An idea to avoid need for a complex H-bridge in 2-way driving electric motor

:o Hello. I am new to electronics. I have looked at a few H-bridges which all seem so complex in design.
I did some thinking and "came up" with a simple idea that may possibly avoid the need for an full H-bridge, and uses just two transistors. It worked when I tried it, but I have not yet attached it to the arduino for fear that I may have overlooked some simple aspect of electronics and eventually blow up the arduino! As stated, I am a beginner and would really appreciate expert feedback on this circuit diagram before I try attaching it to my arduino! Many thanks in advance!

My old design (several first post responded to and showed to be shorting):

[later edit which does not short:]
New Design.png

New Design.png

The transistor will short battery to GND. It cannot work.
Basic H-bridge is just 4 transistors. It seems simpler than having two batteries.

There are 2 ways to switch a bidirectional load, either single supply + H-bridge (using 4 switches),
or bipolar power supply and 2 switches - you've rediscovered the latter.

The latter is not normally used as the cost of a whole second power supply exceeds the cost
of two switching devices.

This is also why class-D (switching) audio amps are typically single-supply, though analog
audio amplifiers are commonly dual-supply. For analog amps the cost of a whole second
amplifier tends to exceed that of a second supply (the opposite case).

Consider how possible it is that in 100 years of design for literally millions of motor circuits, somehow thousands of experimenters and engineers missed a fairly obvious idea.

If you are interested in pursuing this hobby, would be a good idea for you to learn how to draw schematic diagrams. There is a pretty good tutorial at How to Read a Schematic - learn.sparkfun.com.

Here is a conventional schematic for a two transistor, bidirectional motor driver, requiring two power supplies or batteries. It requires both NPN and PNP types of bipolar transistors (or Darlingtons), and dates back to at least 50 years ago.

Note: this circuit won't work with Arduino, because "direction control" must be negative with respect to ground, for the lower transistor to conduct.

Consider how possible it is that in 100 years of design for literally millions of motor circuits, somehow thousands of experimenters and engineers missed a fairly obvious idea.

This is the Forum, "just when you thought you've seen it all..."

MarkT:
There are 2 ways to switch a bidirectional load, either single supply + H-bridge (using 4 switches),
or bipolar power supply and 2 switches - you've rediscovered the latter.

The latter is not normally used as the cost of a whole second power supply exceeds the cost
of two switching devices.

This is also why class-D (switching) audio amps are typically single-supply, though analog
audio amplifiers are commonly dual-supply. For analog amps the cost of a whole second
amplifier tends to exceed that of a second supply (the opposite case).

Hi Mark. Thanks for your reply. The reason I was thinking of using it is for ant weight (150g) combat robotics. You may know (I discovered by accident also) that a 3.7V Lipo (150mAh) can be put into the 5V input of an ESP32 CAM and last for at least the 3 min needed for a competition before the voltage gets too low. These batteries are so small (5g) that they are not really much bigger than a TIP120 transistor. As well as that, (with the 3 batteries) I figured I would only need 8 transistors to control 4 independent wheels. Can you let me know if you think Smaldalf (above) is right about the short circuiting? It definitely does work (unlike he says) but he might be right about the short circuiting and thus some battery damage (e.g short circuiting one battery while the other is operating the motor??? Thanks..

There are SOP8 H-bridges like A4950 or L9910. I think those may be easier to use with comparable size to the discrete transistors.

Just FYI, if you're not an electronics engineer or technician and you want to experiment that's fine but if you are going ask engineers a question it would save time if you start with :
"I'm not an engineer. I need a micro-H-Bridge motor driver for a robot contest. What do you recommend ?

Also, for the future, when you post a schematic,
please label transistor pin numbers. We know which ones they are supposed to be but not if you don't
label them we don't know if that's what you meant.

Also, a TIP120 (besides being crap) only has 3 pins, not 4 (lower one shows four pins , one NC)
All I could find on the web is "Revision-3: 2004" (don't know when Revision-1 was)
This is 2021 (almost) . Everyone uses low Ron mosfets. BJTs have their place but there are so many better alternatives for motor control, ESPECIALLY if power consumption is a concern ,
as in your case.

