that's a neat link. I used 24awg and what looks like 26-30 feet.

where does the logic part stops in your schematic? right before the transistors?

 

yes right before the transistors.

The easiest would be to use the steel bar as the rotor.
the second easiest is to wrap the same amount of wire on the bolts and to have two coils on each bolt as discussed a few posts earlier. (then using the bar magnet as rotor.)

Winding 50 m of wire on each each bolt will be quite some work to do...

Since the power supplies you have apparently have a current limiter you could use this feature to your advantage. (using lower coil resistance and only n-channel MOSFETs.)

 

... I used 24awg and what looks like 26-30 feet.
...

That should be fine I think.

It does not have to be a high efficiency high power motor suitable for long term use!

Basically you only need to make a strong enough magnetic field for the rotor to turn reliably, and maybe a bit extra magnetic strength so a student could "push" on the rotor and feel the magnetism working.

You coils as they are will probably work fine. All you need to do is put a resistor in series with each coil pair to limit max current to a safe level (like say 300mA). At 12v the resistor might be about 47 ohms, and you should use 5W resistors as they will get a bit hot.

That will be an easy solution to set a fixed coil current when testing, and might be all the solution you need in the final device.

...
Edit: something else I just saw. The current for the gate charge of the MOSFETs comes from the CMOS gate outputs. They cannot provide very much current which will slow down the FET gate charge, it will possibly also cause an increased power dissipation in the CMOS gate output stages, although I cannot say to what degree. In other words, this also puts a limit to the maximum rotational speed.

That's a valid point, although in this case the motor only needs low speeds. CMOS gate outputs should drive FETs easily up into the 20kHz range, which with this motor would be a speed of 10000 RPS or 600000(!) RPM.

For demo use the motor will probably need 10 to 200 RPM?

 

All you need to do is put a resistor in series with each coil pair to limit max current to a safe level (like say 300mA). At 12v the resistor might be about 47 ohms, and you should use 5W resistors as they will get a bit hot.

That will be an easy solution to set a fixed coil current when testing, and might be all the solution you need in the final device.

This is why it takes much longer to invent anything by yourself than when you are working in a team! A resistor, of course!.

Too bad the OP didn't say yet what current is needed with the coils he has to obtain the desired magnetic field strength. A bit of experimenting will be needed.

Great

 

Funny, this morning, I was messing around with varying types of resistors in series of the coils, to see how much resistance I need not to trigger the safety on the power supply. However the only resistors I have are the 0.25 watt resistors, and they just blew.

not sure what kind of magnetic strength I need. is that something I can play around with by using different combinations of resistors on the coils?

I also went and bought 400 feet of 30 gauge magnet wire, I was going to wrap 66 feet on each bolt to increase the resistance. I haven't opened the packs yet, I wanted to hear from you guys if it's worth rewinding the coils. I wouldn't mind doing it. I can just set them on a drill and make short work of it. what do you think?

this is what I did today. I don't have the package yet, this stuff I bought at fry's and radio shack.

 

Good idea to use sockets.

The necessary wattage of the resistors you can simply calculate with V*V / R. Current is pulsed so if you just take this value it's ok.

Yes, to find out what would be the magnetic field strength needed for your rotor to turn you can play around with the resistor values. Keep in mind that depending on the wire gauge you used there is a current limit.

Since I have no idea if the field strength your coils are poducing is anywhere near the one you need or very far away it's hard to say what would be the best option.
RBs idea of putting resistors in series is great (for this project), however the more turns you wrap around the core the stronger will be the magnetic field (provided the current is constant.. that's the case if it's limited by your power supply)

So in order to verify if the coils are strong enough I guess it would be a good idea to connect them alternately to the power supply you used that limited the current to 1A (or less if that's adjustable) and see if you get the rotor turning. If that's the case then the resistor in series is the way to go. If not you will need more turns or change the physical layout,i.e. decrease the distance between the magnetic bar and the coils.

 

I was laying in bed this morning trying to figure out how to get the coils to work with that power supply. Your multiple comments, that you were missing something with the resistance of the coils and the applied voltage it should have yielded higher amps, and because it was more than the power supply could handle, it limited voltage output. So, if I can wind enough wire to increase resistance in the coils and have the amperage under the 5 amps then it wont limit the voltage.

by using ohms law and that website to calculate wire resistance in 30 awg wire, with power source of 13 volts (I turned to 13.02v) and a desired 5 amps, I would need 2.6 ohms total resistance.

