4.5 volt to 12volt

[jerryb]

Leds still working. What I want to know is what resistor I need? I cannot seem to find any info that will give me a resistance value. Every calculator I try says there are too many leds but how can that be if they will run off 4.5 v?

 

[jerryb]

Just looked at another set of the same lights, there is a 20ohm resistor but it appears to be on the negative wire that leads into the on/off switch.

 

[Martaine2005]

The resistor can be placed in either leg/side.
Your set is either in parallel or two in series then paralleled.
Use a multimeter across the LEDs to get voltage and then in series with the set to get current. Then the resistor can be calculated.

Martin

 

[Martaine2005]

Leds still working. What I want to know is what resistor I need? I cannot seem to find any info that will give me a resistance value. Every calculator I try says there are too many leds but how can that be if they will run off 4.5 v?

They run on 4.5V because the current is very low (560Ω) resistor and 5mA.

Martin

 

[Audioguru]

The LEDs appear to produce white light (a bit yellow) then their voltage cannot be less than about 3V and they are probably 3.2V.
With 4.5V of new battery cells the 20 ohm resistor will have a current in it of (4.5V - 3V)/20 ohms= 75mA. The resistor heats with 75mA squared x 20 ohms= 0.11W. Each LED has a current of 75mA/20= 3.75mA. 3.2V LEDs will use less current and all the LEDs will use less current as the battery voltage runs down.

(12V - 3V)/75mA= 120 ohms. The resistor will heat with 0.68W then a 1W resistor will be very hot so use a 2W or 5W resistor.

They run on 4.5V because the current is very low (560Ω) resistor and 5mA.

Martin

The 20 LEDs are all in parallel and the resistor is 20 ohms (post #22).

 

[Martaine2005]

Not only is my math terrible but I thought a 570Ω resistor was mentioned. I obviously chose a 560Ω to be the closest. I also assumed a 1.8V fV and 4.5V supply. Totalling 100mA for all 20 @ 5mA each.
But I stand corrected.

Martin

 

[jerryb]

With the 4.5v supply and no resistor I get a reading of 4.5 v and if I'm reading the meter correctly 0.7 amps with the meter set to 10amps. How do I use this info to use a 12v supply?

 

[Audioguru]

You do not use a current meter to short-circuit a supply. Instead you connect the current meter in series with the supply, the current-limiting resistor and the load. If you are measuring the current of the 20 LEDs and the reading is 0.7A then each of the 20 LEDs is drawing 0.7A/20= 35mA which will soon burn them out.
The battery on my car can produce hundreds of Amps to start the engine on a cold day. If I connect a current meter to short-circuit the battery then the meter will probably draw thousands of Amps which will explode the meter and/or the battery.