Question One:
How the hell do we provide enough power to them?
Stall Current on these things is about 5 Amp each.
So I figured we needed a power supply that could provide 10Amp,
at 7.2 Volt (or something around that).
Our plan was to use a ring transformator, a rectifier brige and some capacitors to built a unregulator power supply for this thing.
It seems very hard to get those parts... (Transformator at 6V, 10Amp is not so easy to get here appearantly...)
Does anybody have cheap, simple alternatives for powering those suckers? 10 Amperes seems like an unreachable goal here
Then, controlling them.
We tought about using MOSFETs, an IRF520 and just sending a PWM signal from the Arduino to control them...
Does that seems like a decent idea?
If anybody has input on this, especially the power supply at this point, i would be glad to hear it.
Well the motor stall current rating is a worst case condition for a motor, called locked rotor, and shouldn't be a normal situation. Your motors might only draw half that in most normal cases, but it really depends on the mechanical demand placed on the motors. You could use a transformer with a lower current capacity and fuse it at the transformers max current rating.
The fact you are considering an AC transformer says you don't need portability from AC mains. Batteries (Li-po 2 cell) would have no problem handling 10 amp loads depending on the mah rating of the cells, however their costs plus charging equipment will exceed the cost of a AC transformer/rectifier/filter. So unless you require portability your first plan is more economical.
A N-channel MOSFET is the best switch device for this kind of application. However it's made easier if you use a logic level N-channel MOSFET as the Arduino can only supply +5vdc as a turn on voltage and a normal MOSFET requires 10vdc to fully turn on. Also the MOSFET should be rated at several times the worst case current draw, so 25+ amp ratings would not be overkill.
Here is an example of a nice logic level high current mosfet:
The IRF520 is NOT a logic level MOSFET, an Arduino output pin cannot fully turn it on without additional external components. That's why I stated using a LOGIC LEVEL N-channel mosfet is easier to interface with an Arduino.
OK being in EU. Transformers are sometimes much cheaper if you can find surplus dealers, rather then full retail new stock.
Are the motors to be single direction or bidirectional? If bidirectional the parallax link list the motor controller that works with the motors. If you need a strong power supply, then something like below might be of more interest than buying seperate components. You may want to look at the surplus electronics sites for power supplys that can supply the motor needs.
Ok, but as a "motor driver", the MOSFET should be enough, right?
The thing i'm stuck at now is where i'm going to get those 10Amps.
Thanks for the superquick replies guys!
No, its not beefy enough. You need to chose MOSFETs by the Ron rating - the IRF520 has 0.27 ohm rating, so at 5A it's dropping 1.35V (about 20% of your supply!) and dissipating about 7W - at the very least you'd need largish heatsink. I'd suggest going for a device with Ron less than 0.05 ohms, and with logic-level switching.
The gate threshold spec is not the spec to go by. One really has to look at the graphs showing source/drain current flow Vs gate voltage when the gate is at the 4.5-5vdc that an Arduino can supply. True logic level MOSFETS will have a gate threshold voltage as low as 1.5vdc or so. Gate threshold spec is the gate voltage where the mosfet just starts to conduct, and not it's fully saturated gate voltage.
I was trying to generate a list of plausible parts because Farnell's search is rubbish, but Farnell have outlets in most countries, so it is a useful reference. The idea was to illustrate how low-cost a solution might be.
(and RS are broken at the moment)
[edit]I apologise, that Farnell search would have confused, and not helped.
[/edit]
WHY don't people put their location into their profile? It would save a lot of time when trying to respond to someone appropriately for their location?
[edit]I'm okay with country, but otherwise agree. I prefer to give helpful answers if I can, and knowing the country helps identify electronic parts.[/edit]
