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'[EE] PIC PWM to 0-10V analog out 500mA (opamp sele'
2011\03\16@092818 by alan smith

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I've used a PIC PWM to generate a analog out (RC filter on the output) but its always been at the rails of the processor..ie...3.3V

So now I'm required to generate a 0-10V, so I'm thinking just an opamp for this with a gain of 3, with the rails of the opamp at least 12V (but I have 24V so it will probably be that)..but they need 500mA drive.

So...looking for an opamp that will give me 500mA drive, or is it better to use a transistor to drive that much current?


2011\03\16@095021 by Olin Lathrop

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alan smith wrote:
> I've used a PIC PWM to generate a analog out (RC filter on the
> output) but its always been at the rails of the processor..ie...3.3V
>
> So now I'm required to generate a 0-10V, so I'm thinking just an
> opamp for this with a gain of 3, with the rails of the opamp at least
> 12V (but I have 24V so it will probably be that)..but they need 500mA
> drive.
>
> So...looking for an opamp that will give me 500mA drive, or is it
> better to use a transistor to drive that much current?

If you filter the signal first and then amplify it, you essentially make a
linear regulator.  500mA out at 10V from a 24V supply is going to dissipate
7W.  That's more than even a big transistor can dissipate without some sort
of heat sinking.

A better answer could be to have the PIC PWM drive switches at the full 0
and 24 volt power rails.  Now you essentially have a switching power supply
produce the 0-10V output from the 24V input.  It will dissipate far less if
done right.  If you're driving something mechanical, like a motor or
solenoid, then this is all you need.  Other things might require some
filtering, like a series choke, possibly followed by a capacitor.  It will
depend on the requirements of your load, which you didn't mention.


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Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

2011\03\16@124824 by alan smith

picon face
The load is a small DC motor.

If I understand what your saying, the PWM will drive the gate of a FET that is the switch node, but I would still need the inductor, cap and diode to make act like a switching supply..correct?


2011\03\16@144626 by Oli Glaser

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On 16/03/2011 16:48, alan smith wrote:
> The load is a small DC motor.
>
> If I understand what your saying, the PWM will drive the gate of a FET that is the switch node, but I would still need the inductor, cap and diode to make act like a switching supply..correct?
>
>

If what (I'm assuming) you want to do is control the speed of the motor, then just using PWM with a MOSFET (plus flyback diode) will dissipate far less power than controlling it in a linear fashion, as the MOSFET is either fully on or off.
For instance in a recent project for simple 12V/1A motor speed control I used a 10F with software PWM + MOSFET (TO-220 package), adjustable using a pot read by the ADC. No heat sink was required.
What exactly are you trying to do? How accurate do you need the speed control? Do you want to be able to reverse direction?

2011\03\16@163347 by IVP

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> So...looking for an opamp that will give me 500mA drive, or is
> it better to use a transistor to drive that much current?

Audio amp IC ? Quite a selection in the 10W - 20W range

Analogue is not as good as PWM for speed control if you want
to keep the torque u

2011\03\16@184726 by alan smith

picon face
Getting more info from the customer...wouldnt it be nice if they would just tell me everything...

The 0-10V is the range for the motor to rotate 0 to 180 degrees, so 5V would be 90 degrees, etc.  So it doesnt have to reverse, its simply converted to this. I am trying to get specs on the actual motor


2011\03\16@191549 by Michael Watterson

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On 16/03/2011 22:47, alan smith wrote:
> Getting more info from the customer...wouldnt it be nice if they would just tell me everything...
>
> The 0-10V is the range for the motor to rotate 0 to 180 degrees, so 5V would be 90 degrees, etc.  So it doesnt have to reverse, its simply converted to this. I am trying to get specs on the actual motor
>
>
>
That doesn't sound like the 0 ... 10V is really driving the motor, but feeding a controller of servo + gearbox or stepper.

2011\03\16@192831 by Isaac Marino Bavaresco

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Em 16/3/2011 13:48, alan smith escreveu:
> The load is a small DC motor.
>
> If I understand what your saying, the PWM will drive the gate of a FET that is the switch node, but I would still need the inductor, cap and diode to make act like a switching supply..correct?


