Consumption of a Led Matrix

[haroldjclements]

This is a pretty basic question, but one that I am not finding (or understanding) the answer.

If I had a string of LEDs in series, lets say 3 LEDs across 1.8v that consume @ 20mA. The three LEDs will still only consume 20mA, but the voltage will increase to 5.4v. (3 * 1.8). Now if I have the same 3 LEDs but in parallel, then the voltage will remain 1.8v, but the current will increase to 60mA (20 * 3). - I believe that this is correct.

Now, if I have a 3x3 LED Matrix, I guess that the current will increase to 60mA, but would the voltage be 5.4v?
And if I am correct, what would the voltage be if the LED were mixed colours (voltages)? R = 1.8v, G = 2.4v and B = 2.6v.
Would you take the maximum of all the LED series - 7.8v? - (if 3 Blue LEDs in series, 7.8v).

Thanks in advance for your help.

Kind Regards,
Harold Clements

 

[Arouse1973]

Hi Harold

Firstly you should use a series resistor for each string that you parallel together. I am not sure if you knew that already. You then need to then calculate the resistor value using the LED forward voltage at the desired current. It then won't matter if the LEDs are mixed in colour.

You just add the forward voltages of the LEDs together and subtract this value from your voltage source. Then using the answer from this calculate the resistor value that will develop this voltage across it at the desired current.

Example:

V-Source = 12 Volts
LED forward voltage @ 20 mA = 2.6 Volts
LED Qty = 3

Resistor Value = (12 V-(3*2.6 V)) / 0.02 A = 210 Ohms

Thanks
Adam

 

[kellys_eye]

Individual colours should be treated as individual LEDs.

Other than that, addiing in series or parallel as you have already correctly done is the correct way.

But for larger matrix sizes it is common to multiplex the LEDs and average current is lower due to the pulsing used.

 

[haroldjclements]

Thank you for your replies...

Is this is my circuit. Adam, are you suggesting "a series resistors" between all the LEDs?

Keeping with the circuit below (just so I understand the basics):

PSU: 9v, 1A
Red = 20mA, 1.8v
Green = 20mA, 2.4v
Blue = 20mA, 2.65v

R1 = 9-(1.8 * 3) / 0.02 = 180R
R2 = 9-(2.4 * 3) / 0.02 = 90R (91R best standard value)
R2 = 9-(2.65 * 3) / 0.02 = 52.5R (56R best standard value)

The current will be (20 * 3) 60mA

What would be the required voltage? Would the 9v PSU be adequate?

 

[duke37]

You have calculated for 9V so this is the voltage which should be used.
At 5.4V only the red led would glimmer. Over 9V the current will be more than 20mA in each string. The more the merrier but the shorter the life. The lower the resistor, the more the current change with voltage so the blue leds will be most susceptible to over voltage.

 

[Audioguru]

LEDs have a range of forward voltage. You said your blue LED is 2.6V which seems too low. Most 20mA blue LEDs are 2.8V to 3.5V and you cannot order one voltage, you get whatever they have.

If your blue LEDs are actually all 3.5V then three need 10.5 plus a few volts for the driver and current limiting resistor (13V or more). If the resistor value was calculated for a 2.8V blue LED then the 3.5V LEDs might not light or will be dim.

 

[haroldjclements]

OK, Audioguru, I understand your comment that Blue LED might not be the right voltage. But what I am trying to find out is the formula for finding the correct voltage using my diagram above. Is it the highest voltage from all the series of LEDs (in my case the blue series, 7.95v) ?

Red - 1.8 * 3 = 5.4v
Green - 2.4 * 3 = 7.2v
Blue - 2.65 * 3 = 7.95v

Once I understand the basics, I can tackle the 'school boy' errors in my circuit.

As always, I appreciate your comments and help. Thank you...

Harold Clements

 

[Arouse1973]

Thank you for your replies...

Is this is my circuit. Adam, are you suggesting "a series resistors" between all the LEDs?

Keeping with the circuit below (just so I understand the basics):

PSU: 9v, 1A
Red = 20mA, 1.8v
Green = 20mA, 2.4v
Blue = 20mA, 2.65v

R1 = 9-(1.8 * 3) / 0.02 = 180R
R2 = 9-(2.4 * 3) / 0.02 = 90R (91R best standard value)
R2 = 9-(2.65 * 3) / 0.02 = 52.5R (56R best standard value)

The current will be (20 * 3) 60mA

What would be the required voltage? Would the 9v PSU be adequate?

