Connecting 12v dc motor to a rc circuit car

[kiti]

hey i need help im currently working on a personal project, i want to connect a pair of 12 dc motors to a rc controlled circuit.
The point of doing this is to have more torque output as the given motors in the toy wasn't enough to produce the desired torque. the question is. do i have to make changes to the circuit so it is able to run the dc motors without any restrictions to the power output due to the circuit?.

given data:
2 dc motor
minimum of 8 amps needed per motor

thanks.
kiti

 

[KrisBlueNZ]

Yes! If the original circuit was designed to drive a less powerful motor, and the new motors you have need 8A each, you'll need to make some changes to the drive circuit.

Start by posting details of brands, model number etc for the RC receiver and the motors, and upload some photos of both. See https://www.electronicspoint.com/threads/how-to-take-photos-of-circuit-boards.267883/

 

[kiti]

Hey Kris thanks for that info about posting up the circuit. i have posted the circuits with this posting hope will this be good enough

 

[davenn]

1) you have a battery that is capable of more than 16A that will last for more than a few minutes that
is small enough to fit onto that small car ?

2) those 2 rows of small transistors along the bottom of the board in pic 1 and 3 are the motor driver transistors
They most likely wont handle 8A ... the circuit will require some significant modification/addition to for it to be able
to handle the extra current required

Dave

 

[KrisBlueNZ]

Yes, as Dave says, it's not just the circuit that will need to handle a higher current - you'll need a pretty big battery too.

I guess you want to use this receiver with a different car? One with bigger motors and a bigger battery?

Those 12 motor driver transistors will not be able to handle 8A but you can make an external buffer board that is driven by the outputs from the board, and drives the larger motors.

The buffer board could also drive the motors with a higher voltage than the original board uses.

What is the supply voltage at the moment?

 

[kiti]

thanks davenn and KrisBlueNZ for the insight. Basically im just using the circuit to drive 2x12v motors. even though i may require more voltage to drive the motors, I want to increase the current that flows to the motor. since it application would be used for torque. After doing alot of reading. I've come to know that as voltage increases, so does the rpm and torque increases as current does. so therefore torque and rpm are inversely proportional. With the motor i have im not putting much emphasis on the voltage but the current that the circuit can take. Hence the 8 amps, oh and the rc circuits voltage is 4 1.5v batteries connected in series which i assume would be around 6v input to the circuit.
Anyway with your given insights do i have to switch the transistors to bigger ones? so comparing the two solutions, which would be easier for a beginner like me;. going with the buffer board or making changes on the circuit.

thanks

 

[KrisBlueNZ]

After doing alot of reading. I've come to know that as voltage increases, so does the rpm and torque increases as current does. so therefore torque and rpm are inversely proportional.

That doesn't necessarily follow.

I think you would be best to decide on your requirements then work out how best to meet them.

DC motors have an optimum operating point where input power (voltage x current) is most efficiently converted into output power, and you should be aiming for that point. That might mean using gearing. This should allow you to choose the operating voltage for the motor. I have very little knowledge of this. No doubt there are others who can help. Perhaps duke37?

the rc circuits voltage is 4 1.5v batteries connected in series which i assume would be around 6v input to the circuit.

Right.

Anyway with your given insights do i have to switch the transistors to bigger ones? so comparing the two solutions, which would be easier for a beginner like me;. going with the buffer board or making changes on the circuit.

If you need to run the motor at a voltage that's different from the 6V that the RC board is using, then you will probably need an external buffer. An external buffer is also simpler because you don't need to reverse-engineer the board (reconstruct the circuit diagram) and figure out how it's designed.

I would like to reverse-engineer the board anyway, because I'm interested and because I could be more confident in designing a buffer board (or changing the components, if that's feasible). But I would need some better photos, and I would need to know the markings on the transistors, and what each wire connects to, to be able to start reverse-engineering it.

I can design a buffer board without that information, but without reverse-engineering the board, I can't tell whether you will be able to replace some components to get a higher output power or whether an external buffer will be needed.

Do you need the motor speed to be variable? Or is just forwards/backwards at full speed enough?

