Help please, space needle resistor rating?

Essexman · Nov 30, 2006

Help please. My first build.......
I'm building a space needle using a Lux V (WWOT) on an hotlips heat sink, mag 2D, six AA' s. I plan to use alkaline, which means running at 9V. I'm going to use a 1 ohm resistor to drop the voltage down, so how many watts should the resistor be rated at? At the moment I can only find a 2.5W lying around at work, will that do?

(Yes I know direct drive is not best practice, I plan to get a driver soon)

Essexman · Dec 2, 2006

.......and the tumbleweed rolls by........................

modamag · Dec 2, 2006

You need to drop the voltage by ~1V so try one of those ceramic 1/2 Ohm - 5W resistor . See what the current is and adjust from there. The effective voltage under load for 3xCR123 is roughly 7.8V @ 1A draw.

Keep in mind that if you switch to a different battery chemistry (Li-Ion) you'll also need to change the resistor.

As for the resistor rating it should be at least 1W. Your 2.5W will work too. Just remember to Artic Alumina that to the Mag body to drain the generated heat.

Happy Modding,
Jonathan

Niteowl · Dec 2, 2006

Essexman said:
......I'm building a space needle using a Lux V (WWOT) on an hotlips heat sink, mag 2D, six AA' s. I plan to use alkaline, which means running at 9V.........

I was under the impression that alkalines would not supply enough current for 5w emitter. I was going to use a DB917 with six C/D alkalines, but was discouraged by response to my posting.

Essexman · Dec 3, 2006

Thanks for the response guys.
I went for the 1 ohm 2.5 watt resistor. It works! I was hoping to get 1Amp current draw, but when I measured the current I only got 770mA.
Now I'm not sure if this is because of the resistor or the fact that I'm using alkalines?
I'm going to try some other resistors next week and see if it makes changes to current.

Best of all, my nest door neigbour saw it and wants one. Any idea how much I should charge him?

winny · Dec 3, 2006

Essexman said:
I went for the 1 ohm 2.5 watt resistor. It works! I was hoping to get 1Amp current draw, but when I measured the current I only got 770mA.
Now I'm not sure if this is because of the resistor or the fact that I'm using alkalines?

Alkalines.

They don't supply high currents very well and 5 W from 6 batteries is on the edge. If you want to use alkalines, you can of course do that but keep in mind that you would most likely gain some serious runtime if you used NiMH's instead.

Essexman · Dec 3, 2006

OK. This must have been covered before? The space needle is years old.
Out of intreast I'll try NiMH's and see what I get. The only problem is that I'm stuck with 6 AA's, so I'll be dropping from 9V to 7.2V. But with increased current draw. Swings and rounabouts I think.
I'll post back with my finding in the next few days.

Help please, space needle resistor rating?

Essexman

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Essexman

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modamag

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