Hi guys!
I want to build an electromagnetic stirrer with as few components as possible. Now, what is an electromagnetic stirrer, you may ask?
Well, a typical laboratory stirrer has a dc motor with a wheel carrying two magnets, with north and south pole upwards. Above this you have a beaker with a small teflon-coated magnet (the stirrer bar), which then locks to the magnets on the motor below and gets turned around. Hence the stirring effect.
The new kid in town is the electromagnetic stirrer, which replaces the dc motor and the permanent magnets with 4 electromagnets.
You can wire the diametrically opposed electromagnets together, so that when you run a current through them, one has the north and the other has the south pole upwards. You then shut them off, and do the same with the two other electromagnets. After this you run the current the opposite way through the first two electromagnets, and lastly you do the same on the other set of electromagnets.
The net effect is a "rotating" magnetic field, which the stirrer bar gets locked to.
I want to avoid using a hall-effect sensor, if possible. Surely this can be done by using a microcontroller, but I want to avoid that too.
I want to do this using only logical circuits, like nand and nor-gates, or something like that. Or any other clever method that requires just a few transistors. The goal is to be able to make this work using something like max. 10 universal components like transistors and diodes.
I may be able to figure this out during some weeks of pondering, but I have a hunch that someone "out there" has some experience that I do not have in this exact area - so here is your challenge
A simple and smart solution to create a rotating magnetic field using very few components.
A computer fan dc motor typically uses a hall-effect sensor to sense the position of the permanent magnet rotor, to regulate the frequency of pole activation. In this case, the stirrer bar is the rotor, and of course we cannot start the rotating field at e.g. 500 rpm right away, as the stirrer bar would not be able to stay locked to the field at all - it will drop off right away.
One might be able to avoid the need for feedback on the stirrer bar position if one started the rotating field at a very low rpm and gradually speeded it up to its regular rpm.
But right now I have just started working on how to combine and use TTL circuits to make this work. There are 4 different states involved:
1. Coils 1 and 3: current forward - coils 2 and 4 off
2. Coils 2 and 4: current forward - coils 1 and 3 off
3. Coils 1 and 3: current backwards - coils 2 and 4 off
4. Coils 2 and 4: current backwards - coils 1 and 3 off
I am also pondering how to use transistors to be able to shift the polarity of the current. DPCO relays would be nice, but the frequency is too high. Small solid state relays would be fine, but no DTDP relays exist as far as I can see. Surely I can use a network of transistors for just this polarity reversal function, 4 for each coil set just to stop the current at each side of a central current control transistor - but I want fewer than 5 transistors for each coil set, if possible. Well, 5 are not that much - I guess it would be ok.
I can use a 555 timer to produce a timing pulse, and modify the external 555 components to create a sloping start. The 555 could drive some TTL logic stuff which keeps track of the 4 states and triggers the transistor networks for each coil. Oh - if I only could "download" the ready-made circuit from "above" like Beethoven got his sonatas!
If any of you folks have already "downloaded" it, it will save me some weeks of pondering, I guess. Any suggestions are welcome!
I want to build an electromagnetic stirrer with as few components as possible. Now, what is an electromagnetic stirrer, you may ask?
Well, a typical laboratory stirrer has a dc motor with a wheel carrying two magnets, with north and south pole upwards. Above this you have a beaker with a small teflon-coated magnet (the stirrer bar), which then locks to the magnets on the motor below and gets turned around. Hence the stirring effect.
The new kid in town is the electromagnetic stirrer, which replaces the dc motor and the permanent magnets with 4 electromagnets.
You can wire the diametrically opposed electromagnets together, so that when you run a current through them, one has the north and the other has the south pole upwards. You then shut them off, and do the same with the two other electromagnets. After this you run the current the opposite way through the first two electromagnets, and lastly you do the same on the other set of electromagnets.
The net effect is a "rotating" magnetic field, which the stirrer bar gets locked to.
I want to avoid using a hall-effect sensor, if possible. Surely this can be done by using a microcontroller, but I want to avoid that too.
I want to do this using only logical circuits, like nand and nor-gates, or something like that. Or any other clever method that requires just a few transistors. The goal is to be able to make this work using something like max. 10 universal components like transistors and diodes.
I may be able to figure this out during some weeks of pondering, but I have a hunch that someone "out there" has some experience that I do not have in this exact area - so here is your challenge
A simple and smart solution to create a rotating magnetic field using very few components.A computer fan dc motor typically uses a hall-effect sensor to sense the position of the permanent magnet rotor, to regulate the frequency of pole activation. In this case, the stirrer bar is the rotor, and of course we cannot start the rotating field at e.g. 500 rpm right away, as the stirrer bar would not be able to stay locked to the field at all - it will drop off right away.
One might be able to avoid the need for feedback on the stirrer bar position if one started the rotating field at a very low rpm and gradually speeded it up to its regular rpm.
But right now I have just started working on how to combine and use TTL circuits to make this work. There are 4 different states involved:
1. Coils 1 and 3: current forward - coils 2 and 4 off
2. Coils 2 and 4: current forward - coils 1 and 3 off
3. Coils 1 and 3: current backwards - coils 2 and 4 off
4. Coils 2 and 4: current backwards - coils 1 and 3 off
I am also pondering how to use transistors to be able to shift the polarity of the current. DPCO relays would be nice, but the frequency is too high. Small solid state relays would be fine, but no DTDP relays exist as far as I can see. Surely I can use a network of transistors for just this polarity reversal function, 4 for each coil set just to stop the current at each side of a central current control transistor - but I want fewer than 5 transistors for each coil set, if possible. Well, 5 are not that much - I guess it would be ok.
I can use a 555 timer to produce a timing pulse, and modify the external 555 components to create a sloping start. The 555 could drive some TTL logic stuff which keeps track of the 4 states and triggers the transistor networks for each coil. Oh - if I only could "download" the ready-made circuit from "above" like Beethoven got his sonatas!
If any of you folks have already "downloaded" it, it will save me some weeks of pondering, I guess. Any suggestions are welcome!
