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very simple motor control

I am a software guy who easily gets over his head in circuits. When it comes to controlling motors, I like the ESC because I understand how to use it but it adds a lot of cost. I look into H bridges and most seem overly complicated. This little circuit seems simple anough to understand and implement:


So what's the downside? This is not a hypothetical question. I bought a boat on closeout that should be here tomorrow and it is that "middle grade" of RC - it has an all in one receiver driver board, but has "real" 380 motors, decent enough prop linkage and enough volume to do some USV testing. Possible ways to drive it are to figure out where to tap in to the existing board, get one of the dirt cheap import ESCs and hope it doesn't catch fire or make a circuit to run it. I am not planning on running it at high speed, but more than slow trolling.

I ordered a couple of these:


just as an afterthought while I was ordering a 1280 MiniMega, which you may still see on special there. Did I waste $2? Won't be the first time...

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The thing called "board" in your schematic is a microcontroller, not a rc receiver. The micro controller can output a PWM signal of any frequency and duty cycle. This is what controls the speed of the motor. The RC receiver outputs little PWM pulses that a servo can read and "translate" whereas a regular motor can not. In terms of the MOSFETS. These can be some tricky things to work with. 


Are you planning on having a microcontroller on-board to drive the MOSFET? As far as my Google-fu can help me the '380' motors you're referring to are brushed DC, so I'm going to roll with the assumption that they are, and that there's going to be a micro nearby to control the motor driver, for the rest of this post...

Well, you won't have a BEC included, although it's not like you can't just install a discrete regulator if you in fact need one. Apart from that the ESC does SFA (another nice abbrv. for ya) when controlling a brushed motor.

Are you just driving the prop motor forward all the time? If so, the fan controller circuit you posted is totally fine. The Vgs (Gate to Source Threshold Voltage) required to turn on the MOSFETs you bought is only 2.5V at most, so as long as your 1280 is applying at least that voltage to the gate of the MOSFET through the gate resistor the MOSFET will turn on.
Instead of varying the pulse width from 1ms to 2ms for the full speed range like you would for a servo-style ESC, you'll be more likely to work directly with the duty cycle of your PWM output. Pick a nice frequency for the PWM generator (the optimum varies with motor and driver attributes, I like to start off high and work my way down), set the duty cycle to maybe 50% and start adjusting the PWM frequency until you get the motor running quietly and efficiently .A simple tachometer comes in handy for tuning, but you can do it by eye/ear too.
Once the PWM frequency is tuned, you just have to vary the PWM duty cycle to control the speed... 0% for 0% power, 100% for 100% power. Too easy.
You may need to limit the max PWM duty cycle if your motor is too power hungry for your batteries/MOSFET... or just get more/bigger batteries/MOSFETs.

If you want to run the motor forwards and backwards you'll need at least 4 MOSFETs in the classic H-Bridge config. Either that or use a relay/etc to flip the motor contacts and reverse polarity. I'll go into more detail on those options if you actually need them.

Lastly the fan circuit shows a discrete diode across the motor terminals, plus another 'body diode' which is incorporated in the MOSFET (your MOSFET has one too). I've often just made use of the body diode to protect the MOSFET, but with a bigger, noisier motor the discrete diode would not be a bad idea. As you're already aware, caps will help if you find the circuit too noisy for other components.

yours came in while I was typing. I was way too low on details, though maybe not since you answered with a qualified yes. I don't care about backing up. 

I have a brushed ESC on my AGV, so I have some experience with that end of things. I got a decent deal on an old chassis with motor/ESC to get that. it was not an educated conscious decision, just good luck.

Forgot to answer that part...

The boat is a 7.2v and will feed VIN nicely for the uC boards I am considering. Since you mentioned my cap experience, you know that BECs have lost a little of their luster from my POV...

Sounds like you're on the right track then.

When your boat turns up I suggest you grab a multimeter and blast the motor with a fully charged battery - see how much you can expect the motor to draw when running freely. Then disconnect the motor and measure the minimum winding resistance with your multi, that way you can also calc the worst-case stall current draw.
From there you can work out the max. heat dissipation that the MOSFET will be taxed with, max. drain on the batteries, etc etc.

I tried to think of a way to phrase that without sounding like an idiot, but well, yeah...

I use my meter to check voltage, continuity and resistance (meaning to double check and often correct my band reading to get the right resistor; nothing fancy). That gets you pretty far these days, but I think I just stepped off the end of the pier and would appreciate some swimming instructions...

First off, has your meter got a current option?
You'll probably want to dial it up to the max. in this case, assuming you have different current metering range options on there. All you have to do is throw the meter into the circuit in series with the current loop you want to measure. Note that this is different to measuring voltage or resistance, where the leads of the meter are in parallel with the target. If you only have two leads and they're fixed it's as easy as that. If your meter has several leads, or several ports for the leads, you may have to change them around for current metering mode. Hopefully your meter has a manual, if not the at worst you can post a photo and we'll decipher it.
To measure the motor current you'll only have one loop, so the setup is easy: battery terminal -> motor terminal, other motor terminal -> meter lead, other meter lead -> other battery terminal.
Which part goes in which place, and what polarity you add it in, do not matter in this example. Current from the battery flows through the motor, and that same current flows through the meter, so the meter will read the same current as what the motor is drawing from the battery. You might get a negative current reading but of course here we just need to know the magnitude.

If you don't have a current-capable meter you can slap a high-power, low-resistance resistor (sometimes known as a 'shunt' for these applications) in series with the motor. By reading the voltage across the resistor with your meter, can can calc the current through it, and therefore through the motor, since you know what the resistance is ahead of time: V= IR therefore I = V/R.

Your wish:

I am guessing/hoping it is that big range on the bottom right of the picture...

Your DCA (DC Ammeter) setting only covers 200μA - 200mA, but you've also got a 10A setting down the bottom-left there which might be enough. According to the legend on the lead ports, you need to move the red/positive lead into the top spot to use the 10A DC ammeter setting.

Just in case you'll be drawing more than the rated 10A it'd be best to measure the winding resistance and estimating the motor stall current before you fire up the motor with the ammeter in the loop, otherwise you could end up with no current reading but plenty of smoke =)
If the calculated stall current is a bit greater than 10A that's alright - the motor will draw close to the stall current for a fraction of a second at startup but after that it'll drop away considerably, so the ammeter should handle it without issue. If the stall current is really large however (greater than 20A) you may want to consider using either a heftier multi-meter or the shunt resistor method I mentioned earlier.

So I Googled. It seems that 380s are generally rated at 3A and below. I saw one post recommending that you limit the available power to 9A to keep them from overheating. These are RC guys trying to win races. They want it to barely not explode. I want to hold back a little more than that. I want to make decent speed with good maneuverability but still let the motor have a long and happy life.

FYI, it is a boat exactly like this one:



and I would be quite happy with the speeds that guy is getting. Even a little less than top speed would be fine. He seems to be timid about keeping it a high speed. You just get a hint of it. I noticed the comment from someone that burned one up and went to his page. He has a video of Barbie being pulled on a tube behind the boat. I suspect that sort of activity may have had something to do with its demise. 

There are probably better platforms out there, but this was being closed out at $11.68 delivered.