Opamp Transistor Driver Assistance

[EdV]

I need to speed control a 12 VDC 1 amp brushless fan using some
somewhat limited resources. I have:

1. 6 open collector transistors that I can control from a SW
interface. I plan to use that as sort of a DAC.

2. Radio Shack for parts - IRF510 N channel fet for drive, LM324, caps
and resistors.

3. A preference toward driving linear rather than PWMing because
adding another noise source to test setups is kind of a pain. I know
the brushless fan commutating will cause some but I have to have the
fan.

My question:

I know(imagine?) the IRF510 is going to heat up and increase
resistance. Does using a simple circuit similar to the power
amplifier shown in National LM324 datasheet applications cover the
bases for thermal drift? I probably also need to filter the
commutation noise I would see on the feedback.

Any thoughts, comments and suggestions are welcome.

Thanks!

Ed V.

 

[John Popelish]

I need to speed control a 12 VDC 1 amp brushless fan using some
somewhat limited resources. I have:

1. 6 open collector transistors that I can control from a SW
interface. I plan to use that as sort of a DAC.

2. Radio Shack for parts - IRF510 N channel fet for drive, LM324, caps
and resistors.

3. A preference toward driving linear rather than PWMing because
adding another noise source to test setups is kind of a pain. I know
the brushless fan commutating will cause some but I have to have the
fan.

My question:

I know(imagine?) the IRF510 is going to heat up and increase
resistance. Does using a simple circuit similar to the power
amplifier shown in National LM324 datasheet applications cover the
bases for thermal drift? I probably also need to filter the
commutation noise I would see on the feedback.

Any thoughts, comments and suggestions are welcome.

The LM324 opamp will probably be plenty stable in this
application. The big problem to solve is, if you want speed
control, you want to measure speed, not something only
approximately related to speed, like motor voltage, or even
motor voltage compensated for by adding a motor current
factor to it. Do you have a linear, analog speed
measurement signal available for the opamp to force into a
match with a speed setpoint value you provide, in a closed
loop motor voltage controller? If so, the DAC will be
needed only to produce the setpoint value.

 

[EdV]

The LM324 opamp will probably be plenty stable in this
application.  The big problem to solve is, if you want speed
control, you want to measure speed, not something only
approximately related to speed, like motor voltage, or even
motor voltage compensated for by adding a motor current
factor to it.  Do you have a linear, analog speed
measurement signal available for the opamp to force into a
match with a speed setpoint value you provide, in a closed
loop motor voltage controller?  If so, the DAC will be
needed only to produce the setpoint value.

--
Regards,

John Popelish- Hide quoted text -

- Show quoted text -

I just tried a really simple circuit of just "DAC" to the gate of the
IRF 510. As long as I can get 5 stable "settings" I think I can be
ahppy with that.

Perhaps speeed control was too strong of a term.

This circuit is going to be in airconditioned cabinet so the IRF510 is
going to heat up to whatever it heats up in controlled ambient
enviroment. I guess I will use pots for each setting so they can be
"calibrated" when needed.

Thanks!

Ed V.

 

[John Popelish]

I just tried a really simple circuit of just "DAC" to the gate of the
IRF 510. As long as I can get 5 stable "settings" I think I can be
ahppy with that.

Perhaps speeed control was too strong of a term.

This circuit is going to be in airconditioned cabinet so the IRF510 is
going to heat up to whatever it heats up in controlled ambient
enviroment. I guess I will use pots for each setting so they can be
"calibrated" when needed.

It sounds like most of your problem of stability can be
solved with a large enough heat sink.

 

[JosephKK]

I need to speed control a 12 VDC 1 amp brushless fan using some
somewhat limited resources. I have:

1. 6 open collector transistors that I can control from a SW
interface. I plan to use that as sort of a DAC.

2. Radio Shack for parts - IRF510 N channel fet for drive, LM324, caps
and resistors.

3. A preference toward driving linear rather than PWMing because
adding another noise source to test setups is kind of a pain. I know
the brushless fan commutating will cause some but I have to have the
fan.

My question:

I know(imagine?) the IRF510 is going to heat up and increase
resistance. Does using a simple circuit similar to the power
amplifier shown in National LM324 datasheet applications cover the
bases for thermal drift? I probably also need to filter the
commutation noise I would see on the feedback.

Any thoughts, comments and suggestions are welcome.

Thanks!

Ed V.

What is it? Every doofus thinks that simple voltage or current control
is applicable to DC "brushless" motors. It just does not even apply
(unless there has been interesting changes in the underlying technology
that i have not heard of).
The self adjusting computer fans have a relatively complex IC to drive
them.
If you just gotta have a easy to control variable speed motor or fan get
a RC hobbyist variable speed motor and controller. they can even be
purchased as ducted fans (see "jet" engines).