Reducing DC motor speed w/min components

My new cordless screwdriver runs on 7.2vdc Li-Ion batteries (pm brush motor).
It's not variable but runs at one speed (2000rpm) which is nice when you need
it, but for many tasks it's way too fast.
I like everything about this tool except the speed. I'd like to add a second
speed, half or maybe slightly slower than half.
Obviously space is at a premium in cordless tools like this, so minimum
component count for this modification is paramount.
I know I can just hang a large resistor off of the battery and a large switch
for routing current through it, but I was hoping for a solid-state solution.
Is there any advantage to using a bipolar or MOSFET or such over the
bog-simple resistor method? (ie, it requires a smaller switch; a transistor
might be easier to mount externally than a resistor to cool it, etc.).
Ideas?
Those who think a PIC can be adapted to this requirement, please be prepared
to show your work. (c:
Thanks,
Reply to
DaveC
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A resistor will result in reduced torque as you apply load, as it will waste more power just when you need it. Something which drops a more constant voltage would be better, such as a zener diode, or just 4 or so regular diodes in series, and on a heat sink. Another way would be to use just one of the two Li-Ion cells, which will give half voltage. I'm not sure what effect it would have on the charging if one was significantly more discharged than the other.
Reply to
Andrew Gabriel
*EASY!*
(1) Solder all the I/O pins together & attach a wire. (2) Solder Vcc & Vdd together & attach a wire. (3) Wire as many such chips in series with the battery as required to get the motor speed to where you want it. ;^)
Reply to
Lionel
use a 555 timer, a logic level high current Fet and pulse width drive the motor, this is commonly done in a lot of variable speed drills. the electronics is built in the trigger. 50% duty cycle gives you half speed. the idea of using PWM is that it's either on or off, analog control creates heat build up which is energy wasted and less regulation. You can get fancy enough with the controller and monitor the voltage output between the on pulses to use that has a regulator. The Motor will regenerate back voltage and is a good way to test actual speed. THey call this Armature feed back in most cases.
Reply to
Jamie
What makes a FET "logic level"? Is 2N7224 (a.k.a. IRFM150) a logic level device? I've got a few of them...
Vds is 10v, so does this rule it out for use with 7.2v?
Yeah, I was wondering about heat. Sounds like pwm is the way to go. Now, how to fit it all in...
Don't want to get too fancy... just slow it down. Accurate speed regulation isn't too important.
Thanks,
Reply to
DaveC
Logic level Fets come in different ranges, but you should get one that turns on at about 3 volts. the logic fets normally turn on fully at that point. the Source of the Fet should be on the common side of the supply and assuming that you are using - volts for the common side, you use an N channel type. the Drain of the fet will connect to one of the armature leads while the other motor lead connects to the + side of the battery. use a diode across the armature leads to remove kick back high voltage. the cathode would connect to the + side of the armature.
Reply to
Jamie
On 3/10/07 1:49 AM, in article snipped-for-privacy@news.sf.sbcglobal.net, "DaveC" wrote:
It takes two lithium cells to get 7.2V. Use a single cell and add the means to have the single cell connect properly. That will give you half speed.
Bill -- Fermez le Bush--about two years to go.
Reply to
Salmon Egg
A really simple solution. But the battery pack is modular (encapsulated) and would require modification of the pack. Also, would result in imbalance of charge in the cells. And from what I understand about Li-Ion chemistry. they don't balance very happily during charge.
I think a 555 PWM solution is in my future.
Thanks,
Reply to
DaveC
Jamie sez:
This is now my plan. The only question remaining is re. reversing direction.
How is motor reversing accomplished in commercial cordless drills? I see 2 wires connecting the battery to the speed circuit/trigger, and 2 going to the motor. How can I do it simply?
Ideas?
Thanks,
Reply to
DaveC
The simplest possible way is to add a DPDT switch immediately before the motor, wired to reverse the polarity.
