Author Topic: H-bridge schematic review?  (Read 812 times)

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Offline ErikYTopic starter

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H-bridge schematic review?
« on: January 14, 2013, 10:27:22 AM »
Would anyone be so kind as to review my H-bridge schematic?

I put this together on a prototype board and its not working at all.

I tripple checked all of my connections against this circuit, and nothing at all from my motor.

My assumption is that something is wrong with my circuit design, but I am not seeing it.

I did not design this myself, I got most of it from a book with some very minor changes that I made.

Can anyone see a problem with this?

Offline ErikYTopic starter

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Re: H-bridge schematic review?
« Reply #1 on: January 14, 2013, 08:04:28 PM »
I just noticed that in my sch I have the BHi and BLo reversed for the pin names, I fixed that and still no motor

Offline jwatte

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Re: H-bridge schematic review?
« Reply #2 on: January 14, 2013, 10:15:27 PM »
Do you have a multimeter?

What does it measure on the top, in the center, and on the bottom, as well as for the gates of the transistors?

Offline billhowl

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Re: H-bridge schematic review?
« Reply #3 on: January 15, 2013, 03:44:33 PM »
What voltage you operate? higher then 10V?
MOSFET was not logic level need more then 10V to operate.

Offline ErikYTopic starter

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Re: H-bridge schematic review?
« Reply #4 on: January 15, 2013, 08:46:39 PM »
Guys, thanks for the replies.

I actually got it "kind of" working.

I figured out that my mosfet driver chip was bad. Not sure what I did, maybe I zapped it with static electricity, but it was definitely bad.

By testing the voltage going in and coming out, I saw it was not working.

Fortunately I had another one to use, popped it and it worked.

For some reason which I cannot figure out though, PWM is working for my forward (m1BHI High, and m1ALI PWM) but it is not working reverse, I am only getting one speed with the reverse m1AHI High, m1BLI PWM.

Can anyone make any sense out of that?

Offline waltr

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Re: H-bridge schematic review?
« Reply #5 on: January 15, 2013, 09:43:18 PM »
The P-ch MOSFETs are backwards. Their Sources should be to the +UB rail. Note the diode drawn inside their schematic symbol, they will always conduct as wired.
Their schematic diagram have the source and drain labels on the wrong pins.
Re-check the data sheet for the proper pin-out of the MOSFETs.

Do be careful with the signals on JP1. It is possible to turn on both the top and bottom MOSFET on on or both sides. This creates a short from +UB to ground through the MOSFETs, also known as 'the Smoke state'.
Could this be the problem for only one speed in reverse?

Also, depending on the motor you may need catch diodes across the MOSFETs drain and source.
« Last Edit: January 16, 2013, 09:20:42 PM by waltr »

Offline jwatte

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Re: H-bridge schematic review?
« Reply #6 on: January 15, 2013, 11:49:07 PM »
Quote
Their Drains should be to the +UB rail.

To clarify: The /actual/ drains should be towards the load. The problem is that, what the schematic calls "D" is actually the source of the P-channel MOSFET, as you say:

Quote
Their schematic diagram have the source and drain labels on the wrong pins.



Offline ErikYTopic starter

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Re: H-bridge schematic review?
« Reply #7 on: January 16, 2013, 05:45:02 AM »
Guys,

Thanks for the replies, much appreciated as always.

First off, before I prototyped this h-bridge, I used the actual data sheets for my mosfets that I bought and made sure the pin layouts were correct, meaning I put my p-ch S pin to the +UB and the n-ch S pin to the GND, so that is all good.

The problem was the pins to the MCU, I had two of them reversed, I am an IDIOT!

I spent 2 hours checking every pin on my prototype board, and never once did I look at my MCU connections!

Well, now I have full forward and backward speed control, and stop functionality, the h-bridge is working perfectly.

Unfortunately, I destroyed a $160 linear actuator prior to reading these replies!

Yup, I am really an idiot.

I figured maybe, just maybe my cheap $5 6V DC brushed motor was not responding properly, even though I knew better, so, I decided to test out the only other motor I had in my house, a $160 linear actuator.

Well, that is now ruined.

I even tried direct 12V connection to it, and it is just making very slow, small 1/4" movements and than stopping for a second before continuing.

I clearly messed something up.

I am an idiot!

I would like to know what it was that messed it up, my code was simply to move forward for 2 second at speed 1/4, then 2 seconds at speed 1/2....... stop, reverse 1/4 for 2 seconds......

