Electrical Engineering - FET circuit to switch power to IC?

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FET circuit to switch power to IC?

DaveC

2007-03-19, 9:25 am

A 555 circuit running from battery power needs to be turned on and off. A
switch for this purpose cannot be added due to lack of space. The "on" signal
will be from an existing momentary pb switch (this switch controls an LED)
which can also provide battery voltage (7.2v).

I'm thinking that a cap can be connected to the gate of a FET and charged
when the pb switch is pressed. This FET will power the 555. sThe cap's
voltage will leak off at a constant rate based on the FET's gate
characteristics.

The "on" time isn't critical; something between several minutes to 1 day
would be OK. I just need to preserve the battery between charges.

Questions:

1. Will the FET's gate remain high (once high, always high?) even after the
cap has bled down?
2. Probably need a reverse-connected diode between the pb switch and the
cap/gate (to avoid leakage via the LED)? What kind of diode should I use?

What am I overlooking?

Thanks,--
DaveC
me@bogusdomain.net
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Palindrome

2007-03-19, 9:25 am

Joerg

2007-03-19, 1:25 pm

Tim Wescott

2007-03-19, 1:25 pm

DaveC wrote:

> A 555 circuit running from battery power needs to be turned on and off. A
> switch for this purpose cannot be added due to lack of space. The "on" signal
> will be from an existing momentary pb switch (this switch controls an LED)
> which can also provide battery voltage (7.2v).
>
> I'm thinking that a cap can be connected to the gate of a FET and charged
> when the pb switch is pressed. This FET will power the 555. sThe cap's
> voltage will leak off at a constant rate based on the FET's gate
> characteristics.
>
> The "on" time isn't critical; something between several minutes to 1 day
> would be OK. I just need to preserve the battery between charges.
>
> Questions:
>
> 1. Will the FET's gate remain high (once high, always high?) even after the
> cap has bled down?
> 2. Probably need a reverse-connected diode between the pb switch and the
> cap/gate (to avoid leakage via the LED)? What kind of diode should I use?
>
1. No, unless I'm mistaken about your proposed schematic. The FET gate
is connected to it's external connection, so it's voltage is the same as
the little wire coming out of the package.

2. If you mean a diode to keep the cap from powering the LED -- yes,
you'll need that.

You will find, if you try this, that the FET will have a very
unpredictable and soft turn-off. You won't have much leakage from the
FET gate itself; most of the leakage will either be inherent to the cap
or will be parasitic leakage from wet scunge on the board -- so your
'on' time will vary wildly with temperature and humidity.

You'd do much better to use a resistor and cap in parallel -- the
resistor current would then overwhelm other effects. You'd do better
yet to use a 556 with one side connected as a monostable multivibrator
to keep the power on, and the other side doing whatever you want to do.
This would give you a slightly more predictable discharge, and a nice
'snap' action to the turn-off.

Even better than all of that, consider using an 8-pin (or six; they're
getting small these days) microprocessor. You'll be able to control
that non-critical turn-on time to absurdly fine degrees, and for every
"oh dang, I forgot to consider that" issue you run into you'll need a
bit more code instead of a bit more circuitry.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

DaveC

2007-03-19, 1:25 pm

> You'd do better
> yet to use a 556 with one side connected as a monostable multivibrator
> to keep the power on, and the other side doing whatever you want to do.
> This would give you a slightly more predictable discharge, and a nice
> 'snap' action to the turn-off.

Well, I might just leave the 7555 powered up perpetually (see other reply in
this thread). But if I don't, the multivibrator is a good choice -- schedule
power-off down the microsecond (c: Thanks.

> Even better than all of that, consider using an 8-pin (or six; they're
> getting small these days) microprocessor. You'll be able to control
> that non-critical turn-on time to absurdly fine degrees, and for every
> "oh dang, I forgot to consider that" issue you run into you'll need a
> bit more code instead of a bit more circuitry.

Ah, but I thought all suggestions in this NG of using a u-controller should
be accompanied by code and circuit examples (c;

Seriously, I have not used PICs or such, and have yet to make that quantum
(in my universe) leap. So an electrickery solution is the sole consideration,
for now.

Thanks,
--
DaveC
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DaveC

2007-03-19, 1:25 pm

> t's a " hermetically sealed tilt and motion detector

It's a great idea, but the tool lives almost perpetually in the boot of my
car. That means it'll forever be getting "tilted", probably.

> Use a low power, or ultra low power 555, and you can leave the thing
> powered up, just inhibit it from oscillation eg:

Yeah, I was ready to do that (calculated that using the power spec for the
CMOS part it would run for over a year on one battery charge). But I thought
better of leaving something always powered on. No good reason, I guess, to
not do it.

