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16  Using Arduino / General Electronics / Re: How can I find out why a small DC motor won't work? on: April 06, 2014, 01:37:51 pm
I measured the resistance across the motor terminals, and it is over 100k Ohms.

I have also removed the motor from the gearbox and the motor driver and hooked it up to a 5 Volts power supply, and it does not move at all.  But I was wrong when I said that there was no voltage across the terminals when I hooked it up to the 5 Volts power supply. There is a voltage. I was touching my meter probes to a solder blob on each terminal, not realizing that the solder blobs were not making a connection with the probes.
17  Using Arduino / General Electronics / How can I find out why a small DC motor won't work? on: April 06, 2014, 01:42:06 am
I bought a broken remote control car that I am re-architecting and trying to get to work. The steering for the car is done by a small DC motor. Although the motor is tiny, it runs off the 9.6 Volt battery that powers the rear wheels as well.

The motor worked fine for a couple of months, but now does not turn on at all. It goes through a gear box that actually does the steering. The design of the steering is a little strange. The motor's either full on right or left, or it's off in the center.

So to turn right, for example, the motor turns until it is stopped by the mechanical resistance of the wheel bumping against a block, and the motor then holds that position until the current is turned off. A spring then pulls the wheels back to center. So I would guess that the motor is stalled the whole time it is holding the wheels in a turned position, and pulling the stall current for the motor.

My guess is that the motor burned out because of pulling stall current for too long a time, but how could I check that? If I hook it up to a power supply and check for voltage across the two motor terminals, I get nothing. If I check for continuity across the two motor terminals when no power is being applied, it is an open circuit. That's all I could think of to test.

A visual check reveals nothing unusual. The two motor terminals are attached to a small PCB board that seems to have a few surface mount resistors on it. I have not taken the motor apart.

I cannot see any writing on the motor. I can take a picture of it if that would help.

Any suggestions would be appreciated.
18  Using Arduino / General Electronics / Re: Need help reading a power MOSFET datasheet on: November 07, 2013, 08:08:23 pm

Thanks for the suggestions, dc42. Those do look much better.

What bridge topology are you considering, I'm suspicious you think only one MOSFET
is needed per winding, which would be unipolar drive, which is unusual outside of
low speed stepper motors.

MarkT, I'm embarrassed to say that I forgot all about the bridge topology. I will need current to flow both ways in the winding, so I guess I will need to do a full H-bridge.

What I would really like to do is find some off-the-shelf motor drivers that I could buy to test the concept. Something like the Pololu High-Power Motor Driver 36v20 CS, shown at www.pololu.com/product/1457. But Pololu said that the 50 Volt maximum is an absolute hard limit, so using a battery 48 Volt supply would not work. Also they said even with heatsink the maximum current is about 30 Amps.

[Interestingly, Pololu uses a IPD048N06 MOSFET on that board, and the MOSFET's datasheet says it can handle 60 Volts and 90 Amps: www.pololu.com/file/0J388/IPD048N06L3_Rev2.0.pdf. According to Pololu, the 50 Volt limit comes from the MOSFET driver on the board. But I can't tell which chip that is.]

Pololu was unaware of any motor drivers like theirs that could handle my 48 Volt, 42 Amp requirement. If anyone knows of one, please let me know. Otherwise, I guess I'm on my own.

19  Using Arduino / General Electronics / Re: Need help reading a power MOSFET datasheet on: November 07, 2013, 02:33:49 pm
Thanks to everyone for the information. Your comments helped me better understand some of the things I was reading. That helps a lot.

A few more details about the project. I read about a permanent magnet motor with several stator poles that are electrically independent and each pole powered by its own driver. I'd like to build one and test the concept. I may be biting off more than I can chew, but I figure I might as well bite first and then try to chew.

To that end, I'm looking at using just a MOSFET to control each stator pole. I do plan to use PWM to drive the MOSFET. My tentative design has 17 poles on the stator and 19 permanent magnets on the rotor. Each pole will be powered by the same 48-Volt power supply, and draw up to 42 Amps for a maximum power of 2 kW. But the test-of-concept motor I actually build will probably just have 3 poles and 5 magnets. 

