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12V Multiplex Design

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Suraj143

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I'm going an 16 X 48 LED matrix & the attachment shows how I drive a Row from a PIC micro PIN.Columns are driving from a ULN2803 as well.I have 16 rows, so I'm multiplexing rows @ 1/16th rate.

In this design for 595 Shift registers and to drive row signal from PIC pin I use a 5V supply.But for the LED load I use 12V. Because in future I need to add more LEDs to a single pixel.So 12V is fine.Also it will override the darlington saturation voltages as well.

Is my design ok?Will there be enough current from the row side to drive 48 LEDs?
 

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ronsimpson

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B772 transistor could have the current of 48 LED. If all 48 are on. So what is you LED current then X 48= total current (max).
B772 current gain at 1A is 60min 160typ and 400max. So if you have 1A collector current then you need a base current of 1,000/60=base current. Next question if the base resistor right to get you that current?
 

schmitt trigger

Well-Known Member
....or you could use a P-channel Mosfet instead.

Check the datasheet for any device that you actually select. In particular, Rds(on), and the Vgs(max).
 

Suraj143

Active Member
Hi thanks for the view.

My target is to drive LEDs at 4 mA average current.

Peak currents to led 16 x 4 = 64mA.
Max current Per row = 64 x 48 = 3A.

Is that 4.7K too high?
 

audioguru

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B772 transistor could have the current of 48 LED. If all 48 are on. So what is you LED current then X 48= total current (max).
B772 current gain at 1A is 60min 160typ and 400max. So if you have 1A collector current then you need a base current of 1,000/60=base current. Next question if the base resistor right to get you that current?
Maybe the transistor should saturate so that all rows have the same brightness. The current gain is used for a linear transistor with a Vce of 2V or less so that it is not saturated. The datasheet for the 2BS772 transistor shows that it saturates fairly well with a base current that is 1/20th the collector current.

If the LEDs are 2V red ones and if the 2SB772 transistor saturates then the 150 ohm resistors cause the LED current to be 57.3mA. simple arithmetic shows the max transistor current will be 57.3mA x 48= 2.75A so the base current of the transistor should be 2.75A/20= 138mA which is much more than R1 gives.

The 1:16 multiplexing will look dim at 57.3mA/16= 3.6mA. I do not know if the LEDs can withstand more peak current.
 

audioguru

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If the LEDs were very cheap and bought on ebay or one of its clones then they will probably blow up with a peak current of "only" 50mA.
The Fairchild LEDs I bought have a maximum momentary peak current rating of 200mA.
 
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