If I go the always-on route, why disable oscillation? Oscillation requires
more power? The spec sheet says typical is 100 uA; I presume this is running
current? And in this configuration the MOSFET it triggers (PWM) would also be
running, although not passing current; a direction switch connects the FET to
the motor when it comes time to actually use the tool. Does the FET use any
current when not actually sourcing or sinking current?

(If this sounds familiar, it's my cordless drill that I asked assistance
getting a PWM circuit for it last week.)

Thanks,
--
DaveC
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Joerg

2007-03-19, 1:25 pm

Tim Wescott wrote:

> DaveC wrote:
>
> 1. No, unless I'm mistaken about your proposed schematic. The FET gate
> is connected to it's external connection, so it's voltage is the same as
> the little wire coming out of the package.
>
> 2. If you mean a diode to keep the cap from powering the LED -- yes,
> you'll need that.
>
> You will find, if you try this, that the FET will have a very
> unpredictable and soft turn-off. You won't have much leakage from the
> FET gate itself; most of the leakage will either be inherent to the cap
> or will be parasitic leakage from wet scunge on the board -- so your
> 'on' time will vary wildly with temperature and humidity.
>
> You'd do much better to use a resistor and cap in parallel -- the
> resistor current would then overwhelm other effects. You'd do better
> yet to use a 556 with one side connected as a monostable multivibrator
> to keep the power on, and the other side doing whatever you want to do.
> This would give you a slightly more predictable discharge, and a nice
> 'snap' action to the turn-off.
>
> Even better than all of that, consider using an 8-pin (or six; they're
> getting small these days) microprocessor. You'll be able to control
> that non-critical turn-on time to absurdly fine degrees, ...

But that comes with penalties: 7.2V requires that there be a regulator
to supply the uC. Last time I used one of those new-fangled micro-power
versions we were greeted with a few explosions, leaving dead regulators
with craters in them. The mfg was unable to tell us why and not willing
to throw it all onto SPICE (and that raised my neck hairs). Anyhow, then
I chucked all that and went CD4060. Works from 5-12V unregulated, no
issues whatsoever. As a nice side effect it was also cheaper. A lot cheaper.

> ...and for every
> "oh dang, I forgot to consider that" issue you run into you'll need a
> bit more code instead of a bit more circuitry.
>

True. But I have seen that leading to a wee bit of slacking off in terms
of design discipline. When it's pure hardware one has to be much more
careful to avoid egg in the face when the boards come back from fab.

--
Regards, Joerg

http://www.analogconsultants.com

Palindrome

2007-03-19, 1:25 pm

DaveC wrote:
>
>
> It's a great idea, but the tool lives almost perpetually in the boot of my
> car. That means it'll forever be getting "tilted", probably.

Use a position tilt switch, rather than a motion detector. If the thing
goes vertical during normal driving, you have bigger problems....

Set the time constant so that physically pointing the thing vertical
every few minutes is all that is needed to keep it powered up.

>
>
>
>
> Yeah, I was ready to do that (calculated that using the power spec for the
> CMOS part it would run for over a year on one battery charge). But I thought
> better of leaving something always powered on. No good reason, I guess, to
> not do it.
>
> If I go the always-on route, why disable oscillation? Oscillation requires
> more power?

During normal operation it is charging a capacitor and then discharging
it - both through resistors - a waste of power best avoided.

The spec sheet says typical is 100 uA; I presume this is running
> current? And in this configuration the MOSFET it triggers (PWM) would also be
> running, although not passing current; a direction switch connects the FET to
> the motor when it comes time to actually use the tool. Does the FET use any
> current when not actually sourcing or sinking current?

Depends on the circuit but better to disable switching to save power.

>
> (If this sounds familiar, it's my cordless drill that I asked assistance
> getting a PWM circuit for it last week.)
>
Rather guessed that..

--
Sue

Tim Wescott

2007-03-19, 1:25 pm

Joerg wrote:

> Tim Wescott wrote:
>
-- snip --

>
> True. But I have seen that leading to a wee bit of slacking off in terms
> of design discipline. When it's pure hardware one has to be much more
> careful to avoid egg in the face when the boards come back from fab.
>
Agreed.

But.

I've found that if you're on a tight schedule for a low-production
volume device you can get by with a careful analysis of what's really
necessary from hardware, followed by making sure you put in more insies
and outsies than you think you'll ever need and at least 5x the micro
you think you'll need.

