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2SB772 Transistor Question

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Not an easy question.
There are 4 versions of the part "R,Q,P,E" with a gain of 60 to 400. Depending on conditions and which part the 25mA could be multiplied by 60 to 400.
In one of the graphs they show a condition where there is 10mA of Base current and the C-E starts to open up at 1.3A. (don't know which part)
The more the Base current the smaller the C-E voltage.
With the "R" part I think 1A to 2A. The part was designed to work at less than 2A.
ron s

are you using this transistor as a switch or amplifier? you want to operate the transistor within the safe operating area, and STAY AWAY from the diagonal portion of the SOA. that's the secondary breakdown region, and going past it is instant death for the transistor. it would help, if we had a clue what the circuit is. otherwise it's like asking "if i put the pedal 1/4 of the way to the floor, how fast will the car go?" what car? what engine? what gear is the transmission in? up hill, down hill, or flat?

Like for most transistors, the datasheet clearly shows pretty good saturation when the base current is 1/10th the collector current so 25mA in gives 250mA out if the collector current is limited.
For an amplifier that has at least 2V between its C and E then its minimum hFE is 30 so 25mA in gives at least 750mA out. But if the hFE is its max of 600 and nothing limits the collector current then it will be destroyed.

I'm doing a LED matrix design. 16 rows & 32 columns.

The attachment shows the concept.Per row it has 32 LED's. I'm doing a row scan.For 16 rows the duty cycle will be 1/16.

The problem is the LED's are a bit dim & need more brightness.

I guess the B562 cannot supply enough current? That's why I'm going to shift to B772 transistor
While searching I found 2SA1797 which datasheet mentioned it has "Excellent DC Current Gain Characteristics".

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Current gain is used in an amplifier, but the saturation voltage loss is used for your transistor saturated switch. The 2SA1797 has a little better saturation than the other two transistors but it is still a range where some are better than others.
You will not notice a difference in brightness by using a different transistor because all the other things limit the brightness. Use brighter LEDs but then their viewing angle will probably be very narrow.

If the PIC low is +0.5V and the base-emitter voltage is 0.7V then the 470 ohm base resistor has 8.1mA in it and the transistor saturates fairly well with 80mA of collector current. But the voltage loss of the ULN2803A plus the current limiting by the 47 ohm resistors plus the very low duty cycle produce dim LEDs.

i think the short duty cycle when muxing the LEDs is going to be the main cause of the LEDs to be dim. the usual remedy for this is use LEDs with high forward current and high brightness. a capacitor across the LED and dropping resistor can help by increasing the dwell time of the LED.

Thanks for the replies.

I searched the 2SA1797 on stores, That is bit expensive.After doing a PCB to my design I managed to increase the brightness of my LED's by using the same 2SB562 transistors.I guess my prototype cannot handle the instant current from ground path.(Prototype I did from a phenolic copper boards).

After doing a epoxy PCB I got great results.Anyway I had to find out more bright LED's as well.

Another thing I noticed that unbranded Chinese LED's has lots of leakage current.Specially BLUE & WHITE Led's.When my matrix running I see some unwanted LEDs lights up.So what I do is checking each LED's one by one using an multi meter putting it to X1K range.

Even if you get your LED current up to 50mA, at 1/16th duty cycle the average will be 3mA. You are not going to be able to get them much brighter. Have you considered using something like this? Two of these modules will give you a 32x16 display and only cost about US\$10 for the pair.

Mike.

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