 

[kiti]

Do you need the motor speed to be variable? Or is just forwards/backwards at full speed enough?

speed would be fixed at fullspeed forward/backwards. so do you need further pics or we going with buffer board?

 

[duke37]

Kris is throwing me in the deep end again !

First, you need to know the type of motor that you have. The most common for this application would be a permanent magnet field with carbon brushes to feed the armature.
This motor,if turned, will act as a generator with an output voltage proportional to speed. If it is connected to a power supply at this voltage, nothing happens, no current flows and only the original power necessary to turn the motor is consumed.
Now, slow the motor a little by adding a retarding torque, the motor produces less back EMF so current is taken from the power supply, limited by the resistance of the windings. So the torque is balanced by the current which depends on the DIFFERENCE between the applied voltage and the back EMF. It does not depend on absolute speed or applied voltage.

In your application, you will need to estimate the power required and choose a suitable motor, you could use a small fast motor and gear it down or use a slow big motor that will give the torque required directly.
Then, you need to provide a voltage source to get the required speed and of sufficient current capability to provide the torque.

You are obviously new to this so I would advise to use a separate buffer to match motor and power supply

Does this seem lke sense?

Duke.

 

[(*steve*)]

Also think of running a small DC motor from a higher voltage without changing things as similar to taking the engine out of your 4 cylinder car and replacing it with an engine from a semi trailer tractor. You're clearly going to have issues.

 

[kiti]

yes, thanks duke i think i understand now what determines the current. input voltage-emf/resistance in the winding would give me the current needed.

Also think of running a small DC motor from a higher voltage without changing things as similar to taking the engine out of your 4 cylinder car and replacing it with an engine from a semi trailer tractor. You're clearly going to have issues.

thanks for that insight steve, i know what you mean, but im not using the rc car just the circuit to drive 12v motors.
on another build im doing. i just want to control the 12v motors through remote control. like placing a ecu on the tractor to be more versatile, if i may say.

thanks

 

[(*steve*)]

OK, perhaps I am being swayed a little by some recent threads about people using higher voltage batteries in their model cars.

The best way to increase torque on a given motor is to gear it down. You say the speed is fixed, but does it have to be at the speed it runs now?

 

[KrisBlueNZ]

Thanks Duke. Sorry about that! I just didn't have much knowledge about torque, RPM, voltage and current, in relation to DC brushed motors. I looked up the Wikipedia article, and now I know a little bit more!

I've been working on a design for a buffer stage using MOSFETs. I've come up against two problems: the circuit needs quite a lot of components, and P-channel MOSFETs with low ON-resistance aren't readily available in through-hole packages (the type with wire leads that you can solder into a board).

Then I realised the simple solution: relays. Each half-H circuit on the board needs to go through a relay. If there are two motors, there should be four half-H circuits, so you'll need four relays.

The trouble with relays is that they draw significant current when activated. But here are some possibilities with 5V and 6V coils:

1. Omron G5LE: http://www.digikey.com/product-detail/en/G5LE-14 DC5/Z1011-ND/280371 USD 1.48 coil 80 mA @ 5V; switches 10A.

2. Panasonic JS: http://www.digikey.com/product-detail/en/JS1F-5V-F/255-3620-ND/3885775 USD 1.67 coil 72 mA @ 5V; switches 10A.

3. Panasonic JS: http://www.digikey.com/product-detail/en/JS1-6V-F/255-2221-ND/648574 USD 1.67 coil 60 mA @ 6V; switches 10A.

4. TE Connectivity / Schrack RT1: http://www.digikey.com/product-detail/en/RT174005/RT174005-ND/1128621 USD 4.27 coil 50 mA @ 5V; switches 10A.

5. Potter & Brumfield T75: http://www.digikey.com/product-detail/en/T75S5D112-05/PB442-ND/365877 USD 6.50 coil 43 mA @ 5V; switches 14A.

Items 4 and 5 are more expensive than the others, but have lower coil current consumption. Also, number 6 is rated to switch 14A rather than 10A for the others.

A snubber will be needed to protect the relay contacts from back EMF from the motor.