As I say, I picked out the RFP50N06 as a possibility. Any other suggestions? And any suggestions as to a driver chip or circuit to switch the MOSFET? It seems like Microchip and others sell chips that are specifically designed to drive MOSFETS, but I could not find one that seemed like it would work with the RFP50N06.
20  Using Arduino / General Electronics / Need help reading a power MOSFET datasheet on: November 07, 2013, 01:52:19 am
I'm looking for a power MOSFET that will handle 48 Volts and about 42 Amps continuous. The Fairchild RFP50N06 seems like it will work. The datasheet is here: www.fairchildsemi.com/ds/RF/RFP50N06.pdf

Two questions:

(1) What does the power dissipation max mean? It says 131 Watts, but I'm not sure what that means.

(2) It looks like the switching voltage maximum and minimum are + and - 20 Volts, but the threshold max is 4 Volts. Does that mean I can use an Arduino output to drive it, with 0 Volts turning it off and 5 Volts turning it on?

Thank you.

21  Using Arduino / General Electronics / Re: Trouble soldering onto potentiometer tabs on: August 11, 2012, 07:38:10 pm
Thanks for the suggestions. I tried scraping the tabs, but there is not much there to scrap. Plus there seems to be no oxidation or anything like that. I can't tell what type of material they are made out of, but whatever it is, the material seems to hate solder. Solder just won't stick.

I have not had a chance to try flux yet. I hope to do that soon.
22  Using Arduino / General Electronics / Trouble soldering onto potentiometer tabs on: August 10, 2012, 05:09:51 pm
I bought a small joystick that has two potentiometers attached to it. I had to solder wires onto the 3 tabs on each potentiometer, and had a devil of a time getting the solder to stick to the tabs.

I'm not the most skilled person with a soldering iron, and my iron is a poor one. Still, I have never had a problem like this. The solder sticks to the copper wires that I put through holes in the tabs and then twist to tighten. But the solder does not stick at all to the tabs.

I finally managed to get a workable join on each tab. But is there something I can do in such cases in the future? Does soldering flux help?
23  Using Arduino / Project Guidance / Re: powering the Arduino Uno with a 12V battery on: August 04, 2012, 03:16:20 pm
As CrossRoads says, that will make the voltage regulator on your Arduino run warm. Especially if you are pulling enough current to run two servos. Better to use a 5 Volt source, like the USB output that CrossRoads recommended. They are more efficient, and will save your Arduino from the heat.
24  Using Arduino / General Electronics / Re: Problem using a voltage divider to input a voltage to a low impedance line on: July 30, 2012, 02:00:37 pm
Thanks, Grumpy Mike. That did the trick! I wired up the second circuit, and it looks like it will work. Unfortunately, I cannot test the circuit on the device itself for a couple of weeks. I'll post back then with final results.
25  Using Arduino / General Electronics / Re: Problem using a voltage divider to input a voltage to a low impedance line on: July 30, 2012, 12:17:57 pm
Quote
I can't do that because I only have one wire that goes to my "load." Not two.
No you always have two wires to connect to the load otherwise you can't get any current to flow.
Your other "hidden" wire is either the ground or the +ve power supply.
What do you need to do wit this wire to turn it on. If it is supply +ve then the ground is your other control wire.

You're right, of course. The "hidden" wire in my case is the ground. What I meant, and should have said, is that I cannot connect my load in series between the 12 Volt power supply and the collector of the transistor because the ground wire must be connected directly to the battery's negative terminal. I cannot control the ground wire.

My device has three wires going into it. (Actually, five wires, but two of them are not important here.) One wire is 12 Volt power, and needs to be connected to the battery's positive terminal. One wire is ground, and needs to be connected to the battery's negative terminal. The third wire is the signal wire, which needs to vary between 0 Volts and 12 Volts.
26  Using Arduino / General Electronics / Re: Problem using a voltage divider to input a voltage to a low impedance line on: July 30, 2012, 11:16:18 am
You seem to be making hard work out of this project.

