Boost converter or turn series to parallel in an LED work light.

[rjenkinsgb]

I'm trying to convert it to use my Milwaukee tool batteries, 21V max, 18V nominal.

Just remember the voltage range of the battery.

At full charge, 5 x 4.2V = 21V
By the time it's discharged, it will be down to roughly 3.5V per cell so 17.5V total

With a simple LED + resistor circuit, that's a change of 3.5V, almost entirely across the resistor, which will give a very significant change in brightness.

That's why some form of converter is normally used, so you get a constant output voltage or constant current, regardless of battery voltage.

 

[HR19]

Just remember the voltage range of the battery.

At full charge, 5 x 4.2V = 21V
By the time it's discharged, it will be down to roughly 3.5V per cell so 17.5V total

With a simple LED + resistor circuit, that's a change of 3.5V, almost entirely across the resistor, which will give a very significant change in brightness.

That's why some form of converter is normally used, so you get a constant output voltage or constant current, regardless of battery voltage.

My plan is to use a rheostat so I can adjust the brightness.

 

[Nigel Goodwin]

My plan is to use a rheostat so I can adjust the brightness.

Bit difficult when you can't even decide what current or power it's taking - you've given two wildly different values (1W and 24W), which is it?.

 

[HR19]

Bit difficult when you can't even decide what current or power it's taking - you've given two wildly different values (1W and 24W), which is it?.

One is a calculated usage, one is a rating from the manufacturer. A rheostat could also easily handle numbers like those, not sure why you think there's a problem.

 

[Nigel Goodwin]

One is a calculated usage, one is a rating from the manufacturer. A rheostat could also easily handle numbers like those, not sure why you think there's a problem.

I suspect you don't really understand the wattages involved? - and a rating of 24W isn't only going to draw 1W, and wouldn't be terribly bright if it did. 1W from an LED totch is pretty damn bright, but only in a small area - lighting to cover an area is MUCH more spread out and needs much more power.

A rheostat is an antique term (pre-1960's?) and usually refers to a massive huge wirewound device, such as used to be used in ancient theatre lighting (they used to have them where I went to school ) A modern device is a potentiometer, which is only low power, and designed to control electronics.

I think there's a problem because you appear to be hoping to control 24W with a device that may only be able to control milliwatts - there's also the issue that as you get towards the end of the track you get high wattage across only a very small portion of the track, which futher limits their power capacity.

If you're actually using an antique rheostat, then there shouldn't be any problem - other than inefficiency and heat.

 

[HR19]

I suspect you don't really understand the wattages involved? - and a rating of 24W isn't only going to draw 1W, and wouldn't be terribly bright if it did. 1W from an LED totch is pretty damn bright, but only in a small area - lighting to cover an area is MUCH more spread out and needs much more power.

A rheostat is an antique term (pre-1960's?) and usually refers to a massive huge wirewound device, such as used to be used in ancient theatre lighting (they used to have them where I went to school ) A modern device is a potentiometer, which is only low power, and designed to control electronics.

I think there's a problem because you appear to be hoping to control 24W with a device that may only be able to control milliwatts - there's also the issue that as you get towards the end of the track you get high wattage across only a very small portion of the track, which futher limits their power capacity.

If you're actually using an antique rheostat, then there shouldn't be any problem - other than inefficiency and heat.

I suspect you don't really understand what I've been saying this whole time. The LED array is rated at 24W. I'm running it through an effective 73.3ohm resistor. The voltage across the resistor is 4.41V at the moment. Ohm's law tells us that's about 60mA. I have 12 parallel sets, makes 5mA through each set. 5mA*2.5Vf makes 12.5mW*72 LEDs = 900mW, roughly 1W. It's way too bright to look at directly. Is it bright enough for an area? Doubtful, probably only 100-200 lumens (if it's 2500 lumens at 24W, then probably in the range of 100-200 lumens at 1W since LED output isn't linear).

I know what a rheostat is and I know what a potentiometer is. That's why I'm using the term rheostat, is because I plan to use a rheostat. A rheostat is, functionally, a potentiometer with very low resistance, and higher power rating. For the most part, I do my best to know what I'm talking about when I talk, and to ask questions when I don't. Some times I fail, but mostly I'm pretty good at it.