The whole point is that we simply can't tell what resistance will slow the motor down by how much. Also (and this is a bigger deal) you'll probably find that the motor will lose torque when it is slowed down.
Now, I presume that you'll be doing something like spinning slightly angled mirrors and reflecting the beam from one spinning mirror to the other. In this case the torque isn't going to be much of an issue and you're probably not going to have to vary the speed over a significant range.
However, you first need to determine the range of resistances you need to use, then select a pot with a value near the maximum that you've chosen.
As I mentioned above, there's no magic formula. I also would have thought that a couple of fixed resistors will be a lot cheaper than a fairly high wattage pot.
Incidentally, I just happen to have one of those laser spirograph thingies that I built up from a kit. I believe it switches resistors in series with the motor, but I'd have to check. Hmmm, can only find the April 1992 version, and it's not that one.
Now, I presume that you'll be doing something like spinning slightly angled mirrors and reflecting the beam from one spinning mirror to the other. In this case the torque isn't going to be much of an issue and you're probably not going to have to vary the speed over a significant range.
However, you first need to determine the range of resistances you need to use, then select a pot with a value near the maximum that you've chosen.
As I mentioned above, there's no magic formula. I also would have thought that a couple of fixed resistors will be a lot cheaper than a fairly high wattage pot.
Incidentally, I just happen to have one of those laser spirograph thingies that I built up from a kit. I believe it switches resistors in series with the motor, but I'd have to check. Hmmm, can only find the April 1992 version, and it's not that one.
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