Basic problem with transistors

[giftiger_wunsch]

I fail to see how the diodes offer protection.

Notice that the ground return for the microprocessor circuits is back through the pair of parallel bits rather than to the microprocessor's shared GND pin. If it was emptied into GND, a HH state on any pair of pins would cause a short-circuit in the battery circuit. By grounding the circuit back into the correct pair of terminals, this prevents a current flowing if both are high since there's no actual voltage difference in this situation. The diodes D(n)b and D(n)c are required to prevent a direct short-circuit between the pair of terminals.

Further, the D(n)a diodes allow the battery current to drop down to the bottom and back to battery ground, but block µP current from flowing back up from the bottom wire into a different motor's ports. If not for these diodes, a HH state on one of the pairs would still activate the transistors as long as any one of the other ports were in the L state. The schottkys isolate each µP control circuit.

 

[Nigel Goodwin]

Sorry still don't understand, can you show the connections to the micro?.

I take it you know that to turn the H-bridge ON you take the inputs LOW and not HIGH?.

 

[giftiger_wunsch]

Sorry still don't understand, can you show the connections to the micro?.

I have shown the connections to the microprocessor (or to the prototyping board on which it is mounted, anyway): it's a complete schematic.

I take it you know that to turn the H-bridge ON you take the inputs LOW and not HIGH?.

No, a logic high from port PXn is 3.3V. I'm bridging logic highs and logic lows to use the 3.3V output to control the transistors and switch the H-Bridges.

 

[Nigel Goodwin]

I have shown the connections to the microprocessor (or to the prototyping board on which it is mounted, anyway): it's a complete schematic.

In which post?, #257?

There's hardly any of a circuit there.

No, a logic high from port PXn is 3.3V. I'm bridging logic highs and logic lows to use the 3.3V output to control the transistors and switch the H-Bridges.

Where is the common ground connection?, and what do you mean by 'bridging'?.

 

[giftiger_wunsch]

In which post?, #257?

There's hardly any of a circuit there.

I don't understand what you mean. That's the complete schematic. What is missing?

Where is the common ground connection?, and what do you mean by 'bridging'?.

By 'bridging', I mean 'connecting'. I am passing a current between PA0 and PA1 (or vice verca) to activate the top BJTs.

It just struck me that I could simplify the circuit to remove D(n)b and D(n)c diodes now that only two of the transistors are being activated by the µP directly though; I should now be able to simply connect PA1 to the emitter of Tr1a, and PA0 to the emitter of Tr1b, which wasn't possible with the previous schematic. And of course, follow the same procedure for the others.

In fact, that should remove the need for any of the diodes... eureka it appears hero's modification to my original idea has made my ground return much simpler. I'll post a new schematic shortly.

 

[giftiger_wunsch]

Thanks to hero's schematic, my original idea (before needing to come up with the complicated plan involving Schottkys and whatnot) should work, I think - comments are welcome. This has saved me money on diodes and voltage by avoiding the diodes - thanks Hero!

Again, comments, suggestions, and questions are welcome.

The transistors which need to be activated have essentially just been placed between PA0 and PA1 (and the corresponding ports for the other 4 h-bridges), such that neither are on when PA0 = PA1, and otherwise the correct transistor comes on. My only remaining worry is whether the transistor will pass current from emitter to base - however, based on my understanding of transistors base -> emitter should essentially act as a diode and will therefore not allow a reverse current across these terminals. The other note is that the motor itself has also been placed directly between the two uP terminals, causing a short... but if I move the resistors RXd to the wire immediately above where they're currently situated (I will change the schematic later) the current should be reduced to ~3mA here anyway.

 

[mneary]

Your latest schematic in post #266 does not look like Hero999's schematic in post #256.

You left out two very important transistors, six resistors, and one Schottky diode per circuit.

I think we are all glad you have saved the 'organic ground' invention for later.

 

[giftiger_wunsch]

I give up.

 

[Nigel Goodwin]

I don't understand what you mean. That's the complete schematic. What is missing?

Any connection to a micro to make it work, plus resistors I would consider pretty well essential.

By 'bridging', I mean 'connecting'. I am passing a current between PA0 and PA1 (or vice verca) to activate the top BJTs.

And does that work?, lack of a common ground makes it look like it wouldn't.