It seemed to "die" when I reversed it, which is what was not working on my DC motor, it went forward very nicely.

So, I am assuming it was the reversed mcu connections trying to send PWM to the L.A., but I am not sure.

The L.A. has limit switches built in.

I also did not have the diodes across the mosfets D and S, so perhaps that could be it?

Man, I am so pissed that I did that, very, very expensive learning experience.


Offline ErikYTopic starter

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Re: H-bridge schematic review?
« Reply #8 on: January 16, 2013, 08:54:11 AM »
Another question just came up, if someone could be so helpful as to help me understand something.

Now that I have this h-bridge working, I want to make sure I understand everything about the circuit, since I did not create it.

I don't quite understand how or why the resistors are being used in this circuit.

If I look at R1 , it is a 10k resistor, connecting +UB to the Drain of Q7, a n-ch mosfet designed to take the high output of the mcu pin, and send out a negative voltage into the Gate of Q2, a p-ch power mosfet.

I completely understand the source of Q7 being GND, and turning on that mosfet with the high output from the MCU,  allowing the GND voltage to pass through the Drain, and into the gate of the p-ch power mosfet, switching it "on".

I am struggling to understand the purpose of R1 in this application, lets say I am using this for 12V as my +UB, 10K resistance is going to allow 1.2mA through that connection into the Gate of Q2, which I suppose is not going to turn on the p-ch mosfet, as that needs negative voltage.

So, the way I see it, the only way that Q7 can be turned on, is if the mcu goes high at that pin, which would send a negative voltage through that 1.2mA into the p-ch Q2.

So, if the mcu goes high to the Gate of Q7, its going to send a negative voltage through the gate of Q2, and if the mcu is low, it won't send anything as Q7 will be off.

I know this is being used as a pull up resistor, I just don't understand why in this application.

Same thing goes for R5, it looks like R5 is acting as  a pull down resistor for the Gate of Q7, I just don't understand why it's needed.

Any help would be greatly appreciated.

Offline jwatte

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Re: H-bridge schematic review?
« Reply #9 on: January 16, 2013, 03:15:30 PM »
R1 and R5 (and their mirrors on the other side) are used to "default" the circuit to a known good state when power is being applied or removed.
The gate of MOSFET transistors are small capacitors, that can retain their last switched state for a long time, unless there is something for them to "bleed" that charge into if driving power is removed.

Quote
I decided to test out the only other motor I had in my house, a $160 linear actuator.

If you do not own a digital multi-meter, please buy one! A $20 Radio Shack one will probably do a good enough job. Typically you just need to measure voltage and resistance.

Offline ErikYTopic starter

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Re: H-bridge schematic review?
« Reply #10 on: January 16, 2013, 07:13:55 PM »
R1 and R5 (and their mirrors on the other side) are used to "default" the circuit to a known good state when power is being applied or removed.
The gate of MOSFET transistors are small capacitors, that can retain their last switched state for a long time, unless there is something for them to "bleed" that charge into if driving power is removed.



If you do not own a digital multi-meter, please buy one! A $20 Radio Shack one will probably do a good enough job. Typically you just need to measure voltage and resistance.



Got it, that makes sense, thanks for that explanation!

I actually do own two multimeters, I was just not smart enough to use it!

My DC motor was going forward with PWM working perfectly, and going reverse at just one speed, but going nonetheless.

I assumed (big mistake) that everything would be fine with the L.A., I will be sure to be much more careful in the future!

Offline waltr

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Re: H-bridge schematic review?
« Reply #11 on: January 16, 2013, 09:20:09 PM »
Quote
Their Drains should be to the +UB rail.

To clarify: The /actual/ drains should be towards the load.

Opps your right, brain glitch.
I'm correcting my post above to try and prevent future confusion.

Offline waltr

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Re: H-bridge schematic review?
« Reply #12 on: January 16, 2013, 09:26:50 PM »
Some high wattage resistor are good substitutes for a motor when testing circuits like this. Then just put your voltmeter across this load resistor.

Switched connections, burned out parts. Yep, been there have some times still do that.

Nice you got this working.

Offline ErikYTopic starter

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Re: H-bridge schematic review?
« Reply #13 on: January 17, 2013, 04:46:03 AM »
Thanks much for your help guys, I appreciate it!

Offline jwatte

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Re: H-bridge schematic review?
« Reply #14 on: January 18, 2013, 11:15:21 AM »
Quote
I was just not smart enough to use it!

Good decisions come from experience.
Experience comes from bad decisions.

 


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