Why Won't This Voltage Sag Work...?

[railhead]

Maybe it's because I'm running off about 6 hours of sleep in the last 48 hours, but I can't figure out why voltage sag setup won't work.

Battery positive > pot lugs 1&2 (and I tried just 2) > pot lug 3 to positive probe
Battery negative > negative probe

Here's a pic:

[amptramp]

You have only the very high input impedance of the DMM connected to it.  So if you add, say, 10K in series with a DMM input resistance of 10 Megohms, you don't get much voltage drop.  The sag depends on the current drain through the pot.

[railhead]

There has to be a way to measure the voltage, though...

[MoltenVoltage]

You have the pot wired as a rheostat and are only controlling the current.

You need to wire it as a voltage divider, but be careful because pots can only deliver a limited amount of current before they catch on fire!

Battery + to lug 1
Battery - to lug 3
Probe - to Battery -
Probe + to wiper

[railhead]

You have the pot wired as a rheostat and are only controlling the current.

You need to wire it as a voltage divider, but be careful because pots can only deliver a limited amount of current before they catch on fire!

Battery + to lug 1
Battery - to lug 3
Probe - to Battery -
Probe + to wiper

Yep, I already did that.

I was hoping to use a pot to control the speed of a 9V motor, but when I choke back to slow the motor, the pot heats up.

[PRR]

> use a pot to control the speed of a 9V motor

How much POWER (Volts times Amps) does the motor need??

Most motors need far more power than most radio-pots can handle. Even at part-power, the pot smokes.

You don't need infinite adjustment for a fan motor.... throw half-watt 100 ohms, 47 ohms, 220 ohms resistors in _series_ until you get a happy blow and noise. If the resistor gets hot, find a higher-Watt one. If you need a mild breeze for hand-painting and a strong breeze when cooking crack, use a switch to select various resistors.

If you need steady infinite speed control on a _DC_ motor, series resistors (or reasonable pots) won't do it. Build a solid variable VOLTage power source. Perhaps a LM317 adjustable regulator chip.

AC motors are extrememly different. Speed control is not easy.

[Hides-His-Eyes]

For a motor I'd run a 555 PWM switching through a transistor. Then the pot can control the duty cycle instead of catching fire.

[Cliff Schecht]

If you are looking to efficiently run an AC motor, you'll need to look into some sort of H-bridge driver. DC motors can be run off of a 2N2222 most of the time, unless you have a fattie that requires more than 500mA of continuous current.

[PRR]

> ...I'd run a 555 PWM switching through a transistor.
> ...to efficiently run ...look into some sort of H-bridge...

Both require a better grasp of basic electrics than Maury has at this point in his path.

There's 6 million ways to control motors. We do NOT know what Maury is really wanting to do. Reduce fan noise? Hit 345.6 RPM exactly? Extend battery life?

The selected fixed series resistor is SIMPLE, and broadly useful. Maury can wire that in a jiffy and see if it's what he wants.

If RPM must be very steady, then the standard DC motor with light load may be controlled with VOLTAGE. A selection of well-regulated 3V 5V 6V 9V wall-warts or a fist-full of C-cells will quickly test the basic idea.

If Maury comes back saying the battery life is short, or the speed isn't steady enough, or it breaks tape, or throws hash into the radio, then we can explore more elaborate controllers.

[railhead]

I did totally forget to explain what I'm doing, didn't I?

I'm building a simple etchant agitator. The motor will spin a wheel that will have a rod attached at the edge of the wheel. The rod will be attached to a plate that will hold my etching container. As the wheel turns, the plate will see-saw and provide waving. Everything will be made from lightweight materials, and I'm simply using 9V since that's what I have laying around. I probably only need 3V, which I can get at the local RatShack if need be. I'm controlling the rotation of the motor shaft by reducing voltage.

I was wanting to read the value of the voltage at the pot to help get closer to using fixed resistors. I'll just breadboard the project, though, rather than worry about reading as I originally wanted.

