Changing the Direction of Movement for a 12 VDC Motor

Yup, sounds like you have pretty much answered your own, (very nicely worded) Q.

A H bridge has the advantage of being able to be controlled by a computer/microcontroller or some other electronic circuit.

A mechanical switch is limited to mechanical operation.

Regarding reverse currents/voltage spikes... varistors, back to back zenner diodes, capacitors all play a part in avoiding these. Many FETs have reverse protection diodes in them to help with this. If you look on the web there are plenty of good sites for this stuff.

Del

Del

Thank you Dell the Cat, I am mechanical engineer and I am not expert in electronics. Could you help me with some simple electronic diagram that I will pass to a CAD software to build a PCB.

another question, the H bridge can be only controlled with a PC?, can be controlled with a simple switch?

thanks again.

Yes it can be controlled with a simple switch...
You can build it on strip board quite easilly, unless you are going into mass production however as PWSlack said it depends on the motor size (has it got varistor supression built in? (you can usually tell by looking through the slots by the brushes, the varistor looks like a disk between the windings and the commutator....)) etc
Send a bit more info and I'm sure we can post some simple circuit suggestions.
Del

PS. If you are going to control the H bridge with a switch...then why bother you might as well just use the switch to control the motor directly. Switches are far less prone to unexplained blowing than FETs are .
Simplicity is the greatest engineering virtue.

Del

Hi Dell the Cat,

Here goes more information:

- The motors are 12V to be used in a underwater ROV, are common motors, the body diameter is 40mm, body lenght 65mm, shaft 4 mm.

- The rov is very ligh, no more that 10 Kg (on air)

Please ask any question that you may have!

If the control wires are going to come up to the surface I would definitely say use a switch up at the surface end...you don't want anything down below which could burn out the wiring.
Just use your switch arrangement that you have already drawn and take the motor power wires down to the vehice.... it is so much simpler and safer than any 'clever' electronics....

Also as I said...you don't want any electronics where it could (and undoubtedly will at some point) get wet.

Even my elecric golf cart contrived to run off into a stream.....the water got into the FET drive and it wouldn't stop until it dried out ....I even got a bit of moisture into the control circuitry in a downpour...then it wouldn't go...(but on average it was fine )

Del

Thank you!, very fast solution. I ever want to be simplest as possible, but I heard about the H bridge and I think that it could be better.

there are three motors in the ROV, and another user in the group recently give me next information.

"n the piece of equipment drawn, reversing any of the three motors will cause an instantaneous dumping of the energy from the spinning motor into the other two as it reverses, causing a brief drop in current from the supply, followed by a brief surge in current from the supply as motor #1 comes up to speed in reverse.

Fitting appropriate protection across the power supply terminals might be a good idea. How about a rectifier reverse-biased, just in case? If it were smooth DC, then a large electrolytic capacitor as a 'storage tank' might be a good idea. If it were a battery, then why bother"

I will supply power with a battery on the surface.

It depends upon the size of the motor and what it is driving.

In 45 years' experience of switch-reversing 12VDC motors in models:

the number of failed motors + the number of failed pieces of supply equipment = zero.

In the piece of equipment drawn, reversing any of the three motors will cause an instantaneous dumping of the energy from the spinning motor into the other two as it reverses, causing a brief drop in current from the supply, followed by a brief surge in current from the supply as motor #1 comes up to speed in reverse.

Fitting appropriate protection across the power supply terminals might be a good idea. How about a rectifier reverse-biased, just in case? If it were smooth DC, then a large electrolytic capacitor as a 'storage tank' might be a good idea. If it were a battery, then why bother?

Back in the sixties, when slot cars were a big craze, a big problem was slowing the cars down in the curves. Simply backing off the power didn't slow the cars quickly enough, so a lot of us modified our controllers by putting a reverse-biased battery into the switch circuit. That way, when we cut the +12 volts to the motors, we slapped -1.5 volts across it instantly putting it into reverse. This allowed us to 'slam on the brakes' as opposed to coasting into the curves, giving us a racing edge. See if you can adapt this concept to your ROVs if you need to stop more quickly. Just note that depending on how it's wired, you could limit the max volts coming in or you could be charging the "reverse" battery.

Hi Graebeard,

Nice experience, really I think that could be used in a underwater ROV.

Can you explain me more about "...must note that depending on how it's wired, you could limit the max volts coming in or you could be charging the "reverse" battery."

This "reverse" will injure the battery or the motors?

How can I limit the max volts?

Best wishes,

Esteban

If you have a varistor across each motor ..say a 35v one this will help get rid of back emfs.
Also if your reversing switches have a 'centre off' position this will give a bit of delay between forward and reverse, this also allows better control as you can stop a motor without having to go to another switch.

Del

hi Del the Cat,The switchs have a centre off position, and the switch must pass by this to change the gyration.Can you explain me a little more where should I insert the varistors?, maybe you could recommend me one from www.rs-components.com/

What about that user opinion "...ust note that depending on how it's wired, you could limit the max volts coming in or you could be charging the "reverse" battery...."

thanks

18v DC rated varistor from RS. Connect a varistor across the back of each motor, the motor has two terminals... the varistor has 2 legs, one leg to each terminal... this should supress any excess voltages. It is good practice to connect a 0.1uF capacitor across the back of each motor too, this will help supress electrical noise and arcing at the commutator and arcing at your switch.

Del

Perhaps a simpler approach: Why not just use a 3-position switch where center would be OFF and the motor unconnected from the supply. Consciously waiting the few moments for the motor to come to rest before changing direction?

thanks.

I am decided to use a 3 position switch

As I have readed here, seems to me that the motors will no suffer damage if I pass from forward to reverse if I pass the switch by the center (off) position. Seems to me that is no needed wait.
do you think that really I wait few moments?

Please remember for anyone reading this that some DC motors will not reverse just by reversing the leads to the supply, nobody has mentioned that up to now!

The ones with wound field coils need to have either the armature voltage OR the field voltage ONLY to be reversed, reversing both means that it runs in the same direction always!!

oooppss... I will test my motors!

Thank you!!

Its only "ooopppss" if the test shows the problem, but even then, on such motors the leads for both fields and commutator brushes are often accessible, so you can usually make your fix quite easily!!

Don't worry!!