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Transistor taking square wave input

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king.oslo

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Hello there,

I need a transistor for a circuit. It needs to conduct as long as a square wave 5V signal is present. My signal will look like this: high(1-2ms) --- low (20ms) --- high (1-2ms) --- low (20ms) etc. Even during the time of low, the transistor has to conduct.

Are there any kind of transistors that can do this?

Thanks.M
 
During the low how low is it?, assuming it's a standard logic signal there's effectively no signal, so nothing much to detect.

You really need to give far more detail.
 
Hello there,

I need a transistor for a circuit. It needs to conduct as long as a square wave 5V signal is present. My signal will look like this: high(1-2ms) --- low (20ms) --- high (1-2ms) --- low (20ms) etc. Even during the time of low, the transistor has to conduct.

Are there any kind of transistors that can do this?

Thanks.M


There is no transistor that will do that. There is however a circuit that will. Look up "missing pulse detector"
 
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Signal high will be 5VDC.
Signal low will be zero.

The duration of the high will be between 1 and 2ms
The duration of the low will be exactly 20ms.M
 
When the pulse sequence ends, how quickly must the transistor turn off, i.e., what is the maximum time that can elapse between the last pulse and the transistor turn-off?
 
This is a thermostat for an air-cooled vehicle. The servo regulates the engine cooling. All this will do is to switch on the main power for the servo when a signal is sent for the microcontroller to the servo.M
 
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This is a thermostat for an air-cooled vehicle. The servo regulates the engine cooling. All this will do is to switch on the main power for the servo when a signal is sent for the microcontroller to the servo.M

How much current does the servo draw?

Is the transistor switching the negative (ground lead) of the servo, or is it switching the positive (+12V) lead?
 
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It could be either positive or negative. Circa 3amps at max load @5VDC.M

The "@5VDC" makes no sense. I thought the servo is powered from the 12V? If not, where does the 5V a 3A come from?
 
If the five volt High comes from a PIC pin, then it can source ~20mA. To get from 20mA to 3000mA, that takes a β of 3000/20 = 150 which is hard to do with a single NPN. Use either a Darlington, a Darlington-connected pair of of NPN transistors, or a logic-level NFET. Connect the emitter/source to the same voltage as the PIC Vss, connect the Collector/Drain to the negative lead of the Servo, connect the positive lead of the Servo to +5V. If using NPNs, you will need a current-limiting resistor between the PIC pin and base of the transistor.
 
I ended up buying a BD675AG transistor, a TI SA555P and three L7805CV 5V regulators. How does that sound, you think?

Thanks.
M
 
Ok, I misunderstood, you need to create the pulse train as well as cause it to switch 3A on/off. The parts you listed should work, however, why three regulators? What unregulated voltage are you starting from?
 
That was good to hear. Last project i only had one regulator. So i decided that three could supply more than enough! Hehe :D M
 
You cant just parallel regulators and expect that to work. If you are running three different servos, you could use a separate regulator for each as long as you tie the negative side of the three regulators together.
 
It wont work? That is surpising.M

Not without some dicking around. The problem is that no matter what, the three regulators have slightly different output voltages, which means that one tries to supply all the load current while the others do nothing (until the first one overheats, and shuts down).
 
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