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Topic: Which method should i use to control motor's direction and state? (Read 3915 times) previous topic - next topic

GHarrington44

I am trying to control a 12V  10A DC motor so that it can start, stop, go backward, and go forward. Originally I was going to use a 12V relay with an 25A external power supply since the arduino current is not large enough. I was going to use an H bridge to control the direction of the motor.

I was reading somewhere if I combine a double pole double throw relay (DPDT) with a single pole single throw relay (SPST) then I can control the direction of the motor and also if it is on or off. I will not be caring about the speed so I will not need to use a PWM signal.

What is easier for me to do: use an SPST relay with an H bridge to control the motors direction and its state, or should I use a DPDT in combination with an SPST like I already stated?

And also, about the transistors I need, are those already built into the relays or do I have to buy those?

GHarrington44

Also, which method is cheaper? I am kind of on a budget

CrossRoads

Circuit like this with SPST relay replacing the transistor would work.

I've not controlled that much power thru relays, I'll defer to others on what kind of protection might be needed to prevent the contacts from welding themselves closed.

Cost? I don't know.
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zoomkat

For heavy DC currents, you might want to look at using car starter relays.
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GHarrington44

@zoomkat

Are you saying i shouldnt use SPDT/DPDT relays? Wouldnt a car starter relay be one of those?

zoomkat


@zoomkat Are you saying i shouldnt use SPDT/DPDT relays? Wouldnt a car starter relay be one of those?


I'm saying that you need to use a relay with contacts that can handle your ~high DC current. Auto starter relays are probably SPST relays. If you can find DPDT relays at a reasonable price that are rated for your current requirements, then they would be a simpler setup.

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dc42

A slightly simpler solution is to use two SPDT relays configured as a H-bridge. A side effect of this arrangement is that when the motor stops, it is shorted, which makes it stop faster (assuming it is a permanent magnet motor).
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GHarrington44

@dc42 You seem to be the most up to date on my lack of electronics knowledge as you answer my questions wherever I go haha.

I was initially going to buy an already configured H bridge instead of making one myself to control the motor's direction and then I was going to use that with a relay (I'm assuming SPST) to be able to switch on the external power supply needed for the motor. You think it would be easier to configure two SPDT relays as an H bridge instead? Would that activate the power supply and control the motor's direction? I need to be able to do both.

@Zoomkat, Are you saying if I can find a reasonably priced DPDT relay then I should only use one to get both of my tasks done? Or will I have to buy 2 DPDT relays (one to control direction and one to activate the external power supply)

Thanks for all the help guys. I am sorry if I am a little slow trying to comprehend your advice but I really do appreciate all the help.

zoomkat

Quote
@Zoomkat, Are you saying if I can find a reasonably priced DPDT relay then I should only use one to get both of my tasks done? Or will I have to buy 2 DPDT relays (one to control direction and one to activate the external power supply)


I think the real issue may be finding the appropriate relays at a reasonable price. 10a at 12v might be handled by some medium sized relays. The below SPDT auto relay probably would work . Below is a motor reversing setup that uses two SPDT relays.

http://www.parts-express.com/pe/showdetl.cfm?partnumber=330-073&utm_source=google&utm_medium=cpc&utm_campaign=pla

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keeper63


@dc42 You seem to be the most up to date on my lack of electronics knowledge as you answer my questions wherever I go haha.

I was initially going to buy an already configured H bridge instead of making one myself to control the motor's direction and then I was going to use that with a relay (I'm assuming SPST) to be able to switch on the external power supply needed for the motor. You think it would be easier to configure two SPDT relays as an H bridge instead? Would that activate the power supply and control the motor's direction? I need to be able to do both.


You wire the SPDT relay so that moving contact of each relay is connected to a terminal on the motor; then the "upper" NC contacts of each relay (when it isn't energized) go to the positive rail of your supply, and the "lower" NO contacts go to the ground rail of the supply. Activate only one relay at a time; when the relays are both activated (or deactivated), this acts as a "brake" (because the back EMF of the motor is running across a dead short, bringing the motor to a halt quickly). The upside is you have a built in brake, and few parts needed to implement - the downside is there is no "freewheeling" mode (if that is something that is needed).

This diagram shows what you want to build:

http://letsmakerobots.com/files/userpics/u15005/MotorReverseTransistorRelay.png

SPDT relays with the appropriate current requirements are easy to find - they are available at any auto parts store, and are called "Bosch relays" - you'll want one that can control at least 15-20 amps; just go for the 25 or 40 amp "common" version. It is also possible to find a dual "reversing" relay for automotive use, which are essentially two SPDT relays in one package; they are fairly easy to use, but you may end up paying more for one of those than two standard relays (plus, the standard relays are easier to find in general).


@Zoomkat, Are you saying if I can find a reasonably priced DPDT relay then I should only use one to get both of my tasks done? Or will I have to buy 2 DPDT relays (one to control direction and one to activate the external power supply)


A DPDT relay, by itself, can be used to control the direction of the motor, but not whether it is on or off; you would need an extra relay (either between the positive voltage of the supply and the relay and motor, or on the ground side) to control current flow; it could be a simple a simple SPST relay. In such a circuit, you would have a freewheel mode - but no brake mode, the exact opposite of the SPDT circuit.

To get both brake mode and freewheeling mode with a relay h-bridge, you would need to use four SPST relays, like in the following diagram:

http://kartikmohta.com/tech/hbridge/hb3.png

...of course, if the relays aren't set right, this can allow for "shoot-thru" (presenting a direct short across your power supply), which can be very dangerous (smoke, fire, explosion of battery, etc).

Also - when you use relay-based h-bridges, implementing PWM control can be a challenge (one way is to put an NPN bipolar transistor or N-channel MOSFET between the bridge and the ground rail with the same current handling capability, and control PWM with that).
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