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### current??

Posted: Fri Jul 05, 2013 5:30 pm
Hi, everyone....
I would like to ask a question.
let's say, a 470uf 10v capacitor will be the power supply to 1.5v LED.
by using voltage divider to reduce the LED acceptable voltage, which circuit could make LED light last longer or both just same result?

### Re: current??

Posted: Fri Jul 05, 2013 6:17 pm
sorry, circuit (b) should be 1764.7 ohm

### Re: current??

Posted: Fri Jul 05, 2013 10:37 pm
Why you need a voltage divider? Why don just put a high resistance resistor series with the LED?

### Re: current??

Posted: Fri Jul 05, 2013 11:04 pm
After the degree, I'm not applying much calculation anymore. So, just make it and try-and-error. But practically, it's don't have 1764 ohm in the market, hehe

### Re: current??

Posted: Sat Jul 06, 2013 2:19 am
yes I know I could just use 1.5v AAA battery to light LED.
but I want to research the circuits, nothing different, no pros and cons, or something?

### Re: current??

Posted: Sat Jul 06, 2013 9:27 am
In both cases, the LED will not even get lighted.

1st case, the current from the battery is 840mA, but most of the current is swallowed up by the bottom resistor. Only 0.03mA goes to the LED not enough to light the LED.

2nd case, the 10K resistor is too high, there is not even enough current going into the circuit not to mention the LED.

### Re: current??

Posted: Sat Jul 06, 2013 9:10 pm
ABSF WROTE:In both cases, the LED will not even get lighted.

pic16.PNG

1st case, the current from the battery is 840mA, but most of the current is swallowed up by the bottom resistor. Only 0.03mA goes to the LED not enough to light the LED.

2nd case, the 10K resistor is too high, there is not even enough current going into the circuit not to mention the LED.

thank u very much ABSF, you always solve my problem effectively

### Re: current??

Posted: Mon Jul 08, 2013 5:48 pm
one more question
is there any different put motor upside and downside?

### Re: current??

Posted: Tue Jul 09, 2013 9:29 pm
is there any different put motor upside and downside?

Sure, there are differences.

The one one the left is called common emitter or "CE" config..... It has more gain and when the BJT saturates, the voltage drop across the CE would be lower so the motor would get higher voltage and current.

On the right is the common collector or "CC" config. It has a unity gain (i.e. gain=1) and need more current to saturate as compared to the CE config. Your motor would get less current in this config.

Since you're driving motor load, it is better to use the CE configuration. The CC is normally used for non-inverting buffer meaning the output signal is in phase with the input signal. In CE config, the output signal is inverted compared to input signal.

Allen

### Re: current??

Posted: Wed Jul 10, 2013 12:49 am
ABSF WROTE:
is there any different put motor upside and downside?

Sure, there are differences.

The one one the left is called common emitter or "CE" config..... It has more gain and when the BJT saturates, the voltage drop across the CE would be lower so the motor would get higher voltage and current.

On the right is the common collector or "CC" config. It has a unity gain (i.e. gain=1) and need more current to saturate as compared to the CE config. Your motor would get less current in this config.

Since you're driving motor load, it is better to use the CE configuration. The CC is normally used for non-inverting buffer meaning the output signal is in phase with the input signal. In CE config, the output signal is inverted compared to input signal.

Allen

I see, I see
could you recommend me any fast switching or high sensitive transistor if I use very small current & voltage with high frequency input (like PWM)at the Base? got transistor like this in big watt and small watt?