Reply to
Lionel
ISTM that the right solution here is a plug-on reduction gearbox. Doing it mechanically would also do the required increase in torque. If there is not one on the market already then someone is missing a commercial opportunity.
Say a 2.5 or 3:1 ratio, input/output shafts inline, make both shafts as hex sockets so that it can be used to increase or decrease speed. Supply a hex bar to couple the screwdriver/drill and gearbox hex sockets.
Reply to
Tony Williams
A two pole c/o series/parallel (slide) switch... and the modification is usually pretty trivial. The slide switch can be very small and won't require anything like the space that an electronic unit (including its own switch/control) will need.
Reply to
Palindrome
Using only one of the two cells can cause problems as you end up with one cell completely discharged in series with a partly charged cell.
You don't need a zenner diode. You just need one or two or three "regular" diodes. Each will drop from .5 to .9 volts depending upon diode size and load.
Reply to
John Gilmer
Thanks, Sue.
Modifying the battery does look like the obvious route. As does the series/parallel switch (what's "c/o"?). I'll have to find one to handle max current, of course, in such a small package...
The main objection I have to applying this mod is that it means I'll have to rely on my memory to place the switch in series before placing the pack in the charger. Charging two 3.6v lithium-ion cells in a 7.2v charger is Not A Good Thing (sm). I'll have to balance the pro's and con's. Unless you have an idea to help protect the cells & charger from that possibility... (c:
FYI, it's this tool:
Reply to
DaveC
2000 rpm 7.2V cordless screwdriver? I don't beleive it.
Bye. Jasen
Reply to
jasen
jasen sez:
Actually 2300 rpm:
Reply to
DaveC
That's an impact driver and I suspect that it needs the rpm to get the inertia to ride through the impacts. Its the impacts that torque the screw, not the direct motor speed/torque. Reducing the speed may not have the effect you desire.
Notice the next driver down, which is not an impact, and which does the nearly the same torque by means of a larger step down in the gearbox.
Reply to
Tony Williams
Thanks for your comments, Tony.
I want to double the use of this tool by providing lower speeds. Yes, the impact is useful for driving/undoing tight screws, but it's useless (way too fast) when, for example, installing small screws in covers on residential light switches or receptacles.
By providing low-torque low speeds, I make this tool useful for small screws, too. Yes, I could buy another tool to cover that purpose, but for a few bucks, (and having a bit of fun in process of design and implementation) I can do this.
(And I already own it...)
Thanks,
Reply to
DaveC
But this means that another switch must be put between the battery and the PWM circuit to turn it off. This spoils my "plug n play" solution. I was hoping to simply insert the circuit between the battery and the existing reversing switch but now must either add an on-off switch or...?
I thought over the ideas suggested by some re. a rotary switch and dropping diodes. But I can't find a small rotary switch to handle the current (3A constant, 5A peak on start) -- there's precious little room for a HD rotary in there...
I like the MOSFET w/speed pot idea best, but 1) it gives only 90 percent max, so I thought to use a switch on the back of the pot to short across the MOSFET to give 100 percent 2) the circuit needs a switch on the battery supply conductor to turn it off. Would be nice to use the switch on the pot, but I'd like to use that for max speed...
More ideas?
Thanks,
Reply to
DaveC
I've thought about this method (without zeners). If each diode drops ~0.7v, to give 4 speeds (full speed plus 3 speeds obtained with diodes) with 7.2v battery would mean 3 diodes / speed, so:
low speed 1 = 3 diodes (2.1v) low speed 2 = 3 more diodes (4.2v) medium speed = 3 more diodes (6.5v)
That's 9 diodes in a pretty small space.
Doesn't using a 1.8v zener (or one close to this v) for each 3 diodes perform the same purpose (with the added advantage of more effectively giving me 3 evenly-spaced speeds)?
Configure as a "diode ladder" and pick off voltages to switch to the motor? I presume I'll have to use a high-current zeners.
Thanks,
Reply to
DaveC

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