Big, Fat Electronics question for all y'all.

[footfootfoot]

It's got a 300° rotation, in practice if I pot it down more than 1/10 of a turn the fan stops spinning. I can't start the fan from lowest setting on the pot and bring it up, it's off until it hits full power. When the pot is on full power I can dial it down only about 1/10 of a turn before the fan stops. That's the point when it starts smoking.

It seems like not enough voltage is making it from the pot to the fan to get it to turn. I thought the whole point of a DC motor is that it can be variable speed by changing the voltage. If I understand correctly, the pot is reducing the voltage to the fan by increasing resistance. In this case, with the slightest rotation, it is dropping the voltage too dramatically. What I want is a gentle slope and what I've got is a precipitous cliff.

Is this not a voltage issue? Is there a difference between 10K Ohms and 10k Ohms? So the pot is creating too much resistance? I need a lower Ohm rated pot to get less resistance? I think I may be catching on, but don't tell the short bus to drive away without me yet.



I've powered the fan through the pot with both a 6V and a 12V wall wart, only the 12 gave me any adjustment at all. The 6 was either on or off.

[tw]

Originally Posted by footfootfoot

It's got a 300° rotation, in practice if I pot it down more than 1/10 of a turn the fan stops spinning.

Reread those previously posted numbers. Your resistor must vary by maybe 30 ohms. You are using a resistor that is 10,000 ohms. You cannot possibly adjust a 10K resistor between 1, 5, 10, and 30 ohms. And if you did, the 10K resistor must burn.

Trying to do this stuff without a layman tool is frustration. A multimeter that any layman should have for anything electrical (even a car battery) sells even in WalMart for maybe $12. So required for simple problems that a meter sells in almost every store that also sells something more expensive - hammers. Using that meter would have made that problem obvious. Also make numbers in my previous post just as obvious. Even explain 6 volt and 12 volt behavior.

You also have another complication to consider. Electric motors require a high startup torque. Just another reason why the 555 solution can be so useful. And again, another reason why that digital multimeter is needed.

[BigV]

now *that* was payback for being previously unhelpful.

thanks tw!

[footfootfoot]

I found at Radioshack a 25-Ohm 3-Watt Rheostat. It seems like that is the right tool for the job, does that seem right? I will try that.

[BigV]

careful to keep the magic smoke on the inside, mkay?

[tw]

Originally Posted by footfootfoot

I found at Radioshack a 25-Ohm 3-Watt Rheostat.

A hypothesis is based in those two equations and numbers provided for the fan. To know whether that solution works means experiimental evidence (numbers) a multimeter. It might work. Without those numbers, I cannot say anything useful. No other tool is more needed when doing anything electrical. From solving a dead car battery to fixing an appliance to doing a unique design.

[footfootfoot]

Originally Posted by tw

A hypothesis is based in those two equations and numbers provided for the fan. To know whether that solution works means experiimental evidence (numbers) a multimeter. It might work. Without those numbers, I cannot say anything useful. No other tool is more needed when doing anything electrical. From solving a dead car battery to fixing an appliance to doing a unique design.

I've got to find my multi meter. It's around here some where.

I'm to check the resistance of the fan?

[tw]

Originally Posted by footfootfoot

I'm to check the resistance of the fan?

Fan is a non-linear load. Measuring resistance will not say anything useful.

A brushless fan contains transistors that power cycle (fully off or on) to spin.

Start with the circuit wired. And resistor set to some desired fan speed. Then measure voltage across fan and voltage across 25 ohm adjustable pot. And finally, measure current through pot and motor. Or remove the pot to measure its resistance.

Those numbers will confirm parts are working per the hypothesis.

BTW, also take an ohm measurement of any 120 volt incandescent bulb. That number will be used to demonstrate another non-linear load.

[infinite monkey]

Your face is a non-linear load.

Just kidding. I'm channeling jim.

[footfootfoot]

Originally Posted by tw

Fan is a non-linear load. Measuring resistance will not say anything useful.

A brushless fan contains transistors that power cycle (fully off or on) to spin.

Start with the circuit wired. And resistor set to some desired fan speed. Then measure voltage across fan and voltage across 25 ohm adjustable pot. And finally, measure current through pot and motor. Or remove the pot to measure its resistance.

Those numbers will confirm parts are working per the hypothesis.

BTW, also take an ohm measurement of any 120 volt incandescent bulb. That number will be used to demonstrate another non-linear load.

Great. I found my multimeter last night, I'll perform those tests tonight.

[tw]

Originally Posted by infinite monkey

Your face is a non-linear load.

Resulting in a non-linear smile.

[footfootfoot]

It's working! I'm using that rheostat and a 4.74v wall wart. Gonna build a case for it and I'll give it a whirl.

[tw]

Originally Posted by footfootfoot

It's working! I'm using that rheostat and a 4.74v wall wart.

Appreciate the world is ternary. Of three possible conditions, you have either good or undefined. Undefined means it is working but (maybe) overstressing parts. The rheostat will typically consume most power in some half way point. Confirm voltage across the rheostat times its current never exceeds 3 watts. It could be exceeding that at some point - the undefined zone.

That's what the multimeter does. Confirm the non-linear nature of that motor is not overstressing the rheostat.

Measure a rheostat voltage at a midpoint setting. And then set the meter to current (maybe its 2 amp settting) to measure rheostat or motor current. W=VI. W must be less than 3.

Or measure its voltage. Then disconnect the rheostat to measure its resistance. W=Vsquared/R.

A few random midpoints on the rheostat to make those measurement should confirm that smoke or fire is not a long term option.

IOW to confirm the circuit is in the good category and not in the undefined category.