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

[svdsniper]

Hi

I am new here and i came because i seek an answer. I have a device that draws 3.5Amps and i have a 6v sla 4.5Ah battery that i can run it from, problem is the device is of a 1964 vintage and i dont want to dammage it as it was designed to run from a 4.5v battery that is no longer produced and i can not find anything suitable so i would lie to add a component in the positive side of the supply circuit that wil reduce the voltage to around 4.5v and still keep the current. Please can someone help me to select the correct component?

Thank you very much for any help you can offer.

svdsniper

 

[blueroomelectronics]

What is the device?

 

[svdsniper]

Hi

The device is a Soviet Infra red night vision viewer. The actual IR lamp is a 20w lamp.

svdsniper

 

[Hero999]

It depends on what the device is?

If it's just resistive i.e. a heater, lamp or motor, then connect a resistor in series, otherwise you might need a propper voltage regulator.

There again, the device might be robust enough to be powered from a 6V battery.

 

[svdsniper]

Hi Hero999

Could you advise on a suitable resistor?
would the resistor generate much heat?

thanks

svdsniper

 

[Hero999]

Sorry, I missed your post.

Are you from the Eastern block?

That's probably quite expensive.

Is it just an IR lamp or does it include a viewer too?

If it's just a lamp then you should just be able to use a resistor, use Ohm's law.

Hint: Work out the current, then the required voltage drop and finally the resistance.

All I know is you'll need a >5W resistor and I haven't even done the calculation.

 

[svdsniper]

Hi Hero999
I am in UK , i am a collector of Soviet opto electronics and related items. yes this item includes an Infra red illuminator so that the viewer can detect the infra red light and convert it into an image that can be seen .
i run the **broken link removed** website.

svdsniper

 

[colin55]

Put 3 x 5amp diodes in series. I am very surprised a 3.5 amp device runs on 4v5.

 

[Hero999]

That's interesting, seems like an unusual thing to collect.

Here's a page with all the formulae you need.
Ohms Law - Electric Circuit

Hint: the voltage is the voltage dropped across the resistor.

Post what value you calculated and how you worked it out and I'll check it for you.

 

[svdsniper]

Hi Colin555

would you expect a 3.5 amp device to run from a 6v battery?
it is said that either can be used but i had 3 x 4.5v lantern batteries in series and it was brighter than just 2 in series so i need a much higher capacity.

svdsniper

 

[svdsniper]

Hi Hero999

i just did the calculation as follows:-

i want to drop 1.5v so i done 1.5 / 3.5 amps (what the device draws ) and i got a resistance figure of 0.4285714 Ohms

I have tried a 470kohm (band colours are yellow,violet,black,red and finally brown) resistor and it only dropped the voltage by 0.29v

svdsniper

 

[Hero999]

I though you said it's rated for 20W which, at 6V, is a higher current than 3.5A?

If you measure the voltage across the lantern batteries it will probably drop to 4.5V because they're overloaded.

If you go for the diodes then make sure they're rated for at least 5A.

Does it need to be portable?

You could use a 5V, 5A (25W) mains power supply and a single diode, if you're worried about the voltage being too high, although I think you'll be fine with 5V.

 

[svdsniper]

Hi Hero999

the actual lamp has 4.5v 20w marked on it. When we tested the draw of this item with the viewer and lamp turned on it was drawing 3.5amps i want to run it at 4.5v from a 6v battery and yes it has to be mobile as the battery compartment is belt mounted.

svdsniper

 

[Hero999]

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.

 

[svdsniper]

when we tested this i had 3 of the 4.5v lantern batteries in series to give greater capacity and the voltage was 4.5v , the draw was confirmed as 3.5A but although the lamp is marked as 20w it was not lighting very bright, it was lit but nowhere near its correct brightness so this may explain the problem with violating ohms law.

svdsniper

 

[Hero999]

Three 4.5V batteries in series would give 13.5V so the voltage must be dropping to 4.5V because the batteries were overloaded.

Are you sure you didn't connect them in parallel?

 

[svdsniper]

sorry yes you are right , my mistake.
i have them in front of me and they are in parralell

svdsniper

 

[Hero999]

That sounds more like it.

If your voltage and current measurements are correct then the real power rating is 15.75W.

I suspect it's nothing more than an incandescent lamp with an infra-red filter to remove most of the visible light.

 

[svdsniper]

yes you are correct , it is an incandescent lamp with an IR filter, there is no visible light comming through the filter. it works well for an old device.

svdsniper

 

[Hero999]

In that case, why not simply replace the bulb with a 6V bulb of a similar wattage?

I suppose the problem is getting one with the right fitting.