The motor itself will be your inductor. You don't need the capacitor,
only the flyback diode.
You don't need to keep the voltage on the motor constant, only the
current (with some ripple).

Isaac

2011\03\16@193420 by Oli Glaser

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On 16/03/2011 23:14, Michael Watterson wrote:
> On 16/03/2011 22:47, alan smith wrote:
>> Getting more info from the customer...wouldnt it be nice if they would just tell me everything...
>>
>> The 0-10V is the range for the motor to rotate 0 to 180 degrees, so 5V would be 90 degrees, etc.  So it doesnt have to reverse, its simply converted to this. I am trying to get specs on the actual motor
>>
>>
>>
> That doesn't sound like the 0 ... 10V is really driving the motor, but
> feeding a controller of servo + gearbox or stepper.
>

I thought so too - which also makes me wonder whether 500mA is really needed.
More info needed, knowing the motor model number would help (plus details of controller chip/board if present)

2011\03\17@091307 by Olin Lathrop

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alan smith wrote:
> Getting more info from the customer...wouldnt it be nice if they
> would just tell me everything...

Grow up and stop blaming the customer.  You're the one who screwed up.  Your
job as engineer is to gather the requirements if these are not already known
before designing something.  Most customers don't understand what you need
to know.  After all, that's why they hire you.  They also tend to imagine a
solution and then ask for that implementation.  Good engineers will ask
questions until they are satisfied they know what is really needed and what
will solve the customer's true problem, not necessarily the one they first
presented.

If you can't handle this, you should be working for a large company with
senior people around.  Don't try to be a consultant.

> The 0-10V is the range for the motor to rotate 0 to 180 degrees, so
> 5V would be 90 degrees, etc.  So it doesnt have to reverse, its
> simply converted to this. I am trying to get specs on the actual
> motor

Stop and think.  From this description you should be able to tell you're not
dealing with a raw motor.  All previous suggestions are therefore
unapplicable.  It also means new suggestions are pointless until you get the
true interface spec.  Do you know what "servo" means?


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Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

2011\03\17@121621 by Bob Blick

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On Thu, 17 Mar 2011 08:13 -0500, "Olin Lathrop" wrote:
{Quote hidden}

Hey! Up in your lofty perch, can you hear me? Down here, yeah!
There's more than one way to do things. You are comfortable with it one
way. Maybe his customer doesn't like being put through your meat
grinder. Maybe they already talked to you and liked Alan's personality
better. I know you won't deal with customers that can't come up with a
complete specification. That leaves plenty of customers that have money
to pay and need help developing the spec as well as designing the
product.

I don't see how Alan is not a "good engineer" even by your definition.
He is preparing his next round of questions for the customer. We're
helping. There's nothing wrong with that. It's actually a more
up-to-date way of doing it, utilizing the hive mind as a resource.

Bob

-- http://www.fastmail.fm - Access all of your messages and folders
                         wherever you are

2011\03\17@123320 by Olin Lathrop

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Bob Blick wrote:
> There's more than one way to do things. You are comfortable with it
> one way. Maybe his customer doesn't like being put through your meat
> grinder.

Getting at the real requirements isn't a meat grinder, but is a essential
part of engineering.  You can't design something if you don't know what it
needs to do.

> I know you won't deal with customers that can't come up with a
> complete specification.

Not at all, but when a customer can't describe the requirements it's my job
to ask the right questions to get them.  Often by understanding the system
one or two levels up you discover that there is a better way to solve the
overall problem.  In any case, you don't start designing before
understanding the system.

> I don't see how Alan is not a "good engineer" even by your definition.
> He is preparing his next round of questions for the customer.

The point was that these were questions he should have asked and gotten
answers to before jumping in and attempting the design.  It was a design
problem he originally came here with.  He should have seen for himself he
wasn't ready to ask that question yet.

His latest information raises more questions than it answers.  What he is
telling us is inconsistant with itself.  Motors don't go to fixed positions
based on a voltage by themselves.  That requires a control system around
them.  That means he's really talking to the control system and not driving
the motor directly, so the motor's raw specs don't matter.  This means he
doesn't know what is really being driven, which again means its premature to
design the drive circuit for it.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

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