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[SimonCornes]

Hi,
I'm after a bit of opinion please. I have 7 small LED's wired up to a 3v Duracell pack with a rheostat/variable resistor in the negative feed.


I have noticed that the rheostat is getting hot - I first noticed the 'hot' smell! I've taken the pair of AA cells out of the battery holder so I'm safe enough now but what have I done wrong? Most of the LEDs are 3mm 'soft' whites and I turned the lighting 'down' to make them more like 1930's tungsten filament bulbs. I think the maximum resistance of the rheostat is 1 ohm and the rheostat is useful because, as the voltage drops, the LEDs go out so I turn the brightness 'up' and they come back on again!


This must be fairly simple but I'd be grateful if you could give me a bit of advice please!
Thanks


Simon

[Martin (Admin)]

Depending on the type of rheostat/variable resistor, pot (potentiometer) you're using, it sounds like it too low a wattage / current rating.
Maybe it's worth looking at a simple voltage regulator instead.

Eg. https://tinyurl.com/35p2jzmc ( this one has everything built in )

[SimonCornes]

Thanks Martin
I am using a 16mm 1 ohm linear variable resistor. A mate has suggested that I need at least a 10 ohm one so I have ordered a 100 ohm to see if that will do the trick. You wouldn't think that a pair of Duracell's would cause any kind of overheating or that 3mm LED's would pull any significant current? The 100 ohm resistor is a pretty cheap fix but I may use your suggestion as a Plan B as it would complicate my wiring some!

[GG]

Simon,
        I'm guessing that the LED's are wired in parallel with the battery, in which case the current drawn will be the sum of the individual LED currents.  The AA Duracell's will probably cope with this but not the variable resistor if it is one of those small low power ones which were never intended to act as current reducing resistors.


If you want to duplicate the golden glow of 1930's filament bulbs, perhaps using Yellow LED's would be better than trying to dim White LED's? 


Glynn Guest

[roycv]

Hi the Buck DC -DC converter was for a fixed o/p voltage. 

I bought one from Component shop with a variable o/p and I am running an electric motor from an o/p of 2.2 volts.  It was very cheap but it was not on their website when I last visited.
Roy

[SimonCornes]

Simon,
        I'm guessing that the LED's are wired in parallel with the battery, in which case the current drawn will be the sum of the individual LED currents.  The AA Duracell's will probably cope with this but not the variable resistor if it is one of those small low power ones which were never intended to act as current reducing resistors.


If you want to duplicate the golden glow of 1930's filament bulbs, perhaps using Yellow LED's would be better than trying to dim White LED's? 

Too late for that Glynn - fitting into inaccessible places!! I’m hoping that a 100 ohm variable pot will be okay!!

[JimG]

Increasing the size of the resistance by 100 will not help with the overheating as you will still only be using a small portion of the track. The problem is that most of the lower wattage variable resistors are made with a thin carbon coating on a thin section of circuit board. Any more than a minimal current will cause the carbon track to heat up leading to overheating of the backing giving the burning smell. The only way round this would be to try and get a suitable higher wattage wire wound resistor, these however are much larger.
Jim

[SimonCornes]

Increasing the size of the resistance by 100 will not help with the overheating as you will still only be using a small portion of the track. The problem is that most of the lower wattage variable resistors are made with a thin carbon coating on a thin section of circuit board. Any more than a minimal current will cause the carbon track to heat up leading to overheating of the backing giving the burning smell. The only way round this would be to try and get a suitable higher wattage wire wound resistor, these however are much larger.
Jim

""xxxxx"" - Thats a technical term! Jim, could you suggest a suitable wire wound resistor then and maybe what wattage would do? I find it hard to believe that 7 LEDs cause so much trouble! Originally I would have used tungsten filament grain of wheat bulbs but I was seduced by the idea that LEDs would last forever - at this rate the rest of the boat won't!
Ive just checked the LED's - 20 milliamps apiece so thats 140 mA in total. Is that a lot for a simple rheostat?

[roycv]

In a quick answer Yes it is a lot for a probably small component.  For a 100 ohm variable resister it needs to be current sqared times the resistance 2 watts.Use a variable o/p Buck DC-DC converter and forget about it.
Regards
Roy

[Martin (Admin)]

Like this one?     https://www.amazon.co.uk/DollaTek-Converter-0-5-30V-Adjustable-Regulated/dp/B08P37TKHY/ref=sr_1_6?keywords=Buck+Boost+Converter&qid=1644974589&sr=8-6

[C-3PO]

Two sources of info

LEDS - resistors in parallel and series and how to calculate values

https://modelshop.co.uk/static/LEDs-resistors

Series Resisitor Calculator

https://www.digikey.co.uk/en/resources/conversion-calculators/conversion-calculator-led-series-resistor

C-3PO

[grendel]

a lot depends how you are using the resistor, if using it as a voltage divider, ie the battery connected to each end and the LED's connected to the middle leg, you will automatically draw 3A across the potentiometer on a 1 ohm resistance, to calculate the resistance needed you need the current flowing to be correct for the LED output, ie 140mA, so a resistance of approximately 22 ohm would be correct when using a potentiometer as a voltage divider.

[SimonCornes]

I'm using it as voltage control. Positive wire in one side and positive wire out the other and thats it.
Maybe a wire wound resistor is the best answer?