pot would be wired in series with the sense resistor, it'll need to stay close to the converter chip. Not sure if this is convenient for you.
So many aspects of adding a pot in series with the sense resistor are messy and could cause circuit problems. It wouldn't be my first choice.

The best way to dim the LEDs is to do what both Semiman and the datasheet recommend... run a pwm signal into the "Adj" input and vary the pwm duty cycle. If it was me, I'd just throw together a circuit with a 555 timer chip or something. There may be some sort of module that does this, perhaps??

 

[mercrazy]

good one Mark...i hadn't thought of that...lol.

now i understand the pwm on adjust pin. seems i would still need a pot and it adds more circuit complexity and i'm crowded for space already. i'm also leery of pots in an outdoor/moist application. i must have almost zero chance of failure. do they make a waterproof pot?

 

[Steve K]

if you go to any of the electronics retailers such as Digi-key, Mouser, Newark, etc., you can do a parametric search for different kinds of pots. They will also provide names of the manufacturers, and you can go to their web sites and do more detailed searches.

I haven't dealt with waterproof pots. If you are talking about running a shaft through a housing, with a knob on the outside of the housing.. that could be a challenge.

You'll probably want to be familiar with the IP rating system for moisture resistance..
https://en.wikipedia.org/wiki/IP_Code

Maybe go visit the Dive Lights subforum and ask them?

 

[beley]

According to the datasheet (Section 2.3) the ADG pin can use a DC signal to vary the output current, this method would allow the use of any value of pot (within reason) and doesn't add the extra complexity of a PWM circuit.

And yes, they do make waterproof pots, you'll pay quite the premium for them though from what i have seen.

 

[DIWdiver]

Unfortunately, PWM is the only way to dim that controller in the range of 0-25% of nominal output. It is possible to adjust the nominal output by changing the sense resistor, but anything you put there has to be able to handle the full output current. Small pots just aren't made to handle currents like 450 mA.

There are plenty of small inexpensive (a few dollars) 'trimmer' pots that are washable, meaning they can be put through what's essentially a fancy dishwasher. They rarely come with IP ratings, but 'washable' is approximately equivalent to IP65 or IP66, though it isn't an official designation based on specific requirements and backed by testing.

I think your best bet to achieve what you are looking for is to choose a different controller, one which is designed for dimming with a pot, then choose a washable pot of suitable value.