Is this circuit going to be a problem?

B

Bob Engelhardt

Guest
I want to control a small motor with a sensor (variable resistance).
The sensing works fine, but I'm wondering whether I can power the motor
with the full-wave, unregulated source while the control power is
regulated. I'd like to do this so the regulator doesn't have to carry
the motor current. Will the MOSFET have a problem with the unregulated
voltage on its drain?

https://www.dropbox.com/s/9kvvfu4mks2wl38/lastscan.jpg?dl=0

Thanks,
Bob
 
On Sat, 06 Feb 2016 13:55:32 -0500, Bob Engelhardt
<BobEngelhardt@comcast.net> wrote:

I want to control a small motor with a sensor (variable resistance).
The sensing works fine, but I'm wondering whether I can power the motor
with the full-wave, unregulated source while the control power is
regulated. I'd like to do this so the regulator doesn't have to carry
the motor current. Will the MOSFET have a problem with the unregulated
voltage on its drain?

https://www.dropbox.com/s/9kvvfu4mks2wl38/lastscan.jpg?dl=0

Thanks,
Bob
Click to expand...

How does that work?

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
In article <n95fiq015df@news4.newsguy.com>, BobEngelhardt@comcast.net
says...
I want to control a small motor with a sensor (variable resistance).
The sensing works fine, but I'm wondering whether I can power the motor
with the full-wave, unregulated source while the control power is
regulated. I'd like to do this so the regulator doesn't have to carry
the motor current. Will the MOSFET have a problem with the unregulated
voltage on its drain?

https://www.dropbox.com/s/9kvvfu4mks2wl38/lastscan.jpg?dl=0

Thanks,
Bob
Click to expand...

Should work, just don't let the main supply voltage get down
to low near the gate voltage.

Btw, when drawing out PTC/NTC or what ever the senser type is, its
customary to use proper symbol so not to confuse others. Most know
by the drawing that it's simply a variable R device, many love to know
more about it.

Jamie
 
On Sat, 06 Feb 2016 13:55:32 -0500, Bob Engelhardt wrote:

I want to control a small motor with a sensor (variable resistance). The
sensing works fine, but I'm wondering whether I can power the motor with
the full-wave, unregulated source while the control power is regulated.
I'd like to do this so the regulator doesn't have to carry the motor
current. Will the MOSFET have a problem with the unregulated voltage on
its drain?

https://www.dropbox.com/s/9kvvfu4mks2wl38/lastscan.jpg?dl=0

Thanks,
Bob
Click to expand...

I won't say that WILL work, because there's not enough detail, but it
certainly COULD work. Particularly if the design intent is to have the
motor either on or off, but nothing in between.

I'd put a catch diode across the motor to absorb the inductive kick of
the motor when the FET turns off.

--
www.wescottdesign.com
 
On 2/6/2016 2:36 PM, M Philbrook wrote:
Should work, just don't let the main supply voltage get down
too low near the gate voltage.
Click to expand...

What would happen if the motor supply was less that Vg? I.e., if its
ripple was such that at times it was less.

Btw, when drawing out PTC/NTC or what ever the senser type is, its
customary to use proper symbol so not to confuse others. Most know
by the drawing that it's simply a variable R device, many love to know
more about it.

Jamie
Click to expand...

That is the best sensor symbol, I think. The sensor is 2 electrodes
immersed in water, to a varying depth. Resistance varies inversely as
depth.


On 2/6/2016 4:49 PM, Tim Wescott wrote:

I won't say that WILL work, because there's not enough detail, but it
certainly COULD work. Particularly if the design intent is to have the
motor either on or off, but nothing in between.

I'd put a catch diode across the motor to absorb the inductive kick of
the motor when the FET turns off.
Click to expand...

"will":"could" - fair enough. I was really asking "Will this circuit
NOT work?" I wanted to know if there was something inherent that would
prevent it.

In thinking about it more, I realize that the "large" motor current
would cause a lot more ripple on the power cap and the regulator could
drop out. Rather than worrying about the cap size needed to prevent
that, I have isolated it with a diode. The motor works "fine" with
unfiltered full wave.