Hello raschemmel.

Thanks for your reply. I find it helpful that your mentioned TIP120s are outdated, and the "better alternative" Ron mosfets. I have also updated the schematic to include BCE on transistors.

My intention however was not to look for a full H-bridge unless someone could show me a defect in the above design, which from my perspective would simplify things rather than having to go with the "traditional" full H-bridge.

What I currently looking for a answer to is "Does this schematic short circuit one battery while the other is driving the motor?" If so, and to damaging proportions, I will have to give up and go with a full H-bridge.

Thank you.

looking for a answer to is "Does this schematic

The circuit shown in the OP "short circuits" the battery to ground.

Please DO NOT modify your original post after replies have been posted. It makes the replies difficult to impossible to follow.

calebtaylor:

For example, if pin 7 goes high (voltage) with respect to 'gnd', then the upper transistor turns 'on', right? Which basically connects the + terminal of battery #2 to the - terminal of battery #2 via the 'C-E' route.

And this is not a desirable situation. That's what is meant by the circuit won't work ..... as in short circuiting of the battery. Not good.

SOP:
state objective
" experience
" parts available
" corcuit requirements

Southpark, thanks for the comment:
"For example, if pin 7 goes high (voltage) with respect to 'gnd', then the upper transistor turns 'on', right? Which basically connects the + terminal of battery #2 to the - terminal of battery #2 via the 'C-E' route."
Yes, but that is obvious (I know you probably wont like me saying that). The important bit is it shorting to "damaging the battery"? - e.g perhaps (1) passing through the transistor reduces the current "enough" and/or (2) the voltage passing through the transistor combined together (note batteries 2 and 3 are in parallel through the transistors) may even possibly reduce the shortage effect because they are completing. Yes I've probably got all this terminology wrong - the point is maybe the "short" circuit doesn't really matter here. But I'm not sure yet...

There exist people who are all too willing to throw out all traditional lines of thought, , and on the other side people who are engrossed in traditional methods they can't see beyond into new possibilities. Yes, I may be wrong in this instance but would like to investigate. Hope there's nothing "wrong" in that!

calebtaylor:
Southpark, thanks for the comment:

"For example, if pin 7 goes high (voltage) with respect to 'gnd', then the upper transistor turns 'on', right? Which basically connects the + terminal of battery #2 to the - terminal of battery #2 via the 'C-E' route."

Yes, but that is obvious (I know you probably wont like me saying that). The important bit is it shorting to "damaging the battery"? - e.g perhaps (1) passing through the transistor reduces the current "enough" and/or (2) the voltage passing through the transistor combined together (note batteries 2 and 3 are in parallel through the transistors) may even possibly reduce the shortage effect because they are completing. Yes I've probably got all this terminology wrong - the point is maybe the "short" circuit doesn't really matter here. But I'm not sure yet...

There exist people who are all too willing to throw out all traditional lines of thought, , and on the other side people who are engrossed in traditional methods they can't see beyond into new possibilities. Yes, I may be wrong in this instance but would like to investigate. Hope there's nothing "wrong" in that!

Most welcome Caleb.

The first thing to do is to read up on (ie. google) things like 'dangers of short circuiting battery'. Even if the danger situation were 'ignored' ...... then there will be other shortfalls. Let's consider everything.

Short circuiting a battery (for batteries having no inbuilt protection features) basically allows the energy in it to be drained relatively fast ...... as in could be within seconds ....... and the battery and/or the wire could get very hot due to the relatively high current flowing through the wires ...... could result in burning of wires, and burning of battery, explosions. People getting hurt. Homes getting burned right out etc.

And ---- for batteries that have inbuilt protection against shorts ---- that's if they have such protection in them (some batteries) ----- a short circuit condition will mean the battery and system operating away from normal desired conditions ..... which will mean the system is not going to perform as expected (or as desired). When a battery becomes shorted, and if that battery is meant to power the system (circuit), then the system won't work ----- due to the short circuit, because the battery won't be able to carry out its intended function ---- as in to supply power to the circuit/system.

