p7 mag problem

Hi I recently built a 3D P7 maglite and it has worked fine until today when I tested the voltage actually getting to the bulb. I usually just test the batteries with my multimeter before I put them into the light but today I put the batteries int hte light and touched the positive and negative wires on the bulb and turned the light on. Outside the light the batteries measured 4.02v but inside the light they measured 3.37v which even though the C bin P7 I have is rated for 3.25-3.50 volts I would like to get at least 3.6 volts. The circuit I am using is this http://www.kaidomain.com/ProductDeta...ProductId=1770 is the circuit consuming the voltage?

The batteries I'm using are two CTA 12,000 mah D cell batteries and one radioshack 4,500 mah D cell battery ( I know I should use batteries with matching mah outputs) All are 1.2v NiMH's.

can someone please explain what is happening or why

3D black mag

Hi there;
i think there is a faulty battery there,droping its voltage when current is asked.You should test them.

Hi synergic .I do not think it is a faulty battery because I also did the same thing using three energizer D cells that measured 4.5 volts outside the light But when I put them in the light and measured them they measured 3.04 volts I do not know why.:shrug:

any other suggestions or explanations?

3D black mag

You are measuring the P7's forward voltage, not battery voltage. Your numbers look fine.

The voltage drop is normal. The battery has an internal resistance which will cause the voltage drop when the current rises.

Alkalines have higher higher internal resistance than NimH batteries.

Edit:
like justin case said your measurings seems to be ok since you are measuring voltage at the led and not at the battery

OK so my bulb is being run at around the 3.7 that the driver should put out?

3D black mag

No, you're bulb is running at the current set by the driver. The voltage does not really matter

3D black mag said:

OK so my bulb is being run at around the 3.7 that the driver should put out?

3D black mag

Click to expand...

Each LED has its own "V-I" (forward voltage vs forward current) characteristic curve. If you send a certain amount of current to the LED, then LED voltage across its terminals is whatever the Vf has to be for the If being sent to the emitter.

For your P7, whatever current it is that you are feeding to the LED using your NiMH cells, the Vf happens to be 3.37V. When you use the alkalines instead, those cells can't deliver the current that the NiMH cells can so the Vf for that current being sent to the LED is 3.04V (Vf decreases as current decreases).

With your NiMH cells, you are probably sending close to 2.8A to the LED since the Vf you measured is in the ballpark of the I-bin spec of 3.25V to 3.50V. With your alkalines, you are sending less than 2.8A. My guess is that it is close to 1A.

Oh ok I get it know so the voltage adjusts according to the current.

Thank you der witchell and justin case

3D black mag

I didnt read that you were measuring voltage in the led.....

Your setting is running well.

Thanks :laughing: