Lowering voltage to a motor

[Wrenow]

This question came up in another forum and got me pondering.

You have a 5v system.
You have a motor rated at 3V max (max draw at stall is between 1-2A).

You want to drive the motor both forwards and reverse within your 5v system, but deliver only 3v to the motor.

If it were unidirectional, I could see several simple fixes Like powering it through a diode or two in series. But that does not work for going the other direction. Perhaps an H-bridge or similar arrangement would provide a similar drop, but allow bi-directionality?

I also though of running off couple of resistors set up as a voltage divider, but if you run the resistors across V+ and V- and run the motor off tap and V-, then in the other direction, you would be running the motor off tap and V+. I am thinking that would not work.

Any other thoughts out there?

Cheers,

Wreno

 

[BeanieBots]

You can diode drop AND have bi-direction.
Two ways of doing it.
1) Diode in series with supply, then relay to reverse.
2) Back-to-back diodes in series with motor.

A typical H-Bridge will lose a volt or two anyway.
A darlington driver will drop 1.2v

I'd go for darlington driver (allowing PWM control) plus series diode with relay to reverse.
That would drop about 2v and still give you bi-direction but keep an eye on the power dissipation in the darlington.

 

[boriz]

Or you could just PWM the power to the motors, making sure that the duty% never get's above the appropriate proportional fraction. IE: 3v motor from 6v = 50% max PWM. That’s what I do. And it’s more efficient than series diodes.

The only drawback is: What happens if the Picaxe crashes leaving the PWM output high. But there’s ways around that.

 

[westaust55]

You can diode drop AND have bi-direction.
Two ways of doing it.
1) Diode in series with supply, then relay to reverse.
2) Back-to-back diodes in series with motor.

A typical H-Bridge will lose a volt or two anyway.
A darlington driver will drop 1.2v

I'd go for darlington driver (allowing PWM control) plus series diode with relay to reverse.
That would drop about 2v and still give you bi-direction but keep an eye on the power dissipation in the darlington.

Think that back to back might be better expressed as diodes in parallel but opposing.

Back to back
    | /|    |\ |
    |/ |    | \|
----|  |----|  |----
    |\ |    | /|
    | \|    |/ |

 

[Wrenow]

OK, a little more info is probably required.

I forgot to mention that he is planning on using a hobby ESC (electronic speed controller), so there is no requirement to come up with your own PWM etc. The issue then becomes that you need to limit the ESC output to the motor's limits.The ESC will be controlled by standard RC Hhobby servo type signals which command direction and speed).

Hence, a potentially 6v source, with reversing polarity, that needs to feed the 3v motor's inputs.

As for putting diodes back-to-back - thought the diode prevented flow of electricity in one direction, kind of like a check-valve? if you put check-valves back to back, than nothing will flow in either direction. Is this not the same with Diodes (unless you overwhelm them, of course)?

Cheers,

Wreno

 

[BeanieBots]

"back to back" is the 'common' term used for such an arrangement.
However, I can see how that might cause confusion.

 

[westaust55]

Guess I am more used to "back to back"
with respect to capacitors, for example to create a very large bi-polar cap.

a simple case: http://www.siliconchip.com.au/cms/A_101869/article.html

Then again, for "back to back diodes": see figure 5 here: http://www.elexp.com/t_test.htm
and elsewhere also for back to back diodes we have as BB suggests (see figure 2) at: http://www.analog.com/library/analogDialogue/archives/41-05/input_protection.html

So not as clear as one might think

 

[QuIcK]

a 3v regulator? 3.3v is a pretty common voltage, if you could tollerate that extra 300mv

 

[Wrenow]

Hmmm. In re-reading Westaust55's earlier post about the diodes, he indicated "parallel but opposing", while the diagram showed serial and opposing. I am thinking parallel and opposing would, indeed, be the simple solution.

As for a voltage regulator, remember, the power supply can change polarity (to reverse the motor), so a regulator would only work if it were not polarity sensitive (don't happen to know of any that can work both ways around, offhand).

"back-to-back" to me would have indicated witing them in reverse polarity to each other, but in serial. Glad I asked the question, as I have learned several things that may prove useful in the future.

Thanks,

Wreno

 

[BeanieBots]

To clarify:

"back-to-back" when used to describe the connection of diodes means connected in parallel such that conduction can take place in both directions with a single diode volt-drop in either direction.

It is a common term in the analogue electronics field and used for many purposes.

The regulator suggestion is valid. Simply use it as the SUPPLY voltage for whatever you use to drive the motor. eg, the H-bridge.

The simplest bi-directional speed controlled motor drive is a single transistor for PWM and a relay to reverse. Such an arrangement would only require diode(s) in series with the supply to drop multiples of 0.6v.

An H-Bridge such as the L293D will drop about 2v anyway.
A darlington driver such as the ULN2003 will drop about 1.2v.
The 'normal' problem is how to bi-directionally drive a 5v motor from a 5v supply!

 

[Wrenow]

BB, the problem with using the regulator on the supply is that it lowers the voltage to the ESC. Since, many people use a BEC (Battery Elimination Circuit) in the EXC to power the RX (Receiver), servos, etc. If you reduce the voltage to the ESC supply below 438v (5.5-6v on some), some no longer operate, and the RX and servos may not operate either. I have several RXs that will not operate below 3.8?v.

Thus, to make sure everything operates within specs, you need to operrate the supply side of the ESC at 4.8v-6v or more (many ESCs prefer 7.2v to give enough overhead). In fact, most of my equipment is set up for a 7.2v supply, which the BEC in the ESC drops to 5v for the radio and servos.

Thus the question on dropping the voltage on the "downhill" side of the ESC (the side that supplies the motor), which is already a PWM which reverses polarity and which can, at full throttle, supply the full voltage given at the input.

Cheers,

Wreno

 

[BeanieBots]

Ah, these little bits of info that come out of the woodwork!
I was under the impression you had scope to manouver here.

If it's a bi-directional RC ESC, then your ONLY option is to lose volts by putting something in series between the ESC and the motor.

Get yourself a full-wave bridge rectifier.
Connect it between ESC and motor using the AC pins.
Short together the + and - pins.
Instant bi-directional 1.2v drop.
Remove the +/- short and fit a single diode (Cathode to -) between the + and - to get a further 0.6v drop.