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Author Topic: simple transistor switch help  (Read 7609 times)

trials_modder

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Re: simple transistor switch help
« Reply #10 on: July 20, 2006, 05:32:21 PM »

By the way, do I have a definative answer to the diode question? Is the way it is configured in the schematic the correct way? The motor, having inductor-like properties, will tend to keep the current flowing in the way it was after it is switched off, right (according to electron current flow, not conventional current flow)? So the diode should allow the motor to continue the flow with itself in the little loop...

EDIT: Whoa, bad news. The link you just sent me was awesome, thanks. Unfortunately, my circuit looks almost identical to the "bad example" he says not to build.
« Last Edit: July 20, 2006, 05:45:51 PM by trials_modder »
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viridius

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Re: simple transistor switch help
« Reply #11 on: July 20, 2006, 06:59:20 PM »

Yeah, right now, you're just shorting it out through the diode.  Diode should go the opposite direction; induced back-EMF from the windings will be the reverse of the normal polarity.
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Skylined

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Re: simple transistor switch help
« Reply #12 on: July 20, 2006, 10:57:02 PM »

EDIT: Whoa, bad news. The link you just sent me was awesome, thanks. Unfortunately, my circuit looks almost identical to the "bad example" he says not to build.
What are you talking about?
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trials_modder

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Re: simple transistor switch help
« Reply #13 on: July 20, 2006, 11:51:45 PM »

You'll need to rig an H-Bridge.

I was referring to this link.

Apparently it is incorrect to connect the load to the emitter of the transistor, I don't exactly know why though. I though it would pretty much be the same as connecting it through the collector, but apparently not.
« Last Edit: July 20, 2006, 11:58:40 PM by trials_modder »
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trials_modder

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Re: simple transistor switch help
« Reply #14 on: July 20, 2006, 11:54:25 PM »

Yeah, right now, you're just shorting it out through the diode.  Diode should go the opposite direction; induced back-EMF from the windings will be the reverse of the normal polarity.

So you agree with the schematic from page 1.
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viridius

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Re: simple transistor switch help
« Reply #15 on: July 21, 2006, 12:05:41 AM »

No, the diode should be flipped around.  The reason you don't connect loads to the emitter is that the voltage across the load can rise high enough to bring the transistor out of saturation.
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trials_modder

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Re: simple transistor switch help
« Reply #16 on: July 21, 2006, 03:09:51 PM »

Ok, so I've created my own H-bridge. I've used LEDs temporarily in place of the motor just for testing purposes.
Here's the schematic:

I don't really see a difference with those "pull down" 10K resistors that are supposed to keep the transistor off.

I guess the next step is where to place the diodes. The guide http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt_analysis.html I'm following has components that contain some "freewheel diode" built in to the transistor, which I do not have.

I can't see how the "optional" diodes he has are any use to my circuit as mine lacks those internal diodes.

1) I don't really see how I could place the diodes with this set up as I don't think the 2n3904/6 has any of this freewheeling diode he speaks of.
2) I don't know how I'm going to control the H-bridge as it will be done via parallel port, and I don't know if its good to source and sink directly to it.
« Last Edit: July 22, 2006, 12:41:07 AM by trials_modder »
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trials_modder

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Re: simple transistor switch help
« Reply #17 on: July 22, 2006, 10:37:28 AM »

Searching around on Google (and even in this forum), I've seen designs without this back-emf shorting diode protection.

It appears like this design only works if the transistors have a freewheeling diode built into them. Is there a workaround?
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viridius

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Re: simple transistor switch help
« Reply #18 on: July 22, 2006, 12:23:30 PM »

Keep the pull-down resistors in there, but the protection diode is unnecessary.
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trials_modder

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Re: simple transistor switch help
« Reply #19 on: July 22, 2006, 08:38:06 PM »

Protection diode unnecessary? Is that the equivalent of saying you should drive without a seatbelt on? Won't the voltage generated by the motor cause damage, in the long run?

Also, how do I make a circuit that can connect its input to either 5v or ground? Apparently the microcontroller chip that can control a h bridge like that can do that.

EDIT: as for the diode configuration, I found this page http://www.dprg.org/tutorials/1998-04a/ that at the end shows the recommended config. I don't see the path of the motor's generated current however. It appears to flow into the battery.

SECOND EDIT: the bottom/top pair of transistor switches must be opened to allow safe back EMF absorbtion, I figure. However, if the power to the H-bridge's transistors were suddenly removed, it would cause the problem.

In the meantime, I'm still stuck on how I'm supposed to control this bridge with my parallel port.

THIRD EDIT: I think i've found another way to run the circuit. It will require two parallel port pins per bridge to control forward, back, and brake. coast will be done by the PWM pin, giving me a grand total of three pins per motor. This might work.
« Last Edit: July 22, 2006, 09:50:44 PM by trials_modder »
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