And 9V is also because I can just plug a wall wart into things and get cranking -- or a battery in a pinch.

[R.G.]

The advice you're getting on motors, electronics, power, current, etc. is good. Read those posts carefully and wherever you don't understand them, stick a little red flag in the ground. You're going to need to watch that point.

I have some less related views.

Water is not a lightweight material, particularly. Ask anyone who has ever hauled water either by hand in a bucket or in a loaded vehicle.

Nor is water which is filled with permanently-staining corrosive materials a pleasant material to deal with. Please_please_please_please_please_please_please try whatever you do with plain water first. 

A primarily aqueous fluid in a tank is a mechanically resonant circuit. Excite it at the right (or wrong) frequency and it will ring - that is, the oscillations will build up without limit until something limits them, like sloshing over the top.

What you really want is to tip the tank so slowly from side to side that you're guaranteed to be below the resonant frequency of the fluid in the tank. The right motion for an agitated etch tray is about 1/2 to 1 cycle per second. You want slow flowing motion, giving plenty of time for the fluid resistances to damp the fluid motion and prevent a resonant buildup of motion.

You also want the mechanics to be inherently limited in motion, so the whole tank can't possibly suddenly invert, spilling etchant. Especially spilling it on anything which staining will damage, or anything aluminum. Ask me know I know this... 

One mechanical setup I have used is a big plastic tray holding the actual etching tray inside it, just in case, the whole mess on a tipping platform which rocks to and fro over a pivot point. I used a plastic rod about 3/8" diameter. The setup was such that you could drop one end from level and it would not spill. I drove it at about 1/2Hz with a gearhead motor using a coupling rod on a disk attached to the output of the motor, much like what I think you're describing.

I got this running, and after a few uses went back to manually tipping the tray. Home etching is hardly ever done consistently enough that you can go away and come back with the timer dings. At least mine wasn't. And if I had to watch it all the time, I might as well rock the tray, too.  At least that was my reasoning.

Put your efforts into getting photo exposure on presensitized PCB material for good resolution and figuring out how to drill holes efficiently. They're far bigger problems.

[railhead]

I have all my PCBs fabricated, so that's a huge benefit.

The etch transfer method I use is static: 3 minutes under my iron on photopaper through my laser. My different shells with different art have their etch times logged in my Black Book, and I shoot right at about 1-2 minutes every time, consistently, etc. All this is done with no rocking at all, though I do pull the shells out at 5-6 minute increments and clean them off. Point being, I've been really successful in getting my etching system down to a consistent process, thankfully! I'm just doing the agitation as a fun side project that may, actually, be beneficial as well -- if I get it to work how I want.

[R.G.]

I have all my PCBs fabricated, so that's a huge benefit.

It is. I couldn't tell your level of build sophistication. No offense intended.

The etch transfer method I use is static: 3 minutes under my iron on photopaper through my laser. My different shells with different art have their etch times logged in my Black Book, and I shoot right at about 1-2 minutes every time, consistently, etc. All this is done with no rocking at all, though I do pull the shells out at 5-6 minute increments and clean them off. Point being, I've been really successful in getting my etching system down to a consistent process, thankfully! I'm just doing the agitation as a fun side project that may, actually, be beneficial as well -- if I get it to work how I want.

Ok.
In that case, think about the etch tank resonance thing. Consider using a rocking platform with only about 1/4" to 1/2" of free travel at 6-9" from center. Rock it at under 1Hz with a smooth motion. The eccentric arm thing works. Use a motor with much more torque than you think you'll need. An AC-DC (or "universal) powered gearhead type motor is great. Look up "Surplus Center" for some great mechanical deals, including gearhead motors. Don't dink with slowing down motors with a pot. Hit the surplus places, Harbor Freight, or tool places and get a cheap "router speed controller" for changing the motor speed.

As an alternative, consider a bubble etcher. Much simpler mechanically.

[PRR]

What R.G. said.

