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Author Topic: simple transistor switch help  (Read 7612 times)

trials_modder

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Re: simple transistor switch help
« Reply #20 on: July 22, 2006, 10:26:52 PM »

I think I have the solution to all my problems. Well, the only one left is whether I used the correct resistor values. Hopefully most would be usable values in voltage input of 5 - 20 volts. Of course, I'd be replacing the transistors dealing with supplying current to the motor with higher power ones, like the TIP41C/TIP42C.
What do you think?

Schematic pretty much self-explanitory. I tried to design it so it would be impossible to have both transistors on either side of the H bridge open simultaneously. The NAND gate is reliable enough to not allow that due to delay right?
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viridius

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Re: simple transistor switch help
« Reply #21 on: July 23, 2006, 02:37:26 PM »

I don't know why you are using 2n3906's and 2n3904's, they have the same current ratings.  Also, why do you need the inverters?  The pull-down resistors will take care of turning off the appropriate transistors.  Lose the diodes.  Right now, your power supply will short out through D1/D4 and D2/D3.
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trials_modder

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Re: simple transistor switch help
« Reply #22 on: July 23, 2006, 09:28:18 PM »

I will be swapping the current transistors that handle the motor's current directly with ones of higher current ratings (like the TIP41C/42C. I put the diodes in the wrong direction, their symbols should be pointing up. The diodes are supposed to allow the motor to short itself out when I want the motor itself to stop, presumably to prevent the voltage generated by the still spinning motor from breaking something. Hopefully that is what would happen if for some reason the H-bridge loses power. The inverters (NAND gates you're referring to) ensure transistors on the same side of the H bridge NEVER are saturated at the same time.
Pin 2 will be controlled by computer and is pulsed to give Pulse Width Modulation.
Pin 3Pin4Transistors onEffect
00Q5, Q6Motor shorted, effectively a brake at high rpm
01Q5, Q4Positive current through motor, forward rotation
10Q6, Q2Negative current through motor, reverse rotation
11Q2, Q4Motor shorted, effectively a brake at high rpm


EDIT: By the way, is there any special way I have to treat the NAND gate chip? I know it cannot source a lot of current, or sink much either. The resistor values I used are mostly guesswork.
« Last Edit: July 23, 2006, 09:41:33 PM by trials_modder »
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trials_modder

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Re: simple transistor switch help
« Reply #23 on: July 27, 2006, 07:44:06 PM »

Well, the circuit is pretty much complete, except I can't get the NAND gate to work. I did what the data sheet said to do, putting 5V in Vcc, then wiring the appropriate inputs, output, and ground.
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viridius

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Re: simple transistor switch help
« Reply #24 on: July 27, 2006, 08:05:13 PM »

Ground out the inputs of the unused gates and put a decoupling capacitor across the power pins.
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trials_modder

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Re: simple transistor switch help
« Reply #25 on: July 28, 2006, 03:13:21 PM »

Really? The data sheet described the other logic gates as "independant", so I didn't think you'd have to do anything to them. What do you mean by decoupling the power pins and why would that be necessary?
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viridius

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Re: simple transistor switch help
« Reply #26 on: July 28, 2006, 07:27:47 PM »

It's just standard procedure to force the inputs of unused gates high or low to prevent excessive power consumption due to rapid switching when the gates pick up noise.  Decoupling capacitors smooth out the power rails in the vicinity of the chip, so current draw in another portion of the circuit won't make the supply to the chip dip or if a gate turns on, the rail won't dip either.
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trials_modder

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Re: simple transistor switch help
« Reply #27 on: July 29, 2006, 07:11:42 PM »

Sure, for optimization. But the chip appears to not be working at all. I've tried swapping it with others of the same kind.
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viridius

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Re: simple transistor switch help
« Reply #28 on: July 30, 2006, 01:36:07 AM »

And you're measuring the output at the output pins of the chip?  5 volts into pin 14, ground out pin 7, pull 1 and 2 low and get...what on pin 3?  Honestly, I don't know why you're going through all the trouble to essentially invert your signal twice before driving your 2N3906.
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trials_modder

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Re: simple transistor switch help
« Reply #29 on: July 30, 2006, 07:41:23 PM »

Oh oh. By pulling the inputs "low" does that mean I have to ground them? I thought that simply disconnecting the inputs would cause the output to be +5v. I want to use the parallel port as the input, so I don't know how its off state works.
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