And in your case ------ you mention that the motor still works. But with that particular wiring diagram you showed, it is difficult or impossible to believe that your motor actually works with that particular wiring you have in the opening post. But ----- let's just say that your motor does turn. The prediction would then be the batteries will have their stored energy depleted real quick. But even then ........ the safety recommendation is to not use that circuit you showed in the opening post ..... because seriously, as raschemmel pointed out somewhere in his/her post, things could end badly.

And one teaching here is ...... a short circuited battery will mean that the bulk of the current will flow through the short circuit and not much current will flow through the rest of the stuff (ie. devices, components etc) that the battery would normally supply current to. And if not much current flows into your circuit (such as your motor), then the motor (or your circuit) is just not going to work. But ----- not working is fine. The bad thing is that a fire or exploding battery can cause some nasty damage to people and property. So be real careful when working with electronics. Make sure to get some fundamentals (circuit theory) under the belt first. Look after yourself and others and property first. Very important.

If you have multimeter connect it to battery #2 and read the voltage when you turn on the TIP120
at the top of you diagram.

Shorting a Lipo Battery

raschemmel:
If you have multimeter connect it to battery #2 and read the voltage when you turn on the TIP120
at the top of you diagram.

Shorting a Lipo Battery

Ok, here's the results to the experiment. It may surprise you (or maybe not...)
Disclaimer: I used two 5g 150mAh for batteries #1 and #3, BUT I substituted battery #2 for two standard rechargeable in-parallel 1.5 V batteries (ie 3 V approx) for the "shorted battery" to avoid the dangers of short circuit to a Lipo.
Initial charge reading on both Lipos was 3.86 V.
Initial charge reading on battery #2 (not Lipo) was 2.95 V.
With battery #1 Lipo disconnected, and BOTH the other two connected, the "voltage reading" across battery #3 went immediately to 3.22 V and stayed at that value. (It appears to be an average of #2 and #3?). Note that the base pins of both TIP120 had not been activated yet. It may be interesting that that reading slowly increased to 3.28 within about a minute - it appears the Lipo #3 was charging the 3V batteries (#2)?
Next step - activate 3.86 V Lipo #1 onto the circuit (left one on your diagram, upper one on mine) -
Left everything in that state for 3 minutes. Motor stated going, and continued going all 3 minutes...
At the start of the 3 minutes the TIP120 was cool. The "voltage reading" across went immediately to 1.0 V)
After 3 minutes (with base still activated) the voltage reading across the "shorted battery" read as 0.93 V, and the TIP120 was very hot but NOT smoking or smelling...

Final reading on battery #2 after all circuit disconnected was 2.81 V. On #3 it was 3.85 V

It appears the "shorted battery" lost 0.14 V over the the course of being short circuited for 3 minutes.

Now - I am still very cautious - Yes the TIP120 is overheating, and may be sustaining damage in the long term... (but their easy and cheap to replace though). My main concerned would be if I used a Lipo instead of the standard 3V rechargeable. Would the loss of 0.14 V on that "blow it up" - here's where I'd appreciate your advice before doing it... Would the Lipo discharge faster that 0.14 V over minutes or not ?

Thanks...

Yes, whatever topology you have to place the switches betweed the power and the load of course :slight_smile:

To sum up:

Caleb ........ I recommend to not use the circuit that you showed (in your original post). Use the configuration(s) shown by MarkT and/or other members in this thread.

Your circuit from the original post is unsatisfactory - and practically unworkable, and can be hazardous as well. You have now seen it yourself that the battery became drained relatively quickly. That's because your design has issues with it. It's unsatisfactory for what you want to do, and is absolutely not recommended.

A course in electronics of some sort will likely help you to get on top of this sort of thing ----- toward understanding circuits like this.

Working with certain kinds of electronics without having adequate understanding of where and how much current flows through say wires or devices, and how much power is dissipated in wires or devices ....... is a safety issue. Just got to watch out for that.