At that point "we'll fix it in software later" is actually achievable
(unless you underestimate, in which case you have that egg on your face).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Palindrome

2007-03-19, 1:25 pm

Tim Wescott wrote:
> Joerg wrote:
>
> -- snip --
>
> Agreed.
>
> But.
>
> I've found that if you're on a tight schedule for a low-production
> volume device you can get by with a careful analysis of what's really
> necessary from hardware, followed by making sure you put in more insies
> and outsies than you think you'll ever need and at least 5x the micro
> you think you'll need.
>
> At that point "we'll fix it in software later" is actually achievable
> (unless you underestimate, in which case you have that egg on your face).
>

"The customer is always right" rarely applies when the customer is
writing a specification. Yet the designer is expected to provide what
the customer needs, not what the customer says he wants. Requirements
change, even after the unit has entered service and has been in use for
several years. Being able to field-program the entire system, with a
down-time of minutes rather than days, is worth quite a lot. With the
recent hike in energy costs, a whole lot of systems are now being
re-programmed for economy, rather than performance. Even F1 cars have an
eocnomy button, these days..

--
Sue

Joerg

2007-03-19, 5:25 pm

Palindrome wrote:

> Tim Wescott wrote:
>
>
> "The customer is always right" rarely applies when the customer is
> writing a specification. Yet the designer is expected to provide what
> the customer needs, not what the customer says he wants. ...

True. And then there is feature creep. "The boss just came up with this
idea that if we made a tiny change we could also use it for...". So
yeah, there are clients for whom I keep things rather flexible. Because
I just know that it'll be coming. When they call in for the umpteenth
change and apologize I can tell them that it's perfectly ok.

> ... Requirements
> change, even after the unit has entered service and has been in use for
> several years. Being able to field-program the entire system, with a
> down-time of minutes rather than days, is worth quite a lot. With the
> recent hike in energy costs, a whole lot of systems are now being
> re-programmed for economy, rather than performance. ...

With many of my designs the units are just chucked and replaced. Of
course, that isn't necessarily good from an environmental point of view
but technicians would often refuse to touch things that have been used
in a catheter lab for a long time. It can be quite gross.

> ...Even F1 cars have an eocnomy button, these days..
>

Wow. I definitely didn't know that.

--
Regards, Joerg

http://www.analogconsultants.com

Joerg

2007-03-19, 5:25 pm

Palindrome wrote:

> DaveC wrote:
>
>
>
> Use a position tilt switch, rather than a motion detector. If the thing
> goes vertical during normal driving, you have bigger problems....
>

A friend in Botswana told me that in some areas this could be a quite
normal modus of vehicle operation. T'is why they all have winches with them.

--
Regards, Joerg

http://www.analogconsultants.com

Tim Wescott

2007-03-19, 5:25 pm

Palindrome wrote:
> Tim Wescott wrote:
>
> With the
> recent hike in energy costs, a whole lot of systems are now being
> re-programmed for economy, rather than performance. Even F1 cars have an
> eocnomy button, these days..
>
That reminds me of a stunning stock car race that I watched a couple of
decades ago. Richard Petty went into economy mode about 50 or 100 miles
short of the finish. He suckered the other leaders into not going in
for pit stops, then drifted across the finish line about the same time
he ran out of gas. They ran out earlier.

There was much wailing and gnashing of teeth afterward about what a
tricky bastard he was. It made for good theater -- but he still won.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

DaveC

2007-03-19, 5:25 pm

>> It's a great idea, but the tool lives almost perpetually in the boot of my
[color=darkred]
> Use a position tilt switch, rather than a motion detector. If the thing
> goes vertical during normal driving, you have bigger problems....

LOL! Point taken...

> Set the time constant so that physically pointing the thing vertical
> every few minutes is all that is needed to keep it powered up.

So, the circuit as described previously (FET, cap, diode), now fed 7.2v by
the tilt switch?

Thanks,
--
DaveC
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Palindrome

2007-03-19, 8:25 pm

DaveC wrote:
>
>
>
>
> LOL! Point taken...
>
>
>
>
> So, the circuit as described previously (FET, cap, diode), now fed 7.2v by
> the tilt switch?
>

Not quite. I can't remember the whole thread but assume that you are
using the 555 as a variable duty-cycle oscillator, driving the FET? If
so, don't connect the top of the timing chain directly to the supply,
but connect it via the tilt switch and a small R, and put a C between
the top of the chain and ground. The small R is there to limit the C
charging current to the tilt switch Imax.

When the tilt switch operates, C is charged very quickly via the small R
and will slowly discharge via the timing chain, keeping the oscillator
running until it is discharged (the tool will have to be given a quick
flip to recharge it and keep the tool running). Set the C to a value
that will give a couple of minutes running with the values of resistors
in the timing chain.