I certainly am making hard work out of this project, aren't I. Two or three times, I thought I had it solved, only to have it not work and have to go back to the drawing board.

You can drive an open collector transistor or FET from the arduino. Your 50ohms load is connected between your 12 volt supply rail and the collector of the transistor.

I can't do that because I only have one wire that goes to my "load." Not two.

If there's a datasheet for the device it would be really re-assuring to see it - I don't know how such a thing works and what sort of frequencies of PWM are suitable.


There is no datasheet. There is a manual: www.championtrailers.com/BRAKERITEMANUAL.pdf The wiring diagram is on page 19 of the manual. I called up Titan and they said that PWM will work fine, at frequencies from 200 Hertz to 20 kilohertz.

Also automotive devices often are permanently connected to the chassis/ground and can only be switched on the high-side - if this is so you'll need a PNP or p-channel device to switch and some kind of level-shifting circuit to control this from the Arduino.

I think that is the situation here. The device is wired to ground and I just have one signal wire that I need to work with. I need to source a signal between 0 Volts and 12 Volts using the Arduino, at currents up to about 240 milliamps. At least that is what I have come up with so far.
27  Using Arduino / General Electronics / Re: Problem using a voltage divider to input a voltage to a low impedance line on: July 29, 2012, 10:08:41 pm
Most op-amp aren't designed to supply much output current, 35ma is probably pretty typical. Most circuits needing more current drive from an op-amp stage just wire in a transistor as a voltage follower that can supply higher output current. There are special op-amps designed with higher output current capability but you will pay a premium for it compared to more standard op-amps.

Lefty


Yes, it looks like an op amp is not going to help here. I need to send a voltage signal to a low-impedance device (I think it's about 50 Ohms) that ranges between 0 Volts and 12 Volts. That means the current will range between 0 Amps and 240 milliamps. Well outside the range of any op amps I can find.

Some people have had some good ideas here. I'll wait until the stuff I've ordered comes, try it out, see if anything works, and post again.

Thanks for the help.
28  Using Arduino / General Electronics / Re: Problem using a voltage divider to input a voltage to a low impedance line on: July 29, 2012, 01:29:00 pm
That is normal behavior for many op-amps (esp older types) unless you obtain one that is specified to have 'rail to rail' output voltage capability. So check the datasheet for your specific op-amp type to see what it's output voltage swing spec is.

Lefty

Ah, thought it was good, but looked closer and its not rail to rail and also only has 35ma output max so doesn't help much

Oh well, guess I'll have to get a different op amp. At least I got the op amp I have out and gave it a try, and learned something from it.

I'll order a few chips and try a few things. Like the two chips Magician suggested. And a TI chip I found that might work. And I decided to get a commercial in-cab brake controller after all to take apart and see how they built it.

Any other suggestions very welcome. Thanks for all the help so far.
29  Using Arduino / General Electronics / Re: Problem using a voltage divider to input a voltage to a low impedance line on: July 28, 2012, 09:58:21 pm
I must be doing something wrong. I wired up the op amp as a voltage follower, and used it with the potentiometer wired up as a voltage divider. I get from 0 Volts to 5 Volts coming out of the potentiometer into the + input of the op amp.

But I get .3 Volts to 4.3 Volts coming out of the op amp when I leave the output floating. When I hook up the output of the op amp to the low impedance device input, I get only .3 Volts to 2.5 Volts output.

UPDATE: I should clarify that I am just using a + 5 Volts power source for now. So the 0 to 5 Volts that comes out of the potentiometer is correct. But the output of the op amp has a reduced range, although it seems to vary correctly within that range, smoothly without any jumps.
30  Using Arduino / General Electronics / Re: Problem using a voltage divider to input a voltage to a low impedance line on: July 28, 2012, 03:37:17 pm
How about an opamp buffer? Just wired as a voltage follower so you'll have 1M impedance to the opamp and depending which one you pick you will have a decent enough voltage source

That sounds it will work, thanks. The only problem being that I know less about op amps than I do about transistors. But I'll look into it. I even have an op amp, a OPA277P, but the chances that one will work seem small.
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