If you want to go this way, let me know, and I'll post some more information.

 

[duke37]

A two pole changeover relay will do what the normal H-bridge does i.e. change direction but will need a switch to turn power on and off. This could be done with a mosfet.
The relay would be unenergised in one direction and if this is chosen to be the most commonly used, then relay power consumption would be minimised.
A snubber would be necessary on the mosfet but not on the relay contacts if the relay is switched when there is no current. In any case, it would not be a good idea to reverse the motor when running forward, current consumption could exceed the safe limit.

 

[KrisBlueNZ]

That's not what I was suggesting. If you use two SPDT relays, one driven by each side of the existing H-bridge circuit, there's no problem. When the existing circuit drives both outputs low (or both high), both relays are in the same state and no voltage is applied to the motor.

Of course, you should connect the relay coils so that both relays are de-energised in this case, rather than energised, otherwise you would waste power when the motor is not active.

Good point about not quickly changing between forwards and reverse. The existing driver might include a delay, but if the original motor is only small, it might not!

 

[kiti]

okay i appreciate the input from you guys but i have no idea what is being said. I did little reading on H-bridge i see how it works, would the relay switches closed the desired switches for forward and backward. again sorry if i really didn't get what is being mentioned above. although i would be interested in any solution that would be beginner friendly.

The best way to increase torque on a given motor is to gear it down. You say the speed is fixed, but does it have to be at the speed it runs now?

well if i could reduce the rpm for the current speed then it would be good, as long as there is enough torque. to do this i have decided to go with pulley ratio that would reduce the rpm.

 

[davenn]

and BTW 4 x 1.5V batteries are not going to supply 8A let alone 16A

you really do need to so some serious rethinking


Dave

 

[kiti]

yes dave i do know that 4x1.5v wont be enough to supply that much current. i was planning on just using the circuit but with a different build and different input voltage.

 

[KrisBlueNZ]

I wouldn't power the RC receiver from a different voltage from the voltage it is designed for. But you can use a regulator from your 12V supply to drop the voltage down to 6V for the RC receiver.

The H-bridge motor driver is explained pretty nicely at http://www.botskool.com/tutorials/electronics/general-electronics/motor-controller-h-bridge

Read down to Fig. 3 but not the stuff about the L293 IC.

Now you should understand how an H-bridge drives a motor and can make it run in either direction.

Each half of the H-bridge produces an output voltage that is either high (positive voltage) or low (zero volts). The motor will turn if it sees a voltage difference between one side and the other; in other words, if the two halves of the H-bridge are outputting different states. If the two sides are outputting the same state (either both high or both low), the motor will not see any voltage across it, so it won't turn.

For your application, you need to boost those voltages, so instead of each side of the H-bridge producing 0V or 6V, it produces 0V or 12V.

An SPDT (single-pole double-throw) relay, also called SPCO (single-pole changeover), has a coil that is energised by voltage, and a set of contacts with three connections. See http://www.learningaboutelectronics...a-single-pole-double-throw-relay-in-a-circuit for a good simple explanation.

If you connect a relay coil between the output of one side of the H-bridge and the 0V rail, the relay will follow that side of the H-bridge. When that H-bridge output is low, the relay will see no voltage, and its COM terminal will be connected to its NC terminal. When that H-bridge output is high, the relay will see voltage across its coil, so it will energise, and its COM terminal will be connected to its NO terminal.

So you need to connect the contact to the power supply like this:

NO terminal: to +12V
NC terminal: to 0V (negative side of the 12V power supply)
COM terminal: to one side of the motor.

Then you use a second relay, connected to the other side of the H-bridge, to drive the other side of the motor. Voila! You have a buffer that can drive the motor in either direction, from a higher voltage, and is controlled by the existing outputs from the H-bridge.

Your relays must have 6V coils, because they're being driven from the H-bridge, which is powered from 6V. Their contacts must be rated for at least 8A. You should also have some kind of snubbing to protect the contacts from the back EMF from the motor. Do you understand how the relays will work?

 

[kiti]

thanks KrisBlueNz appreciate the info, reading it now.....