Yep, that's the configuration I had in mind.

 

[Arouse1973]

OK, Audioguru, I understand your comment that Blue LED might not be the right voltage. But what I am trying to find out is the formula for finding the correct voltage using my diagram above. Is it the highest voltage from all the series of LEDs (in my case the blue series, 7.95v) ?

Red - 1.8 * 3 = 5.4v
Green - 2.4 * 3 = 7.2v
Blue - 2.65 * 3 = 7.95v

Once I understand the basics, I can tackle the 'school boy' errors in my circuit.

As always, I appreciate your comments and help. Thank you...

Harold Clements

It's important you check the data sheet of the LEDs to ensure you have the correct source voltage, high enough to allow for the desired current but not too high that you dissipate too much power in the resistor which is a waste.
Adam

 

[Arouse1973]

Apart from using a constant current source for each LED chain (with resistor removed) set to 20 mA, the best way is to power each LED from a current limited power supply and measure the volt drop of the LED. This will give you a true voltage drop for that current. The voltage drop will change dependant on the current. See picture below as an example.
Adam

 

[Audioguru]

The datasheet for a real LED (not an ebay who-knows-who-made-it one) shows its range of forward voltage because it is not a simple light bulb, it is a semiconductor diode and they cannot make them all the same. I said that your blue LEDs might be 2.8V to 3.5V but the datasheet will say exactly the range. But many datasheets do not say the maximum voltage, instead they list minimum and typical. The graphs on a datasheet are for a "typical" device and do not show minimum and maximum voltages.

Some of my blue and green LEDs have a range of forward voltage from 3.0V to 4.6V. Here is a copy of the datasheet of a red LED that I have:

 

[haroldjclements]

Just as a follow up, I did what Adam suggested. I set my variable PSU to 20mA and hooked each colour up in turn (without resistor) and as Audioguru surmised the voltages that I had been quoting are wrong.

Red - 2v, Green - 3v, Blue - 3v

So would that mean that (as a rough guide for the circuit above in my 2nd post) that I will be looking for a 9v power supply that gives at least 60mA? (Y/N)

If 9v is correct, then the R:360R, G:300R, B:300R would be the required resistor values.

 

[Arouse1973]

For running one LED of each type from 9 Volts at 20 mA then your resistor values are correct. So looks like my typical graph wasn't too far out for the blue one then, but I guess you have only tested one.
Adam

 

[haroldjclements]

Ha Ha... You are correct on both accounts

I am very appreciative of educational help...

R1 = 9-(2 * 3) / 0.02 = 150R
R2 = 9-(3 * 3) / 0.02 = 0R (1R best standard value)
R3 = 9-(3 * 3) / 0.02 = 0R (1R best standard value)

Does the 0R (or 1R) suggest that I really need a larger supply than 9v... say 12v?

R1 = 12-(2 * 3) / 0.02 = 300R
R2 = 12-(3 * 3) / 0.02 = 150R (1R best standard value)
R3 = 12-(3 * 3) / 0.02 = 150R (1R best standard value)

Thanks again...

 

[Arouse1973]

If you are getting a value of zero then yes your supply voltage needs to be higher. How high, well you need to ensure that if the LEDs are at the top of their range of voltage drops you need to ensure they light up. So the next common available voltage source from 9 volts is 10.5 Volts (7 AA cells in series) but you may not be able to find a 7 AA battery holder so 12 volts would be my choice (8 AA in series). Or a power supply that's adjustable. Just some ideas!

The brightness will vary between different strings of the same colour as discussed in previous threads. Not all LEDs are made the same (have the same forward voltage). The best solution, maybe for later is to look at a current source.

Adam

 

[Audioguru]

You measured the voltage of only one LED for each color. But they will all be different unless you are VERY lucky.
If most LEDs have the minimum or maximum listed voltage on the datasheet then the resistor value for the one you tested will be wrong. You might need to measure the actual voltage of each of your LEDs and throw away the ones that are different from the rest.

 

[BobK]

You could, just barely, use strings of one of each color to run off 9V. Then the resistor would be (9-3-3-2) /0.020 = 50Ω.

Bob

 

[haroldjclements]

Thank you guys for all your help and education. I got some common cathode RGB LEDs today (again a cheap ebay who-knows-who-made-it type). I guess all the math is the same, just saves on breadboard real-estate...

Thanks again...
Harold