voltage drop...............please help

the problem is that it is a sealed beam unit. and i just imported 3 spares for it lol.
going to try the 5amp diodes in series to see if that does the trick. these devices are said to use from 4.5v to 6.0v on the net but info from U.S is very unreliable as i have had many people enquire about why their sight has stopped working only for them to tell me that they are powering its 6.25v circuit with a 9v battery because they read it was ok lol

svdsniper

Hero999 said:

Did you measure the voltage as well?

It's not possible for a device to draw 3.5ADC from a 4.5VDC supply and the power rating be 20W, it violates Ohm's law - a fundamental law of physics.

20/4.5 = 4.44A

If it's 20W when the current will be 4.44A.

My guess is the voltage is dropping below 4.5V so the current is also dropping, or the rating of 20W is a conservative estimate and the actual power draw is much less.

Click to expand...

i think it over loads the battery and the rest of the power is wasted in the battery when he works with 6V.

why dont you try with NiCd or NiMH rechargable batteries? they cab diliver high current and each cell is 1.2V, so 4 cells in series may be suitable. you can make cells in parellel to make more capacity.

The trouble is they won't last for very long because the Ah rating is this 2500mAh and the current drawn is 3.5A.

If you want to go with NiMH, then I'd suggest using C or D cells with a capacity of 4000mAh to 5000mAh or 9000mAh to 10500mAh respectively and make sure you use real C or D cells, some are just AA cells or C cells with extra padding - always check the capacity. Real D/C cells will always be much more expensive than AA cells, a D cell will be at least four times the price of a single AA cell.

Hi All

Today i bought 3 rectifier diodes rated for 6A , i soldered all three in series and got down to around 5v direct from the battery and it was still too high but when i powered up the device through the diodes i found the voltage reaching the lamp was now too low. I removed one diode at a time and retested and found that by using a single diode i was able to achieve exactly 4.5v at the lamp and this is the voltage that the lamp is rated for, so i am guessing this device is now running at the correct voltages as the image intensifier tube may have a higher voltage circuit inside but as i now have the correct output from the control module to the lamp i expect the internal voltages to be correct too.

Thanks to everyone for all the help, now i just need to wrap the diode in a fibreglass sleeve and heat shrink as it does get quite hot.

svdsniper

The voltage will drop as the battery discharges.

I wouldn't recommend putting too many layers over the diode because it will keep the heat in and make it hotter.