FET circuit to switch power to IC?

Because some of us are not quite as quick as you to know that some very cool and tantalizing things like QTCs exist! (c:

(How can Maplin show such a tempting item and not provide data sheet or additional specs?) Data sheet: shows that resistance is logarithmic. How to map this to achieve linear motor response?

Why not, indeed... except that the QTC's min resistance (or the MOSFET's if I use the QTC to control that) still won't allow full motor speed which is necessary for full range of useful torque and in practical use is required for those really tough screws.

Most tool mfrs utilize a full-speed switch. Pull the trigger on any variable-speed tool (cordless or otherwise) and you'll see the speed increase linearly until about 90 percent speed, then a slight jump to light speed. They all short past the semiconductor for full speed.

Now if I could figure out how to mount the QTC atop a mechanical momentary PB with sufficient spring resistance... at max QTC pressure the switch would close. Voila!

A QTC is quite tempting -- at 10 amps, could be used solo, w/0 555 or such...

The way I was thinking of fixing that was two diodes hooked up to the switch, used as "steering diodes" so you get a positive voltage to one node no matter what direction is pressed.

-Jeff

Where would you put the speed controller?

If at the battery, your diodes would be returning a pulse width modulated positive voltage back which would have an average voltage approaching zero at low speeds.

And, of course, the signal wouldn't be present at all if the signal was being used to actually switch on the speed controller...so you would need a seperate "start" signal.

If after the switch, then the diodes wouldn't be needed at all as the switch would act as the on/off control for the power controller. However, the controller would need to cope with reversed polarity..

...

Right, put the power controller after the switch, and the diodes could "cope with reversed polarity" I think it would work, but I haven't seen a schematic for the tool to be sure.

-Jeff

OK. But with the controller after the reversing switch, it would have to be designed to cope with its power supply input reversing in polarity.

And this is very low voltage -so simply sticking it in the middle of a full wave bridge would not do, due to the diode volt drops.

What if the ground of the controller was connected to the ground of the battery, and the positive of the controller was connected to the two diodes coming from the switch, one goes positive if forward is pressed, and the other goes positive if reverse is pressed. The Fet would probably have to switch the ground that goes to the switch, or one could reverse all of the polarities if needed.

-Jeff

As I stated, there are other ways of doing things! However, putting a pwm unit between the existing battery and existing reversing switch, followed by the existing motor and using a simple tilt sensor to power up the pwm, is one of the simplest.

It certainly isn't the "best" - if the application is the best possible electric screwdriver. One could, for example, sense the motor speed and reduce speed immediately a sudden load decrease was sensed - so that a slipping screwdriver bit wouldn't damage the screw head..

Ooo... don't get me started. Feedback... (c:

Right, more than one way to skin a cat. I just thought it might be easier and cheaper to use two diodes, than a tilt sensor. He might already have diodes, and not a tilt sensor, or maybe not.

I like the feedback idea too, you could trigger it from a high current sense to stop at certain amount of tourque (and make that adjustable). because the motor stall current would be highest.

-Jeff

basically yor have this, centre-off reversing switch amd motor .--- | ---o-------. 7.2>-|--- : | | ---o---(M)-' +--- | 0V>-' do this: .--- | ---o-------+->|--. 7.2>-|--- : | | | ---o-+-(M)-' | +--- | | | `------>|---+ | | `--|. .--+--. || | | .--|`-------------| 555 | | | ETC | | | | | `--+--' | | 0V>-+--------------------+

Bye. Jasen

It's another very creative way to skin the cat!

Thanks!

Nice idea! But...

Resistance range: 10^-12 to 1 ohms. Not sure that this would provide much linear speed control... Perhaps a calibration circuit is needed?