It just struck me that I could simplify the circuit to remove D(n)b and D(n)c diodes now that only two of the transistors are being activated by the µP directly though; I should now be able to simply connect PA1 to the emitter of Tr1a, and PA0 to the emitter of Tr1b, which wasn't possible with the previous schematic. And of course, follow the same procedure for the others.

In fact, that should remove the need for any of the diodes... eureka it appears hero's modification to my original idea has made my ground return much simpler. I'll post a new schematic shortly.

Again, does it work? - nothing in the minimal circuit (or the previous one for that matter) inspires me to think so.

 

[Hero999]

I give up.

You've not listened to half of what we've said.

We told you that it isn't a good idea to use a floating ground because it will only work with one h-bridge.

To offer protection from both inputs going high you should use a Schottky diode or transistor to ground one input when the other is high.

There should also be some base-emitter resistors to ensure the transistors aren't turned on by noise, as Nigel said.

Use the schematic attached with high gain transistors for the NPN devices and you shouldn't go wrong.

 

[giftiger_wunsch]

I don't care. I've given up. I haven't listened to half of what you've said because the only one who's listened to half of what *I've* said has been marcbarker. I'm not interested in completing this project anymore, it's already taken up enough of my time and been shot down repeatedly. Clearly it's beyond my ability. If it was possible to delete owned threads in this forum I would have done so already.

Feel free to continue replying to the thread if you really want but I'm unsubscribing from it now.

 

[Hero999]

I don't see why you should feel like that: no one's shot down your project, just the way you've intended to build the h-bridge.

To me it seems like you've got too much pride in your original idea that you won't accept anyone else's.

I've already solved the problem for you, just use the schematic I previously posted and the transistors I suggested and it will work.

There's no point in asking for help and advice if you're too proud to accept it.

 

[marcbarker]

Erk.... Have I missed something the last couple of days? I just read through the postings. Looks like there has been a parting of the ways..

A big thing I've noticed is that a design concept has been developed by Giftig (at posting #1 apparently didn't know what a transistor was) which is a totally fresh approach, and uses some unconventional techniques which are initially difficult to analyse, but seem to work 'OK'. Out of this, came a "2bjt-2fet organic circuit" which is close to a practical circuit, curiously 'minimalist' in design. So much so that I support it, and have developed his concept further (posted as a new thread) https://www.electro-tech-online.com...ganic-h-bridge-motor-driver.96252/#post776149

I notice there's been another circuit being talked about, which I understand is a 'conceptual' circuit (similar to the 'organic' circuit), but having detail omitted for clarity. This circuit includes countermeasures against 'h-bridge-shoot-thru', by dispensing with the ground link, and using the IC's logic L output as a return instead. The principle is that if the motor control input is differentially connected to PA0, PA1, when they are at H/H they cancel eachother out. To ensure that this holds true when more than one motor is actve, blocking diodes are inserted in the ground returns. The blocking diodes have a voltage drop and are assumed not to leak significantly, this was consdered acceptable.

I remarked earlier posting that if a circuit is very different and unusal looking, as though it's from Mars, the author of it (and the people who support it) are going to get themselves a hard time over it being so unconventional. I added that the supporter(s) of that circuit need to prove it works to satisfy the critics.

Bottom line I thought Hero999's circuit is best so far, meeting all the requirements. I thought that's the practical build one, possibly modified (slightly!) by Giftig according to his requirements.

 

[mneary]

Giftiger_Wunsch,

In a few short weeks you've advanced from claiming that you've never used transistors before, to designing a circuit that the rest of us can't understand.

Now it may be time to build your new invention and show the naysayers like Hero999 and myself what we've missed for the past 50 years. I personally have considerable experience with H-bridge design, including a US patent.

 

[Hero999]

Speak for yourself, I'm under 50.

 

[mneary]

Metaphorically

 

[giftiger_wunsch]

to designing a circuit that the rest of us can't understand

Perhaps if you told me what was wrong with it there wouldn't be a problem. I didn't post the thread to get someone to throw a schematic I don't fully understand at me and say do it this way, I wanted advice on how to improve my schematic to get over its shortcomings. And marcbarker seemed to understand my schematic so that leads me to believe that you don't understand it because you haven't looked at it properly or because you haven't read the detailed explanations which I have written several times and haven't had a single comment on from anyone else. If my logic is flawed, I want to know why. If you think my circuit won't work but won't provide a reason better than that it doesn't all connect directly to the µP's GND pin, then I don't want to know. A comment that my schematic won't work and providing one which will work isn't much good to me. Anyone can blindly follow a schematic, I would much rather understand where I made the mistakes, how to correct them, and how to maintain the functionality I've mentioned several times which to my knowledge the provided schematics doesn't provide anyway.