But that does mean that the motor voltage will drop below Vg. I'm not
clear what happens then. I don't even know what "Vds" means when it's
saturated. Isn't saturated-mode Vds = Ids * Rds(on) ?

Since this is s.e.BASICS, I know that I can ask such basic stuff.

Revised, with more detail:
https://www.dropbox.com/s/9kvvfu4mks2wl38/lastscan.jpg?dl=0

At the risk of provoking answers to questions that I'm not asking, here
is the big picture of what I'm doing:
I have a CPAP machine with a dehumidifier and that dehumidifier has a
water reservoir of a cup (240ml) or so. Which means that it has to be
filled EVERY night. So I intend to sense when the water is low and turn
on a pump to refill it. The sensor is 2 immersed electrodes and the
pump is a auto windshield-washer pump. It will probably run for a few
seconds at a time, a couple of times a night.

Thanks,
Bob
 
On Sat, 06 Feb 2016 20:20:27 -0500, Bob Engelhardt wrote:

On 2/6/2016 2:36 PM, M Philbrook wrote:
Should work, just don't let the main supply voltage get down
too low near the gate voltage.

What would happen if the motor supply was less that Vg? I.e., if its
ripple was such that at times it was less.

Btw, when drawing out PTC/NTC or what ever the senser type is, its
customary to use proper symbol so not to confuse others. Most know
by the drawing that it's simply a variable R device, many love to
know more about it.

Jamie

That is the best sensor symbol, I think. The sensor is 2 electrodes
immersed in water, to a varying depth. Resistance varies inversely as
depth.


On 2/6/2016 4:49 PM, Tim Wescott wrote:

I won't say that WILL work, because there's not enough detail, but it
certainly COULD work. Particularly if the design intent is to have the
motor either on or off, but nothing in between.

I'd put a catch diode across the motor to absorb the inductive kick of
the motor when the FET turns off.


"will":"could" - fair enough. I was really asking "Will this circuit
NOT work?" I wanted to know if there was something inherent that would
prevent it.

In thinking about it more, I realize that the "large" motor current
would cause a lot more ripple on the power cap and the regulator could
drop out. Rather than worrying about the cap size needed to prevent
that, I have isolated it with a diode. The motor works "fine" with
unfiltered full wave.

But that does mean that the motor voltage will drop below Vg. I'm not
clear what happens then. I don't even know what "Vds" means when it's
saturated. Isn't saturated-mode Vds = Ids * Rds(on) ?

Since this is s.e.BASICS, I know that I can ask such basic stuff.

Revised, with more detail:
https://www.dropbox.com/s/9kvvfu4mks2wl38/lastscan.jpg?dl=0

At the risk of provoking answers to questions that I'm not asking, here
is the big picture of what I'm doing:
I have a CPAP machine with a dehumidifier and that dehumidifier has a
water reservoir of a cup (240ml) or so. Which means that it has to be
filled EVERY night. So I intend to sense when the water is low and turn
on a pump to refill it. The sensor is 2 immersed electrodes and the
pump is a auto windshield-washer pump. It will probably run for a few
seconds at a time, a couple of times a night.
Click to expand...

You need to worry about the gate voltage of the FET falling below
threshold, but that's more or less independent of the motor voltage.

As long as your 12V supply doesn't drop below 10V or so the circuit
should work with most power FETs out there.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
 
On 7/02/2016 9:20 AM, Bob Engelhardt wrote:
On 2/6/2016 2:36 PM, M Philbrook wrote:
Should work, just don't let the main supply voltage get down
too low near the gate voltage.

What would happen if the motor supply was less that Vg? I.e., if its
ripple was such that at times it was less.

Btw, when drawing out PTC/NTC or what ever the senser type is, its
customary to use proper symbol so not to confuse others. Most know
by the drawing that it's simply a variable R device, many love to know
more about it.

Jamie

That is the best sensor symbol, I think. The sensor is 2 electrodes
immersed in water, to a varying depth. Resistance varies inversely as
depth.


