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[SOLVED] 74HC32 drive BC817 NPN transistor

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Hello All,

NPN.jpg

I can't make the BC817 working when i using HC32 to drive it. The transistor Collector is always Low, 0V. The NPN nominal ON and Off is just 0.92us. The NPN will work when I directly use the MCU I/O instead of using HC32.

May i know what the problem is? Why the transistor can't work when using HC32 driver? One more thing, i did tried change the R1 value from 220~500~1K Ohm but still not function. Does anyone can suggest me how to overcome this problem?

Thank you..
 
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My guess is the transistor is too slow to come out of saturation. Try adding a resistor from base to ground and a small signal Schottky diode from base to collector. Try a fast switching transistor. A small capacitor across the resistor from the HC gate may also help.

Keith
 
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Just a simple change of transistor should improve things. See attached. The pulse would still be short, but some of the other measures I suggested should improve that.

Keith.
 

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  • BC817.pdf
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You would at least see a change in base voltage, even if the transistor doesn't turn fully off.

Two explanations for "always 0.7V":
- you didn't measure the base voltage
- the HC32 isn't driving low at all (due to some circuit wiring errors)

P.S.: I see about 200 ns storage time for BC817 in a simulation under the said conditions.
 
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Actually, in a simulation it would suggest the base doesn't move much with the BC817. I guess the stored charge holds the base up. If I extend the pulse width the base drops low after around 1.5us.

Keith.
 

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Apparently, your BC817 model is rather different from mine. But also according to a BC817 datasheet, the transistor shouldn't be that slow (fT = 170 MHz, Cbe = 65 pF)

P.S.: The same result (with sligthly different waveforms) with Infineon and NXP BC817 PSpice models.
 
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FvM,

You are right - there is something strange about my model. It claims to be a Philips one. I have switched to a Siemens model and the BC817 works fine.

Keith.
 
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Hello keith1200rs, FvM,

Thanks for your all discussion and

keith1200rs: Ya, your advice is work try adding a resistor from base to ground but we still can play the R1, and R2 value around rather than base pull-low. The transistor is not too slow to come out of saturation because i did try Lo ~800ns pulse for it and the transistor able to saturate.

Sorry, for never reply for so long... because i'm spent much time on doing testing until yesterday nite. Finally i get the thing work. May be late by to day I will capture the waveform with using scope then we analysis the result again and see why the BC817 can't saturate.

Thank a lot...
 

Dear All,

Actually i'm using 8051 MCU to control the driver. Following is the practical result and the waveform.

CCT.jpg


Wave A WA
Wave B WC
Wave C WB
Wave D WQ

8051 port current 15mA
R1 = 2K2, R3 = 2K2, WC = WA = 1.875mA
View attachment WAVE11.BMP
* No waveform on Wave D (WQ).

R1 = 10K, R3 = 2K2, WC = WA = 1.875mA
View attachment WAVE15.BMP
Red waveform (WA) - Waveform getting better
Yel waveform (WQ) - Transistor is not fully saturation, <400mV. HC32 not enough current to drive the BC817 if R3 is 2K2??

R1 = 10K, R3 = 1K, WC = WA = 1.875mA
View attachment WAVE19.BMP
Red waveform (WA) - Waveform getting better
Yel waveform (WQ) - Transistor is saturated and able to trigger my following circuity
 

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