Either way, I have lost interest in this thread. I will try building the circuitry based on my own schematics, and if that doesn't work then i'll rethink them and tinker with them until I come up with a working schematic.

 

[Hero999]

I understand how your previous schematic works or at least this one.
https://www.electro-tech-online.com/attachments/sam-motor-proto1c-jpg.32016/

It will work is a base resistor is added to each low side transistor and the 12R resitor is removed.

The top transistors are configured as emitter followers which will loose 0.7V plus the saturation voltage. The bottom transistors are configured as common emitter switches.

The diodes direct the voltages from the I/O ports to the MCU 0V via the port which is at 0V.

When both inputs are high, no transistors will turn on because there is no potential difference.

The problem is the high saturation loss on the high side, and if there are other h-bridges in the circuit the double high protection won't work because other inputs might be low which will create a 0V for the current to flow to.

Try analysing my circuit.

https://www.electro-tech-online.com/attachments/3v-h-bridge-hi-hfe-gif.32335/

When Tr3 is activated, it will connect Tr6's base to 0V causing it to turn on.

Then Tr4 is activated, it will connect Tr5's base to 0V causing it to turn on.

When both inputs go high Tr4 is activated which connects Tr3's base to 0V via the zener diode.

R3 and R4 help to ensure both Tr5 and Tr6 remain off when tTr3 and Tr4 aren't activated.

I'm not meaning to be cocky but my circuit is better because: it uses PNP transistors for the high side, the double high protection will work regardless of how many h-briges are used and it uses less components.

 

[giftiger_wunsch]

my circuit is better because: it uses PNP transistors for the high side, the double high protection will work regardless of how many h-briges are used and it uses less components.

This is why I used your schematic and added my modifications to that. I implemented the high-gain PNP transistors as specified by your schematic, but I changed my grounding system for HH protection.

I didn't actually realise that your circuit offers HH protection, could you explain how that works? And the purpose of the schottky diode.


I am not disagreeing with most of your schematic, I recognise that it is more appropriate than my original schematics, which is why I took your schematic into consideration when I formulated the last one I posted. Before adapting it to your schematic, my ground system was going to involve several diodes to prevent current escaping elsewhere, but I realised when I attempted to apply this to your schematic that it is made much easier by the way yours works. The fact that the upper transistors trigger the lower ones means that I can place the ground connections between the two sets of transistors, and the lower transistors themselves will prevent the current reaching the other H Bridge's Parallel ports.

I don't understand why my grounding circuitry won't work. If you're all so certain it won't, then could one of you explain why not?

Just to be clear, I have included the schematic to which I'm referring in this post.
And just to be completely clear, as marcbarker pointed out earlier, if any of the ports are H they're connected to the 3.3V µP supply, if they're L they're connected to the µP's 0V GND. So there is a common µP ground.

 

[mneary]

If you think my circuit won't work but won't provide a reason better than that it doesn't all connect directly to the µP's GND pin, then I don't want to know.

This where most of us parted ways.

You footnoted your 'final' circuit with something like - the motor is shorted to PA0 and PA1 but we'll handle that later. I pointed out that the transistors that you removed from Hero999's offering were too important to leave out.

I look back and see that I was harsh in the way that I expressed this, and I didn't give a specific electronic/physics reason. I thought it was covered. Summarized here:

Connecting PA0 and PA1 (8mA) to the motor (1A) shouldn't be done. I don't know how to fix that without the two removed transistors. Similarly, reverse/forward protection is missing. The Schottky diode is the simplest way. Further, the bases should to have stray charges drained (not essential, but cheap insurance). Hence the resistors added from bases to emitters. Collector resistors are needed on the 2 driver transistors to limit their collector current. And, early in the thread, if you are avoiding protection diodes, 0.1 uf across the motors to limit the inductive kickback and noise.

As for ground, suit yourself. It's so fundamental to most of us that maybe we've lost our ability to consider just leaving it open. Your small system will work most of the time. Maybe for a toy we've just set our standard too high.

I believe that marcbarker is enthusiastic about your circuit, but I would still like to see it work.