Problem with MUX

[ruzfactor]

hi ericgibbs

Not in series. They are connected in parallel as your second schematic.

 

[ruzfactor]

hi audioguru,

i'm now using two 9V batteries and using a 7812 for +12V from +18V in the circuit. This +12V is also used in 7806 for +6V supply. Previously I used an adapter for this purpose. How do I resolve this dimming of LED??

 

[ericgibbs]

hi audioguru,

i'm now using two 9V batteries and using a 7812 for +12V from +18V in the circuit. This +12V is also used in 7806 for +6V supply. Previously I used an adapter for this purpose. How do I resolve this dimming of LED??

Which voltage do you want to use for the LED's.?

 

[audioguru]

Little 9V batteries have a low output current and don';t last long.

When you power 2V LEDs from an 18V battery then 89% of the power from the battery is wasted.
You can connect 6 LEDs in series and in series with a 240 ohm resistor for a current in the LEDs of 25mA when the batteries are new and a very low current when the batteries are each 6V.

 

[ruzfactor]

The voltage which would make the LEDs brighter. Really these LEDs driving me crazy. My LFR is finished and because of LEDs, it's not working properly.

So what do I do now? I used two 9V batteries because it's cheap. A 12V rechargeable battery costs a lot!

 

[ericgibbs]

The voltage which would make the LEDs brighter. Really these LEDs driving me crazy. My LFR is finished and because of LEDs, it's not working properly.

So what do I do now? I used two 9V batteries because it's cheap. A 12V rechargeable battery costs a lot!

hi,
If the LEDs are say 2V and the battery is a 9V PP3, allow a voltage from the battery to be 8V say.
Connect three, 2V LED's in series = 6V, leaving 2Volts to be dropped across the resistor.
If the current is 20mA, the Resistor= 2V/0.02 =100R
[Most efficient]

If you connected only two, 2 Volt LED's in series then the resistor would hav e to drop 4Volts, 4V/0.02= 200R [inefficient]

Its just Ohms law.

 

[ruzfactor]

so you are suggesting that I should connect the LEDs in Series? Should I connect all six LEDs in series? What resitance should I use? 100ohm?

 

[audioguru]

Six 2V red LEDs in series need a voltage at least 12V. If they are 3.5V white or blue LEDs then they need a voltage at least 21V.

A 9V beattery drops to 6V when it is considered to be dead so six LEDs in series with a 240 ohm resistor gives a current of 25mA when the batteries are new and almost no current when the batteries are dead. The batteries last much longer with 25mA than if the LEDs were in parallel like you had them before but each has a current of 25ma because then their total current is 150mA.

 

[ruzfactor]

Should I now increase the voltage to connect the LEDs in series? How about I use 18V(from the battery) for six LEDs with 240ohm?

 

[ericgibbs]

Should I now increase the voltage to connect the LEDs in series? How about I use 18V(from the battery) for six LEDs with 240ohm?

Hi,
I am trying to help, but we are going around in circles.

Tell me what is the forward voltage drop of one of your LED's.

If you dont know, tell me the LED part number or type, so that I can get the datasheet.

Without this information it is impossible to calculate the correct connections and resistor.

 

[ruzfactor]

Hi
I don't know the forward voltage drop and also the part number. I can give you a pic of these LEDs. These LEDs emit RED light.

 

[ericgibbs]

Hi
I don't know the forward voltage drop and also the part number. I can give you a pic of these LEDs. These LEDs emit RED light.

OK,
Please do the following test:

Connect one LED, via a 470R resistor to a 9 Volt battery.

Use a voltmeter to measure the voltage across the LED and tell us what you measure.

 

[ruzfactor]

connected a LED to +9.6V (adapter) with a 560ohm (don't have a 470ohm) and measured the voltage across the LED which is +1.70V

 

[ericgibbs]

connected a LED to +9.6V (adapter) with a 560ohm (don't have a 470ohm) and measured the voltage across the LED which is +1.70V

hi,
Connect 3 LED's in series, 3 * 1.7V = 5.1V
Assume a 9V battery, when discharged 7V.
Assume 20mA LED current.

So 7V - 5.1V = 1.9V/0.02 = 100R [if you dont have a 100R a 120R will be OK]

So connect a 100R in series with three LEDs also connected in series.


Question: what colour do the LED's glow when lit.?

 

[ruzfactor]

LED's glow in RED when lit. I have six LEDs. Do I Connect Six of them together in series or just 3 of them?

 

[ericgibbs]

LED's glow in RED when lit. I have six LEDs. Do I Connect Six of them together in series or just 3 of them?

hi,
Connect 3 LED's in series, 3 * 1.7V = 5.1V
Assume a 9V battery, when discharged 7V.
Assume 20mA LED current.

So 7V - 5.1V = 1.9V/0.02 = 100R [if you dont have a 100R a 120R will be OK]

So connect a 100R in series with three LEDs also connected in series.

You will have two separate chains of 3 LEDS and a 120R

Do you follow.?

 

[ruzfactor]

Yes sir!

 

[ericgibbs]

Yes sir!

Look at this drawing...

 

[audioguru]

When the battery is new and the resistor is 120 ohms then the current is 33mA which is higher than the max allowed current for many LEDs (30mA max).

Only one LED was measured. Maybe the others have a forward voltage of only 1.6V and maybe the 120 ohm resistor is 5% low at 114 ohms. Then the current will be 37mA.
Use 150 ohm resistors.

 

[ruzfactor]

It's awesome! LEDs are very bright!!! I've used 100ohms as resistance to connect the 3 LEDs in series. I have a dumb question to ask: Can I connect the LEDs to +12V instead of +9V? Will it burn out the LEDs?

To audioguru: I have 100 ohms and after that I have 220 ohm resistors. Don't have any 150 ohm.