He says to aim below ~~1Hz, 60RPM. The common toy motors like to spin 6,000RPM. In theory they would spin 100 times slower with 100 times less voltage; in practice they won't turn at all below about 1/10th or 1/20th voltage at best.

Also the torque is small. Enough to push a 40 gram car on the level or spin a few-inch fan, not enough to lift a tray of water.

A gear train is the answer to both of these. Lower speed higher torque.

A single pair of spur gears "can" do a 100:1 ratio. But a 7-tooth hardly works, and the mating 700-tooth is a lot of teeth to grind out. And the leverage is bad so friction is high. A worm gear can do a little better; still you don't find them in 100:1 ratio.

I guess you could wrap string several times around a 0.1" shaft then around a 10" crank wheel. If you don't have enuff wraps and tension it will slip; too much wrap and tension will drown in friction (the wrap must slide sideways one thread-width for every turn).

Multi-stage gear chains will do 100:1 and are commonly found on.... surplus motors.

A car windshield wiper is a gearmotor, lots of torque (throw your pot off the bench). Somewhat fast at 12V but will work slower at 6V-8V. The current demand is far too large for wall-wart or easy chip regulation. Might make sense if you, like me, had a 1977 Plymouth out in the woods to donate both the wiper motor and the hefty transistor and heatsink on the ignition module.

You don't need to be anywhere near 1Hz. Seems to me it could be idle for many seconds, gently shift to the alternate angle, and stay there while the slosh died out. That suggests a timer or LFO with ramped square-wave shape. Or an intermittent windshield wiper.

The mechanical question is "so interesting" that a bubbler may be a better path.

Then there is a high-production etcher like a pressure-washer inside a tank. Cuts copper quick. Would make a royal mess even quicker. For one-off work you'd surely spend more time checking, finding, and sealing leaks, and swabbing the room, than it would take to etch at no-slosh speed.

[PRR]

Here's why your experiment did not give the expected result.



The pot and meter make a voltage-divider. The 9.0000 Volts splits across the two resistances according to their values. Say the pot is a 1K value (10K pot set at 0.1 turn) and the meter is probably 10 Meg ohms (10,000K). The result is 10,000,000/10,001,000 of what you put in, or 8.999V. Dang near "no drop at all".

Your intended REAL use is also a voltage divider BUT with a motor in the bottom leg. Say the motor is marked "9V 90mA". Ignoring some motor-facts, that's 9V/90mA= 100 ohms. And ignoring the meter, you have a 1K:100 voltage divider and get something like 100/1,100 of what you put in, less than one Volt.

The motor won't spin with 0.818V, so you turn the pot futher. At 1% rotation the working part of the pot is 100 ohms, similar to the 100 ohm motor. The voltage is about half, 4.5V, and the motor turns nearly half speed.

BUT: you have the other 4.5V in the pot. 4.5V across 100 ohms is 0.045 Amps. 4.5V times 0.45A is 0.2025 Watts. The pot may be rated 1/4 Watt or 0.25 Watts. Seems OK? But that 0.2W is concentrated in only 1% of the whole 0.25W pot. The safe rating for 1% of a 0.25W pot is 0.0025 Watts. Since the other 99% is cold, maybe you can push 0.003W, but not 0.2Watts in 1% of the "0.25W" pot. It will, and did, smoke.

For more fun: a good motor draws little current when UN-loaded and more current when loaded. If you push 2" oak through a table saw, the lights dim when you force a knot. So the motor's equivalent resistance changes with motor load. When spinning a fan in air, the motor load is pretty consistent proportion from "full blow" to "too weak". Fixed resistors may be rough-estimated from the motor's nominal V and A numbers. Electric street-cars can be gently brought up to speed with a rack of series resistors, though it is nice if the motorman judges the empty/full load and shifts resistors accordingly. However when tank-thumping the resistance-slugged motor will turn fast on the easy part and stall when it tries to lift the slosher. Here what you want is constant voltage, not a series resistor.