Now, here's were I need clarification on my math. According to the schematic, at any given time only 2 coils are going to be energized. the coils are wired in parallel and total resistance is less than the smallest resistor in a parallel circuit. Doing the math to calculate total resistance, each coil will have to have 5.2 ohms. Using the website I will need a minimum of 50.2 feet at 70 F (15.3m at 21 C)

If that's correct then 50.2 feet is the bare minimum. wrapping each coil with 60 feet will give a Total resistance of 3.1 ohms. That will drop the amps down to 4.19 well within the limit.

RIGHT?

 

by using ohms law and that website to calculate wire resistance in 30 awg wire, with power source of 13 volts (I turned to 13.02v) and a desired 5 amps, I would need 2.6 ohms total resistance.

You cannot pass 5Amps through 30AWG for more than a few seconds.

I had my doubts what would be practical limits.
I took a wire #30 and tested it here. It starts smelling at 2A. It gets very hot at 3A. Insulation melts at 5A. This is where I stopped.

With 30AWG you will need much more turns.

This is why I mentioned #28 and 1A.

http://www.powerstream.com/Wire_Size.htm

 

oh. but then the amp rating for 30 gauge wire is a little less then 1 amp...

back to square one.

 

oh. but then the amp rating for 30 gauge wire is a little less then 1 amp...

back to square one.

If you could choose your wire gauge and number of turns so that you stay at 1 or maybe 2A at most that would be great.

 

Good idea to use sockets.

The necessary wattage of the resistors you can simply calculate with V*V / R. Current is pulsed so if you just take this value it's ok.

looks like resistors are still the answer.
ideally I need 13 ohms at 13 volts to give me 1 amp.
by your calculation I would need resistors rated at 13 watts. is that correct?

 

looks like resistors are still the answer.
ideally I need 13 ohms at 13 volts to give me 1 amp.
by your calculation I would need resistors rated at 13 watts. is that correct?

Yes. That's big, isn't it?

As I said, I would first determine if the magnetic field will be strong enough to make your rotor turn with the coils you have. Thsi means the rotor part would need to be finished first.

If you buy resistors first and then discover that you really need more turns you might need other resistors or you might find out that you could have bought cheaper ones etc.

 

Good point, however to determine if the magnetic field will be strong enough to make the rotor turn with the coils I have, I will need 13 watt resistors so that the power supply doesn't go to a fail safe and cut the voltage. ahh what a conundrum...

when I was testing them (with the failsafe activated) at 0.66V, 1A and 0.7 ohms the magnetic strength was adequate enough to attract the magnet which was sitting on a scrap piece of 1/4" plexiglass. but keep in mind that that was 0.7 ohms is between 2 coils in series, once I change it to parallel that will change everything. The way I see it I have to rewire those coils.

There are 2 coils that I experimented with super thin wire, the wire was wire cannibalized from the secondary winding of an old automotive coil, (there was well over 1000 turns on each bolt)

Since I have to strip those 2 bolts down anyway, I will experiment with # of turns, resistance, etc to determine the maximum amount of wire I can spool onto 1 bolt. I think that ultimately I would want as many turns as possible and then achieve the desired current (1.5A max) using resistors.

This will be fun to experiment with, my daughter is really taking an interest in this, so we'll be able to experiment and spend quality time together. Win Win!

 

This will be fun to experiment with, my daughter is really taking an interest in this, so we'll be able to experiment and spend quality time together. Win Win!

Great!

If I had a project like yours I would probably try to use coils including core from old 12V relays. Like from a car or something...
But since you want to even build your own coils, go ahead.

 

I have thought about that and the only coils that we have an abundance of are the solenoids for electric starters.

good news, fedex just arrived with the package.

 

Good point, however to determine if the magnetic field will be strong enough to make the rotor turn with the coils I have, I will need 13 watt resistors so that the power supply doesn't go to a fail safe and cut the voltage. ahh what a conundrum...

Not at all. If you want, say 1 amp, then just set the current limit for one amp. If you want two amps, set it for two amps, etc. You can use your current limit without resistors to determine what current is needed to get the required field strength. Just be careful and don't set the limit too high, and burn up your coils.

 

quick question: for the ceramic capacitors 100pF and 100 nF, how do I determine + and - ?

or does it matter how it goes into the circuit?

 

quick question: for the ceramic capacitors 100pF and 100 nF, how do I determine + and - ?

or does it matter how it goes into the circuit?

They are not polarized. You can put them either way.

Only the big electrolytic capacitor is polarized.

 

thanks. and the polarized ones are marked with the negative lead having the stripe or - on it. right?

 

thanks. and the polarized ones are marked with the negative lead having the stripe or - on it. right?

Yes. If you are not sure you can post a picture.