[AVR] Help me with led scrolling message board

[codename25]

Thanks Gorgon, Do you recommend to use power transistors like TIP127 or TIP122?

The frame rate is high. I can't even identify the alphabets. The alphabets keep loading very rapidly like un second.

 

[Gorgon]

Thanks Gorgon, Do you recommend to use power transistors like TIP127 or TIP122?

The frame rate is high. I can't even identify the alphabets. The alphabets keep loading very rapidly like un second.

Regarding the TIP127/122. No, I would not have used either of them. Both are darlingtons and made for slow switching use. I would have used a logic gated mosfet, with a low voltage gate threshold, or a medium power switching transistor.

The frame rate has nothing to do with the scrolling speed. The frame rate indicates how often the screen is refreshed each second, to make it flicker free.

The scrolling rate is how fast you are scrolling the text, or how long each picture is shown before it is moved one step. I would think a scrolling rate of 2-10 Hz or steps / second should be ok. The frame refresh is a background task, running the whole time independent of contents update. The contents of the display is a task in the foreground where you decide scrolling or what to display in the message board.

 

[codename25]

[Moved]Help with current handling in led matrix

Hi,
I'm planning to built a 10X100 led matrix with row scanning using Atmega16 and 74HC595. Since it has 100 led in one row I'm supposed to give 20ma to each led then 20X100=2A is needed. Can I use power transistors here like TIP127,TIP125 etc. I have a doubt since these are used in low speed application, whether this will work in fast scanning Led matrix?

 

[milan.rajik]

Re: Help with current handling in led matrix

Search for high speed switching N-Mosfet with Id max 4A.

 

[codename25]

Thanks for the reply Gorgon, I have a doubt, for Uln2003, 500mA peak current capacity is for each output pin or as a whole? If it is for whole, one pin would be 1/8th of 500mA (since it has 8 o/p pin) isn't? That means it cannot bear 500mA in one pin. Whether this is correct?

 

[milan.rajik]

500 mA per channel.

 

[Gorgon]

One ULN2003 has 7 outputs. The max current on each output is 500mA, but the total current for the whole package is 2.5A. These are absolute max numbers, and before you reach these currents, the package will be too hot to work due to the high output voltage drop of around 1V. This drop will generate heat, and at less than the maximum total current the chip will overheat. I suppose you will not run it at max current all the time due to the multiplexing of the display, but you should keep it in mind.

The voltage drop is also why the ULN2003 driver is not ideal for a low voltage (5V) LED matrix.

Remember that in any datasheet for any component, you can never use the component close to all the abs. max conditions. There will always be a tradeoff between voltage, current, speed and dissipated power. As said before, the ULN2003 is not a good choice for this type of releative high speed switching, due to the fact that it is also a Darlington device. This is also the main reason for the high output voltage drop.

 

[codename25]

Thanks for the reply milan.rajik and gorgon, It helped me well.
I have one more doubt. I use ULN2003 for sinking and not sourcing. Im soucing the 20mA of current for each LED in column from 74HC595 (shift register) via a 150ohm resistor. Since ULN2003 is used for sinking, whether these vales (500mA) is applicable for me?

 

[milan.rajik]

Sinking or sourcing it is the amount of current which flows through the device. More current flows, more heat is dissipated.

 

[FvM]

I have one more doubt. I use ULN2003 for sinking and not sourcing. Im soucing the 20mA of current for each LED in column from 74HC595 (shift register) via a 150ohm resistor. Since ULN2003 is used for sinking, whether these vales (500mA) is applicable for me?

Please add up the typical forward respectively saturation voltages of each device at 20 mA respectively 500 mA

ULN2003 1.7V
74HC output 0.8 V
Red LED 1.7 - 2.0 V

What's the remaining resistor voltage drop at 5V supply? How do you calculate the series resistor?

 

[codename25]

Thanks Milan rajik and FvM, The current I'm suppose to give is 2V and 20mA for the Led. I give 150ohm resistor for the source from 74HC595.

 

[FvM]

Your calculation would be correct if ULN2003 and HC595 had zero saturation voltage. They however have about 2.5V saturation voltage in total.

 

[codename25]

I have one more doubt, if in ULN2003, total pin output is 2.5mA. Then can I join all the output pins to get 2.5mA as a whole and connect it to the row of 120 led in series having a total peak current of 2.4mA (20mA each) as a replacement for mosfets or series of uln2003 each connected to 20 led.?

 

[FvM]

Besides writing mA instead of A, you are apparently also confusing series and parallel circuit. You surely don't have 120 LEDs in series.

I understand that you want to know if an ULN2003 with all outputs in parallel can be used as 2.5A driver. Basically yes, but you still get an inconveniently high saturation voltage, slightly reduced to 1.3 V or so.

 

[codename25]

Thanks FVM, Yes, its parallel not series. And also its is 2.5 A.

What if saturation voltage becomes 2.5v? Also I'm insisting on ULN2003 since I see this in many led matrix circuit. Also many are discouraging the use of mosfets in fast application like led matrix.

 

[FvM]

I'm not the only one pointing out from the start that darlington driver is a bad choice.

MOSEFT is generally the better choice, problems with gate drive current and switching speed might occur if you select unsuitable MOSFETs.

 

[codename25]

Thanks FVM, Could you please suggest me a mosfet for the Led matrix. I have no idea where to search to get the appropriate one.

 

[codename25]

How about 2N2222 npn bjt for rows instead of ULN2003 for 40 leds?

 

[FvM]

You are looking for a switch that can be controlled by a processor port. There are BJTs with high current gain at the intended currents, but 2N2222 surely isn't.

Others suggested MOSFETs in the 4 A range. There's a number of low voltage MOSFETs in small case (e.g. SOT23) that can be used. In case of doubt, use something in TO220 case.