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phatlight help?????

bennett326

bennett326

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hey guys

so i need some help i bought a mag light (2D) and i want to use the regular alkaline cells at first i thought i could just direct drive but with two cells my meter couldn't read it because it was over 10 amps so i need driver options

are there any diy ones or do i need to buy something?

thanks for the help guys
 





Jacob32123

Jacob32123

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bennett326 said:
hey guys

so i need some help i bought a mag light (2D) and i want to use the regular alkaline cells at first i thought i could just direct drive but with two cells my meter couldn't read it because it was over 10 amps so i need driver options

are there any diy ones or do i need to buy something?

thanks for the help guys
Click to expand...

You just need a resistor to regulate current.

r = v/i
v = 3 (2 alkaline batteries in series)
i = 8.1 (a pt54's recommended drive current)
You need a .37 ohm resistor to limit the current to 8.1 amps

w = v*a
v = 3
a = 8.1
Your resistor will need to be rated for >24.3 watts

Hope this helps
 
Last edited:
Gryphon

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Wait, are you saying your 2 alkaline batts are putting out over 10A?! If i may, what kind of load were they under when you measured them? when a battery is put under load from the device its powering, the internal resistance of the battery will cause voltage and current to sag.
 
bennett326

bennett326

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i just tested them with my multimeter....

EDIT: so you think two alkalines wouldn't put out more than 8.1 amps
 
Last edited:
Jufran88

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Wow, 10A with 2 alkaline D batteries? Sounds pretty scary to me, at that point your cells might be heating up like crazy and potentially start leaking.

And to even get that kind of amperage, you would need crazy low internal resistance.
 
Jacob32123

Jacob32123

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Jufran88 said:
Wow, 10A with 2 alkaline D batteries? Sounds pretty scary to me, at that point your cells might be heating up like crazy and potentially start leaking.

And to even get that kind of amperage, you would need crazy low internal resistance.
Click to expand...

I did a similar build with 2 D alkaline batteries, and my DMM read in that the phlatlight was consuming ~12 amps IIRC. However, the voltage dropped to 2 volts with that kind of load

EDIT: Post 405, I am now into the visible spectrum :D
 
Last edited:
Jimmymcjimthejim

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Jacob32123 said:
You just need a resistor to regulate current.

r = v/i
v = 3 (2 alkaline batteries in series)
i = 8.1 (a pt54's recommended drive current)
You need a .37 ohm resistor to limit the current to 8.1 amps

w = v*a
v = 3
a = 8.1
Your resistor will need to be rated for >24.3 watts

Hope this helps
Click to expand...

Both of your calculations are incorrect. In order to calculate how much resistance you need, you must first determine how much current will be passing through the LED. So, lets say you want to regulate it at 10A. Now, using the datasheet, calculate what the voltage drop is at that current. At 10A, it should be around 2.8V. Assuming that your batteries are able to sustain that current, subtract your battery voltage from the LED voltage. So let's say we're using a 4.2V Li-ion battery. The voltage difference is 1.4V. So, since the current is 10A, and the required voltage drop is 1.4V, our resistance needs to be 1.4/10= .14 Ohms. In order to calculate the wattage of the resistor, we need to see how much power is dissipated by using the formula I²R, which turns out to be 14 watts.

Sorry if it seemed harsh, but I just hate it when people give out incorrect information.
 
bennett326

bennett326

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ok so i hooked it it up to the phatlight and i read about 7-8 amps wich is fine for now but i want to use rechargeable batteries later on.

i read the data sheet and im starting to understand my problem is i dont think 2 rechargeable d batteries with have enough voltage . (only 2.4) and i dont think i can direct drive with nimh

/?????help
 
Jufran88

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A fully charged NiMH D cell gives out about 1.4 -1.5V. Did you have the red LED or the green LED?
 
Jufran88

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Alright, the datasheet says the forward voltage of the red is about 2V min, 2.7V typical, and 3.5V max. You could direct drive it with 2 D cells and be fine. However, internal resistance will limit how much current you draw. Just be careful.

I direct-drived an SST-90 using 3 D NiMH cells and overpowered it, but it worked for about 10 minutes and then poofed. :(
 
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