On 2/6/2016 4:49 PM, Tim Wescott wrote:

I won't say that WILL work, because there's not enough detail, but it
certainly COULD work. Particularly if the design intent is to have the
motor either on or off, but nothing in between.

I'd put a catch diode across the motor to absorb the inductive kick of
the motor when the FET turns off.


"will":"could" - fair enough. I was really asking "Will this circuit
NOT work?" I wanted to know if there was something inherent that would
prevent it.

In thinking about it more, I realize that the "large" motor current
would cause a lot more ripple on the power cap and the regulator could
drop out. Rather than worrying about the cap size needed to prevent
that, I have isolated it with a diode. The motor works "fine" with
unfiltered full wave.

But that does mean that the motor voltage will drop below Vg. I'm not
clear what happens then. I don't even know what "Vds" means when it's
saturated. Isn't saturated-mode Vds = Ids * Rds(on) ?

Since this is s.e.BASICS, I know that I can ask such basic stuff.

Revised, with more detail:
https://www.dropbox.com/s/9kvvfu4mks2wl38/lastscan.jpg?dl=0

At the risk of provoking answers to questions that I'm not asking, here
is the big picture of what I'm doing:
I have a CPAP machine with a dehumidifier and that dehumidifier has a
water reservoir of a cup (240ml) or so. Which means that it has to be
filled EVERY night. So I intend to sense when the water is low and turn
on a pump to refill it. The sensor is 2 immersed electrodes and the
pump is a auto windshield-washer pump. It will probably run for a few
seconds at a time, a couple of times a night.

Thanks,
Bob






Perhaps change the reg to a low dropout to be sure ??
Click to expand...
 
On 2/6/2016 7:20 PM, Bob Engelhardt wrote:

In thinking about it more, I realize that the "large" motor current
would cause a lot more ripple on the power cap and the regulator could
drop out. Rather than worrying about the cap size needed to prevent
that, I have isolated it with a diode. The motor works "fine" with
unfiltered full wave.

But that does mean that the motor voltage will drop below Vg. I'm not
clear what happens then. I don't even know what "Vds" means when it's
saturated. Isn't saturated-mode Vds = Ids * Rds(on) ?

Since this is s.e.BASICS, I know that I can ask such basic stuff.

Revised, with more detail:
https://www.dropbox.com/s/9kvvfu4mks2wl38/lastscan.jpg?dl=0
Click to expand...

At the risk of provoking answers to questions that I'm not asking,
Click to expand...

That's a risk!
Will the electrodes cover with gas bubbles and alter your resistance?
Mikek
 
On 2/6/2016 10:00 PM, Tim Wescott wrote:
On Sat, 06 Feb 2016 20:20:27 -0500, Bob Engelhardt wrote:
You need to worry about the gate voltage of the FET falling below
threshold, but that's more or less independent of the motor voltage.

As long as your 12V supply doesn't drop below 10V or so the circuit
should work with most power FETs out there.
Click to expand...

Thanks. I just got around to trying it and it works fine.
 
On 2/7/2016 9:36 PM, amdx wrote:

Will the electrodes cover with gas bubbles and alter your resistance?
Mikek
Click to expand...

I had read that people use AC for electrode sensing, to prevent
electrolysis, but I was hopeful that I had enough headroom in my
ultra-low precision sensing that I could use DC.

I have done some testing and whatever bubbles there might be forming do
not effect my sensing. Probably because my electrodes are about 1 sq in
each and the sense current is less than a ma.

Bob
 
In article <n9ja5202ul7@news3.newsguy.com>, BobEngelhardt@comcast.net
says...
On 2/7/2016 9:36 PM, amdx wrote:

Will the electrodes cover with gas bubbles and alter your resistance?
Mikek


I had read that people use AC for electrode sensing, to prevent
electrolysis, but I was hopeful that I had enough headroom in my
ultra-low precision sensing that I could use DC.

I have done some testing and whatever bubbles there might be forming do
not effect my sensing. Probably because my electrodes are about 1 sq in
each and the sense current is less than a ma.

Bob
Click to expand...

maybe you can make some browns gas!

Jamie
 
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