In the horizontal position, the tilt switch won't close, the oscillator
won't run and the FET will stay off. The CMOS 555 and FET will draw a
(negligable) amount of leakage current.

Put the tool vertical, the tilt switch operates, the oscillator runs and
will continue running for a couple of minutes and then stop - unless the
tool is momentarily turned to be vertical every couple of minutes.

--
Sue

cr500r

2007-03-20, 8:25 pm

> Not quite. I can't remember the whole thread but assume that you are using
> the 555 as a variable duty-cycle oscillator, driving the FET? If so, don't
> connect the top of the timing chain directly to the supply, but connect it
> via the tilt switch and a small R, and put a C between the top of the
> chain and ground. The small R is there to limit the C charging current to
> the tilt switch Imax.
>
> When the tilt switch operates, C is charged very quickly via the small R
> and will slowly discharge via the timing chain, keeping the oscillator
> running until it is discharged (the tool will have to be given a quick
> flip to recharge it and keep the tool running). Set the C to a value that
> will give a couple of minutes running with the values of resistors in the
> timing chain.
>
> In the horizontal position, the tilt switch won't close, the oscillator
> won't run and the FET will stay off. The CMOS 555 and FET will draw a
> (negligable) amount of leakage current.
>
> Put the tool vertical, the tilt switch operates, the oscillator runs and
> will continue running for a couple of minutes and then stop - unless the
> tool is momentarily turned to be vertical every couple of minutes.
>
> --
> Sue

That's a creative solution, but why can't the 555 be powered from the switch
that would normally turn on the drive motor?

Jeff

DaveC

2007-03-21, 3:25 am

> That's a creative solution, but why can't the 555 be powered from the switch
> that would normally turn on the drive motor?

Trying to minimize modification/hacking of the tool.

The switch is a 2-pole (+ and - battery conductors), 3-position design
(fwd-off-rev). The MOSFET output from the 555 circuit is fed to the switch
which connects it to the motor for driving in both directions. How would you
suggest the 555 circuit be powered by this in both fwd and rev modes? How to
drive the motor in both modes? More switches? More FETS?

I don't want to hack the existing assemblies any more than I have to. The 555
and components can easily be placed between the battery and the direction
switch with minimal intrusion into the tool's form factor. I just need to
find an elegant means to disable it when it's not being used. Sue suggested
such a creative solution, as you noted.

Basically I think the proposed design integrates variable speed nicely and
minimizes intrusion into the tool's existing form without having to redesign
it from the ground up.

Now if I could just find that *$_)#! pot with push-switch here in USA. Bourns
makes exactly what I need, but none of their distributors stocks it, and
Bourns is asking minimum 10,000 units for an order. Anyone interested in
9,9999 pots? Guess I'll have to order that one from Maplin in UK. Are there
issues with customs when mailorder electronic components hit our borders? Any
recommendations when Yanks order from Maplin?

Thanks,
--
DaveC
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DaveC

2007-03-21, 3:25 am

> Guess I'll have to order that one from Maplin in UK.

Sorry, it's at Farnell:
<http://uk.farnell.com/jsp/endecaSea...jsp?SKU=1307056>

10A switch in such a small package. Gotta love those Brits!
--
DaveC
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Palindrome

2007-03-21, 3:25 am

cr500r wrote:
>
>
> That's a creative solution, but why can't the 555 be powered from the switch
> that would normally turn on the drive motor?
>

Because the switch that normally turns on the drive motor is usually a
two-pole, centre-off - wired one way to swap the supply connections in
order to reverse the motor.

Whilst there are many ways of dealing with that, the ones I can think of
are fairly complicated, compared to a simple, cheap, reliable, extremely
small tilt switch. Which has the added advantage of automatically
switching almost everything off whenever the tool is laid flat for a few
minutes.

--
Sue

DaveC

2007-03-21, 3:25 am

> Whilst there are many ways of dealing with that, the ones I can think of
> are fairly complicated, compared to a simple, cheap, reliable, extremely
> small tilt switch. Which has the added advantage of automatically
> switching almost everything off whenever the tool is laid flat for a few
> minutes.

