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Seven Segment Multiplexing Transistor

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Suraj143

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I'm driving 9V common Anode seven segments.I need TO220 package transistors specially high speed ones.

Can somebody recommend me very common ones.The below method gives some unbehaviour results.
 
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hi Suraj,
With the ULN2003A the 100 ohm resistors are not required on the inputs.

Consider if all the 7 segments are ON, at say 15mA each thats ~100mA, in saturation the gain of Q2 will be around 10, so a Base current of 10mA is required.

The 4k7 base resistor will allow only ~2.3mA so Q2 will not be in saturation, use a 1K5 base resistor, also its good practice to connect a resistor from Base to Emitter of Q2, this provides a leakage current path.

There should also be segment resistors to limit the LED current, one for each segment, say 100R. On the output pins of the ULN

When you say a 9V LED , I dont follow ???

E.
 
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I'm driving 9V common Anode seven segments.I need TO220 package transistors specially high speed ones.

No need for 'high speed' ones, it's only a slow application.

I would suggest considerably reducing R1 in value, in order to increase the base current to make the PNP switch, rather than working in a linear mode.
 
Hi Eric & Nigel.

Now I got it.I'll reduce R1 to 1.5K.

Eric
"The 4k7 base resistor will allow only ~2.3mA" how did you calculate this?

Note that I'm multiplexing 4 seven segments.The diagram shows only one.Also I have put the current limiting resistors on the input side which is wrong.In the real application I have mount on output side of the ULN2003.
 
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hi,
A 'rough' calculation of 11V [allowing for 1V loss across the transistors] 11v/4700 = 2.3mA
 
For the record Q1 isn't even needed. Remove it and tie R1 directly to RA0.
 
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Jon,
For the record, that is wrong.

Doing that Q2 will never turn OFF.

E.

You are correct. I failed to see that he was driving the anode from a 12V supply.

However...I would also add a collector resistor between Q1's collector and the 12V supply to keep the base of Q2 from floating when Q1's base is pulled low.
 
You are correct. I failed to see that he was driving the anode from a 12V supply.

However...I would also add a collector resistor between Q1's collector and the 12V supply to keep the base of Q2 from floating when Q1's base is pulled low.

The 4k7 base resistor will allow only ~2.3mA so Q2 will not be in saturation, use a 1K5 base resistor, also its good practice to connect a resistor from Base to Emitter of Q2, this provides a leakage current path.

Jon,
Already suggested that.
E
 
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