B562 Vs B772 Transistors

[Suraj143]

Hi,

The PNPs will drive from 74HC595.You mean if I change it to a NPN it will drop only its saturation voltage?But if I use a PNP it will add the drop of Base Emitter drop + 74H595 drop?

Is that correct?

 

[Suraj143]

Ok I am planning to change the transistors to NPN type 2N2904.
I hope it will perform better than ULN 2803.

The NPN s will drive from 74HC595.My PCB gets bigger needs to add NPN base resistors and 47R column resistors.Altogether 64 resisters for 32 columns

 

[audioguru]

Yes, the PNP emitter-follower has a base-emitter voltage loss and the 74HC595 adds its output saturation voltage loss.
The 2N2904 is obsolete and is in an old metal case. Its max base-emitter voltage is 2.6V when its collector current is 500mA.
How much will be its maximum current?

Wait a minute. The 2N2904 was an old PNP, not an NPN.

 

[Suraj143]

Opps sorry I'm going to use 2N3904 (NPN).Sorry for confusing.Hope it will perform better than ULN 2803.

 

[audioguru]

The datasheet of the 2N3904 NPN transistor shows a max allowed current of 200mA and poor performance above about 50mA. Its max collector to emitter saturation voltage is 0.3V when its collector current is 50mA and its base current is 5mA.
Again, how much will be its maximum current?

 

[Suraj143]

Hi I need 4mA of average current. So 4mA X 16 = 64mA.

Per column 64mA peak will be fine.

Is that 2N3904 is a good choice?

 

[ChrisP58]

Hi I need 4mA of average current. So 4mA X 16 = 64mA.

Per column 64mA peak will be fine.

Is that 2N3904 is a good choice?

Probably not.
You need to calculate the transistor current based on the maximum current when they are on, not the average of when they're on/off.

 

[Suraj143]

Probably not.
You need to calculate the transistor current based on the maximum current when they are on, not the average of when they're on/off.

Maximum current is 64mA. So when multiplexing @ 1/16th rate it will be 4mA average.

 

[audioguru]

A 2N3904 works poorly above 50mA. I would use a 2N4401 that works well up to 200mA. Both cost the same at Digikey today.

 

[Suraj143]

Hi Audioguru & Others.

I changed my PCB layout by replacing ULN2803 with 2N3904 transistors.I got some improvement in LED brightness.I think I could lower down the value of the resistor further....!!!

Here how I did.Any comments?

 

[audioguru]

The 2N3904 has about 5mA of base current which should be a little more to get a little more collector current with the same saturation voltage loss.
The 2N4401 or BC337 will be a little better.

 

[Suraj143]

Hi.

For me its base current will be (5-.6)/330 = 13mA.

Doesn't the 2N4401 has more saturation drop than 2N3904?

 

[audioguru]

The output of the 74HCxxx is high at +5V only with no load. It is about +4.2V with a 10mA load.
The typical saturated base voltage of the 2N3904 is shown on a graph at 0.9V. Then the base current is only (4.2V-0.9V)/330 ohms= 10mA but less if the 74HCxxx and/or transistor are not typical.

The 2N3904 has a max allowed current of only 200mA and its typical saturation voltage loss is shown on a graph at 0.13V at only 100mA.
The 2N4401 has a max allowed current of 600mA and its graph shows a typical saturation voltage loss of 0.10V at 100mA.