Very cool design. I'm still entranced. (c: And I'm still waiting to see your
house integration controller...
--
DaveC
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Palindrome

2007-03-21, 3:25 am

DaveC wrote:
>
>
> Trying to minimize modification/hacking of the tool.
>
> The switch is a 2-pole (+ and - battery conductors), 3-position design
> (fwd-off-rev). The MOSFET output from the 555 circuit is fed to the switch
> which connects it to the motor for driving in both directions. How would you
> suggest the 555 circuit be powered by this in both fwd and rev modes? How to
> drive the motor in both modes? More switches? More FETS?
>
> I don't want to hack the existing assemblies any more than I have to. The 555
> and components can easily be placed between the battery and the direction
> switch with minimal intrusion into the tool's form factor. I just need to
> find an elegant means to disable it when it's not being used. Sue suggested
> such a creative solution, as you noted.
>
> Basically I think the proposed design integrates variable speed nicely and
> minimizes intrusion into the tool's existing form without having to redesign
> it from the ground up.
>
> Now if I could just find that *$_)#! pot with push-switch here in USA. Bourns
> makes exactly what I need, but none of their distributors stocks it, and
> Bourns is asking minimum 10,000 units for an order. Anyone interested in
> 9,9999 pots? Guess I'll have to order that one from Maplin in UK. Are there
> issues with customs when mailorder electronic components hit our borders? Any
> recommendations when Yanks order from Maplin?
>
> Thanks,

I'm slighlty puzzled why you want to use a pot at all?

Why not use a qtc pill and have finger-tip speed control?

http://www.maplin.co.uk/Module.aspx...a=pill&doy=21m3

--
Sue

DaveC

2007-03-21, 9:25 am

> I'm slighlty puzzled why you want to use a pot at all?

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

> http://www.maplin.co.uk/Module.aspx...a=pill&doy=21m3

(How can Maplin show such a tempting item and not provide data sheet or
additional specs?) Data sheet:
<http://www.peratech.co.uk/pdfs/p001v004.pdf>
shows that resistance is logarithmic. How to map this to achieve linear motor
response?

> Why not use a qtc pill and have finger-tip speed control?

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...
--
DaveC
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cr500r

2007-03-21, 1:25 pm

"Palindrome" <me9@privacy.net> wrote in message
news:wb4Mh.4080$rN.1132@fe09.news.easynews.com...
....
> Whilst there are many ways of dealing with that, the ones I can think of
> are fairly complicated, compared to a simple, cheap, reliable, extremely
> small tilt switch. Which has the added advantage of automatically
> switching almost everything off whenever the tool is laid flat for a few
> minutes.
>
> --
> Sue

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

Palindrome

2007-03-21, 1:25 pm

cr500r wrote:
> "Palindrome" <me9@privacy.net> wrote in message
> news:wb4Mh.4080$rN.1132@fe09.news.easynews.com...
> ...
>
>
>
> 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.
>
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..

--
Sue

cr500r

2007-03-21, 1:25 pm

"Palindrome" <me9@privacy.net> wrote in message
news:qacMh.10593$rN.10423@fe09.news.easynews.com...
> cr500r wrote:
> Where would you put the speed controller?
....
> 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

Palindrome

2007-03-21, 1:25 pm

cr500r wrote:
> "Palindrome" <me9@privacy.net> wrote in message
> news:qacMh.10593$rN.10423@fe09.news.easynews.com...
>
>
> ...
>
>
>
> 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.

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.

--
Sue

cr500r

2007-03-21, 1:25 pm

"Palindrome" <me9@privacy.net> wrote in message
news:EEcMh.2685$Wg6.1098@fe07.news.easynews.com...
> 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.
>
> --
> Sue

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

Palindrome

2007-03-21, 1:25 pm

cr500r wrote:
> "Palindrome" <me9@privacy.net> wrote in message
> news:EEcMh.2685$Wg6.1098@fe07.news.easynews.com...
>
>
>
> 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..

--
Sue

DaveC

2007-03-21, 1:25 pm

> 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:
--
DaveC
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cr500r

2007-03-21, 1:25 pm

"Palindrome" <me9@privacy.net> wrote in message
news:g4dMh.14471$Jt2.12902@fe04.news.easynews.com...
....
> 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..
>
> --
> Sue

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

jasen

2007-03-24, 8:25 pm

On 2007-03-21, DaveC <me@privacy.net> wrote:
>
> Trying to minimize modification/hacking of the tool.
>
> The switch is a 2-pole (+ and - battery conductors), 3-position design
> (fwd-off-rev). The MOSFET output from the 555 circuit is fed to the switch
> which connects it to the motor for driving in both directions. How would you
> suggest the 555 circuit be powered by this in both fwd and rev modes? How to
> drive the motor in both modes? More switches? More FETS?

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

DaveC

2007-03-28, 8:25 pm

> do this:

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

Thanks!
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DaveC

2007-03-28, 8:25 pm

> http://www.maplin.co.uk/Module.aspx...a=pill&doy=21m3

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?
--
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