Reducing DC motor speed w/min components

[DaveC]

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.

Better yet is to use two 2.4v zeners. This gives steps of 2.4v (one zener),
4.8v (two) and 7.2v (none; full battery voltage).

How best to use these? Is just switching them in series with the motor +
terminal all that's needed? The motor can pull 3A; are there zeners that can
handle this current?

Thanks,

 

[Lionel]

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...?

<shrug> Use a 3 position switch instead, centre-off.

 

[DaveC]

<shrug> Use a 3 position switch instead, centre-off.

So, let's see... the existing fwd/rev rocker switch; a new speed pot; and a
new on/off rocker switch. Getting pretty crowded on this little dude.

My goal is to consolidate functions of the pot and one switch. It looks like
I can do without a power switch if I use a CMOS 7555 timer. It draws less
than 100uA and at this rate the battery (1Ah) will last about a year if left
in its box on idle, w/o charging. Longer if I remember to pull the battery
when putting it away.

I think I'm going to use this approach.

 

[Lionel]

So, let's see... the existing fwd/rev rocker switch; a new speed pot; and a
new on/off rocker switch. Getting pretty crowded on this little dude.

Um. I though you were intending to replace the orignal control system.
Why would you add a new F/R switch instead of using (or replacing) the
existing one?

 

[DaveC]

Um. I though you were intending to replace the orignal control system.

Why would you add a new F/R switch instead of using (or replacing) the
existing one?

The single existing on/direction switch simply connects the battery to the
motor in forward polarity or reverse polarity. I want/need to keep that for
direction control (and being quite custom fit, it requires a bit of a hack to
replace it with anything). If I want a PWM solution, adding a (small) pot
w/switch solves most of the problem: I get speed variation and with the
switch I can short across the MOSFET for full speed (MOSFET only gives 90
percent, or so). But that requires the PWM circuit to remain running (it
needs to be powered in order to fire the MOSFET, doesn't it). So another
switch to turn it on/off? Pretty soon this screwdriver starts to look like
some transport ship on an early Star Wars movie (the later ones look much
sleeker, for some reason...).

So I've decided that since the CMOS 7555 uses only 100uA it will run for
years from this 1Ah battery without turning it off. So that's the route I'm
currently taking.

Now if I can only find a mini-size 500K pot with push/pull or
push-on/push-off switch. Every parts house seems to have huge minimum orders
for a Bourns PTR91 series w/switch or stopped carrying it.

<http://www.bourns.com/pdfs/PTR90.pdf >

Maplin in UK has another brand that might work. But patriotism, y'know... (c;

 

[Lionel]

The single existing on/direction switch simply connects the battery to the
motor in forward polarity or reverse polarity. I want/need to keep that for
direction control (and being quite custom fit, it requires a bit of a hack to
replace it with anything). If I want a PWM solution, adding a (small) pot
w/switch solves most of the problem: I get speed variation and with the
switch I can short across the MOSFET for full speed (MOSFET only gives 90
percent, or so). But that requires the PWM circuit to remain running (it
needs to be powered in order to fire the MOSFET, doesn't it).

Why? - Why not just have the switch bypass the MOSFET completely?

Now if I can only find a mini-size 500K pot with push/pull or
push-on/push-off switch. Every parts house seems to have huge minimum orders
for a Bourns PTR91 series w/switch or stopped carrying it.

For a single unit, it might be easier to salvage one from the junk
box.

 

[DaveC]

Why? - Why not just have the switch bypass the MOSFET completely?

Which switch bypass the MOSFET? (By my count, we're talkin' about 3 switches.
I lost track of which you mean...)

1. The original fwd-off-rev rocker, that's gotta stay. That is the current
means to connect polar voltage (+ or -) to the motor.
2. Pot and switch (plan to locate and install): pot controls duty cycle for
the PWM circuit. The switch bypasses the MOSFET when full-speed is
desired....
3. No plans for a 3rd switch, but I mentioned that if I could figure out how
to turn off the 555 (or 7555) easily, I'd do so. Unless I add a 3rd switch
(which doesn't seem possible with the limited real estate -- both internal
and external -- on this tool) I'm going to use a CMOS 7555 and let it run all
the time.

For a single unit, it might be easier to salvage one from the junk
box.

Your junk box must be fuller than mine. Care to donate to the cause? (c: I
was hopin' to find something useable from a dimmer switch at the hardware
store (a mini push-on/push-off switch and pot combo). But the ones that
aren't sliders (which most of them are) are full-size pots. Dang!

 

[Lionel]

Which switch bypass the MOSFET? (By my count, we're talkin' about 3 switches.
I lost track of which you mean...)

1. The original fwd-off-rev rocker, that's gotta stay. That is the current
means to connect polar voltage (+ or -) to the motor.
2. Pot and switch (plan to locate and install): pot controls duty cycle for
the PWM circuit. The switch bypasses the MOSFET when full-speed is
desired....
3. No plans for a 3rd switch, but I mentioned that if I could figure out how
to turn off the 555 (or 7555) easily, I'd do so. Unless I add a 3rd switch
(which doesn't seem possible with the limited real estate -- both internal
and external -- on this tool) I'm going to use a CMOS 7555 and let it run all
the time.

This whole thing is getting insanely complex. All you really need is a
dial to set the PWM ratio (including down to zero), & a switch to set
the direction. Using a switching pot to cut off all power when the
tool isn't it in use is a good idea. A cute way to change direction
would be to use an integrated H-bridge to drive the motor, & switch an
inverter in or out of the PWM output with a simple SPST switch, or a
single set of contacts on the existing switch. Do all that, & all you
need to do is add the pot, & rewire the existing switch.

Your junk box must be fuller than mine. Care to donate to the cause?

If you live in Melbourne, Australia, no problem. ;^)

(c: I
was hopin' to find something useable from a dimmer switch at the hardware
store (a mini push-on/push-off switch and pot combo). But the ones that
aren't sliders (which most of them are) are full-size pots.

Find or buy one of those cheap little portable radios. The volume
control is typically a tiny 5K or 10K log pot with an integral switch.
With careful design, one should be a good match for an RC time delay.