# Engine speed - need a pot

Discussion in 'The Projects Forum' started by sgomes, Jan 23, 2008.

1. ### sgomes Thread Starter New Member

Jan 23, 2008
4
0
Hi guys,

I've got a little project with a solar kit, that came with a solar panel and a small engine. The specs for the panel say it outputs 0.9V at approx. 400mA.

I want to replace the solar panel with a battery or a cell phone charger. Now, I've used a multimeter to measure the voltage the panel was generating, and I'd say a good "slow" spinning speed would be at 0.2V, while a good "fast" spinning speed would be at 0.6V. I want to use the pot to modify the voltage supplied to the engine by the battery, using those two values as minimum and maximum.

So, if I want to use a 1.5V battery as the power source, I'd need a resistance of (1.5-0.2)/0.4=3.25ohm for the slow speed and (1.5-0.6)/0.4=2.25ohm for the fast speed, right? This leads me to conclude that I need a low resistance pot or otherwise the "slow" and "fast" settings would be too close together.

I looked at pots at RadioShack, and the lowest they have is the 25ohm, 3W. But a 25ohm pot in this case would need to handle (0.4^2)*25=4W, right?

Would using other voltages (say, 9V) make it easier? I think I'm missing something here...

2. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
A PWM circuit would make things much more efficient.

You're assuming that the solar panel puts out 400mA at all voltages; it does not.

At 0.2V, how much current is flowing through your engine?
How much current at 0.6V?

That's how much current will be flowing through the rheostat at those voltages.

If you want the maximum speed limited to that at 0.6v, then get a fixed resistor that will drop 1.1V at the current you're drawing at 0.6v.

3. ### kubeek Expert

Sep 20, 2005
5,584
1,093
Motors don´t usually use regulation by resistors. You should try to use PWM regulation.

4. ### sgomes Thread Starter New Member

Jan 23, 2008
4
0
So is there any simple PWM circuit I could build (or better, buy) for this?

Also, would the actual running current of the engine be important in determining the values of the components, or would the supposedly maximum 0.4A in the panel's specs be enough? I guess I could measure it if necessary, but I would have to borrow the multimeter again

Then again, from the very very little I know about PWM circuits you only need to set a maximum current, not design it for a fixed one, right?

Thanks,
Sérgio

5. ### thingmaker3 Retired Moderator

May 16, 2005
5,073
8
For "buy it" PWM, you might consider a web-search on the following chips:

SG1524

SG3525A

MAX038

U2352B

TL494

UC2638

6. ### sgomes Thread Starter New Member

Jan 23, 2008
4
0
Oh, I'm in waaay over my head

7. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
Thingmaker3, you should cross the MAX038 off your list, as sadly that fine IC is made of unobtanium.

sgomes, if you can get a reading of what kind of current the motor is drawing @ 0.6v, I'll throw together a schematic of a reasonably easy PWM circuit that you can build.

8. ### thingmaker3 Retired Moderator

May 16, 2005
5,073
8
Noted.

I have a bad tendency to use outdated parts for my hobby projects. Surplus house prices are almost always less than new item prices. The downside is pig-in-a-poke selection and random availability.

Sgomes, it might help if you read this: http://www.allaboutcircuits.com/vol_3/chpt_8/3.html which introduces the concept of PWM, and also this: http://www.allaboutcircuits.com/vol_6/chpt_6/9.html which walks one through building a simple PWM from commonly available parts.

I apologize for giving a nerd-speak answer when plain English was called for. Let us know if we can help with any of the concepts as you learn what you need!

9. ### sgomes Thread Starter New Member

Jan 23, 2008
4
0

I've bought myself a new multimeter and measured the current. The motor's consuming 0.2A at 0.6V. I think I'll need to build a circuit from scratch, since most "logic" parts run at about 5V, and I was hoping I could use just a 1.5V battery.

I'll definitely take your offer of help on that one, SgtWookie

10. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
13
If you just want to play around and try it, you can make a pseudo-pot with a 10 position switch.

Based on your 1.5V battery calculations, connect a 2.2 ohm resistor between the wire and first switch position, and 0.1 ohm resistors between each of the other positions. You will be able to switch between max speed of ~2.2 ohms down to a low speed of ~3.2 ohms. But, because 0.4A x 1.2V = 0.5W the first is a 2.2 ohm 1W resistor, the 0.1 ohms can be smaller.

11. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
OK, after fiddling around a bit with Spice simulations, I came up with the attached. Can't guarantee that it'll work, because when running at that low of a voltage, things are rather iffy - however, it does work in the simulation.

Don't have a motor like yours around to test it with, either.

Nomurphy, I have no clue how you arrived at your numbers, but they're not correct.

The motor draws 200mA @ 0.6V, which means at that speed it's about 3 Ohms.
The intended V source is 1.5.
1.5 - 0.6 = 0.9 (voltage remaining to be dropped)
R = E / I
R = 0.9V / 0.2A
R = 4.5 Ohms - now figure wattage
P = E x I
P = 0.9V x 0.2A
P = 0.18 Watts - a 1/4 Watt resistor would work; but 4.5 Ohms is not a commonly available resistance. However, three 15 Ohm 1/8 Watt 10% tolerance resistors in parallel would likely be quite close.

If the current @ 0.2v were known, the remaining resistors could be calculated. Simply assuming it's a linear function would possibly lead to erroneous results.

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12. ### hgmjr Retired Moderator

Jan 28, 2005
9,029
219
Sgtwookie,

I wonder if the addition of a base-return resistor connected between Q2's base and the positive rail would improve the performance of the motor driver by providing a means of turning off Q2 a bit more crisply. At the moment there is nothing to turn Q2 off other than by turning off the base current drive from Q1. While the circuit as designed will work just fine, I think that a base-return resistor, if sized properly, will improve the turn-off time of Q2.

hgmjr

13. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
hgmjr,
Good idea! Done Modified schematic & output attached, added a test point to said BC connection.

I also added a small cap across the reverse-EMF diode to quiet things down a bit. Might be overkill, but the 4093 will appreciate it

Oh, a standard bypass cap should be placed across ground and Vcc terminals of the 4093 IC. 0.1uF should do nicely.

ETA: On the far left of the schematic, on pin 1 of the 4093, there is what looks like a "bubble". This is just a Spice directive to set an initial voltage for the RC portion of the circuit, and is not a "real" component.

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14. ### hgmjr Retired Moderator

Jan 28, 2005
9,029
219
Excellent! I think we have a winner.

Nice job, Sgtwookie.

hgmjr