Replacing batteries with power adaptor?

[baker]

I have a portable 120 led light that can run on 6 X AA batteries or an old sony 7.4 volts battery pack.

I would like to run it on a power adaptor.

From my calculation, it looks like it runs between 6 X 1.2v = 7.2 volts or 6 X 1.5v = 9 volts on AA batteries.

How do I calculate what amps I needs for my power adaptor and when it comes to led lights, does the amps matter or will the lights just be dimmer when you underpower the amps supplied to the led lights?

When it comes to led lights, is it better to be over than under in volts?

I have a 9.5 volt power adaptor that runs 2.7amps. Can I use this or should I buy an 8 volts 2amp adaptor from the store.

Thanks

 

[pucho812]

I have a portable 120 led light that can run on 6 X AA batteries or an old sony 7.4 volts battery pack.

I would like to run it on a power adaptor.

From my calculation, it looks like it runs between 6 X 1.2v = 7.2 volts or 6 X 1.5v = 9 volts on AA batteries.

How do I calculate what amps I needs for my power adaptor and when it comes to led lights, does the amps matter or will the lights just be dimmer when you underpower the amps supplied to the led lights?

When it comes to led lights, is it better to be over than under in volts?

I have a 9.5 volt power adaptor that runs 2.7amps. Can I use this or should I buy an 8 volts 2amp adaptor from the store.

Thanks

 

[pucho812]

most leds at max pull miliamps of current.  you can find those details in a data sheet. A single AA battery can according to energizers can output 2400miliamps for an hour, 1200 miliamps for 2 hours, and so forth and so forth. So your 2.4 amp power supply seems way overkill.  Same with the 2 amp supply

 

[PRR]

> 6 X 1.5v = 9 volts on AA batteries.

Put your voltmeter on the AA-pack while the light is running.

LEDs need a current limiter.

A resistor can be part of a current limiting scheme.

A rather-small (for the load) battery has significant resistance.

The light maker may have *assumed* the source was AA cells with non-zero internal resistance, so didn't do much else to limit current.

The LEDs may even have been selected for higher turn-on voltage to make the scheme work (and because over-volt LEDs are often cheaper).

I do not know this "old sony 7.4 volts battery pack", how much resistance it may have.

120 RED LEDs are not much good. White LEDs run around 3.5V-4.0V. I don't know if they run two in series so that a sucked-down 6*AA pack "works", or if they have limiting resistors on every LED.

I would assume 20mA per LED and 60 strings, 1.2 Amps. Given a 9.5V supply, I would guesstimate 9.5V-8V is 1.5V to lose, is 1.5V/1.2A= 1.25 Ohms, 1.5V*1.2A= 1.8 Watts heat, and grab a 1 Ohm 5W resistor to put in series for smoke-test. (If the light is valuable I might start with 2 Ohms and work down.)

 

[baker]

I managed to find a 7.5 volts 1amp adaptor.

pucho812 if the amps gradually decrease with time with the batteries, does that mean the led lights are able to run at a wide range of amps from 2400ma to below 1000ma? Does that mean the leds will still be the same brightness from 2400ma to 1000ma and lower?

Whats the worst that can happen if my amps is lower than what is required by the led light? Will it just be a lower brightness output, flickering, decrease the led light lifespan, damage either of the two units?

Thanks.

 

[kato]

I managed to find a 7.5 volts 1amp adaptor.

I'd imagine 1amp could overheat and die. (Still might be worth trying.)
Estimating 20mA per LED x 120 LEDs = 2.4 amps.
Safest bet is to get an inexpensive switching supply that can do 4A like this one. You might have to change the plug to fit, and possibly switch the polarity.

 

[baker]

I was told that to calculate the amps needed for a circuit, you would find the wattage and divide it by the voltage.

So the manual sais the led light uses 12 watts, so if it is powered by 6 (1.2 volts) AA batteries  = 7.2 volts, then that would be 12watts / 7.2v = 1.6amps.

I have wired up my dc socket and piggy backed the positive and negative of the battery compartment to power the led lights. I have ordered a 7.5 volts 2amp power adaptor.

What would happen if I have my power adaptor on and it is plugged into my led light and;
- the rechargeable batteries are installed but the led light power switch is off but the power adaptor power is on? Will that charge the batteries?
- the rechargeable batteries are installed and the power switch is on? Will that provide too much power to the led light and it will damage it?

Thanks