Linear regulator

J

Jonas

Guest
Hello

HAs anyone ever tried to use a linear regulator to regulate a PWM
signal?

My situation:
Input signal, 24V PWM 400Hz
I want to have 12V PWM signal 400Hz
I was thinking of using a linear regulator (type LM317) and use the
24V PWM as input and get 12V PWM on the output, does anyone know of
any problems related to this (lifelength, EMC etc).

/Jonas
 
"Jonas" <jonas.adiator@hotmail.com> wrote in message
news:2904ab5d-51b6-4720-9579-f9a6516094c4@m26g2000yqb.googlegroups.com...
Hello

HAs anyone ever tried to use a linear regulator to regulate a PWM
signal?

My situation:
Input signal, 24V PWM 400Hz
I want to have 12V PWM signal 400Hz
I was thinking of using a linear regulator (type LM317) and use the
24V PWM as input and get 12V PWM on the output, does anyone know of
any problems related to this (lifelength, EMC etc).

/Jonas
Click to expand...
Well, sort of...

In the early 1980's when the programming voltage for a 2716 EPROM was
25 volts and the read voltage on Vpp was 5 volts. I used an LM7805 linear
regulator to do the job. The ground leg was in series with a variable
resistor
which was set to provide 25 volts. The junction of the variable resistor
and
the 7805 was connected to an open collector TTL buffer.

I was able to switch cleanly between 25V and 5V at a rate of over 10KHz.

I hope this information helps.
 
On Nov 19, 4:10 am, Jonas <jonas.adia...@hotmail.com> wrote:

Input signal, 24V PWM 400Hz
I want to have 12V PWM signal 400Hz
I was thinking of using a linear regulator (type LM317)
Click to expand...
An LM317 wants/needs 4 mA to bias normally; if you have
that much drive, why not just use a resistor and zener diode?
For better precision, maybe a TL431 shunt regulator?
 
On 19 Nov, 20:12, whit3rd <whit...@gmail.com> wrote:
On Nov 19, 4:10 am, Jonas <jonas.adia...@hotmail.com> wrote:

Input signal, 24V PWM 400Hz
I want to have 12V PWM signal 400Hz
I was thinking of using a linear regulator (type LM317)

An LM317 wants/needs 4 mA to bias normally; if you have
that much drive, why not just use a resistor and zener diode?
For better precision, maybe a TL431 shunt regulator?
Click to expand...
Thanks, well I need to have a stable 12V PWM output since the input
can vary +/- 8V. I have a load of 100mA on the output.
 
Jonas wrote:
On 19 Nov, 20:12, whit3rd <whit...@gmail.com> wrote:
On Nov 19, 4:10 am, Jonas <jonas.adia...@hotmail.com> wrote:

Input signal, 24V PWM 400Hz
I want to have 12V PWM signal 400Hz
I was thinking of using a linear regulator (type LM317)

An LM317 wants/needs 4 mA to bias normally; if you have
that much drive, why not just use a resistor and zener diode?
For better precision, maybe a TL431 shunt regulator?

Thanks, well I need to have a stable 12V PWM output since the input
can vary +/- 8V. I have a load of 100mA on the output.
Click to expand...
A quick and dirty method if your load is relatively stiff is to use a 60/120
voltage divider. This will give you 100mA into your 120Ohm equivalent load
if it is purely resistive. The resistors will dispate about 2.4W and 1.2W
each.

This assume your inpute is 24V and doesn't fluxuate. You said Input "signal"
of 24V then say "the input can vary +/-8V".

If your input does very and/or your load is not stiff(which it can be made
stiff but essentially follows whit3rd suggestion) then you can try to use a
linear regulator with the PWM after the regulator.

The idea is to put a mosfet between the regulators output and the load with
your 24V pwm driving the mosfet.

This way you get regulation and PWM.

Pwr->LReg->Mosfet->Load->Gnd
or
Pwr->LReg->Load->Mosfet->Gnd


This will pretty much suit your needs but is fairly more complex if you
don't know about mosfets and such.

Two main things to note. The mosfet cannot be driven directly from the 24V
PWM since it is too high so some method must be used to reduce the voltage.
Since you have such a low PWM rate a simple voltage divider would probably
work just fine.

High side NCh requires a floating gate which can be achieved through several
means. Because of the low PWM rate you can simply this using a few passives
and zeners.

You can use a low side Nch and probably can achieve the same result without
any issues(depends on what you are driving though as it might not work
well).


The LReg might require a small load for biasing which is easy enough by
adding an appropriate resistor in parallel with the the load + mosfet.

Also, the LReg could be replaced with a switching regulator or other method
since it's only purpose is to supply power.

The main point is that your approaching it wrong and hopefully I've cleared
up how you should go about it. It is possible to use whit3rd method if you
don't wanna go about it this way. It all really depends on what your trying
to do.

It is a bad idea to PWM the regulators power though as it is not designed
for such things. Although I have no real idea if it would work or not. If
you really want to try it though then go ahead and see. Just don't expect it
to work nor expect it to work in general.
 
On Nov 19, 11:34 am, Jonas <jonas.adia...@hotmail.com> wrote:
On 19 Nov, 20:12, whit3rd <whit...@gmail.com> wrote:

On Nov 19, 4:10 am, Jonas <jonas.adia...@hotmail.com> wrote:

Input signal, 24V PWM 400Hz
I want to have 12V PWM signal 400Hz

An LM317 wants/needs 4 mA to bias normally; if you have
that much drive, why not just use a resistor and zener diode?

Thanks, well I need to have a stable 12V PWM output since the input
can vary +/- 8V. I have a load of 100mA on the output.
Click to expand...
If the power dissipation is bothersome, add a transistor; collector
of NPN to the 24V pulse input, base to 12 V zener, emitter drives
the load with 12V maximum...

A linear regulator is slow to initialize, you'll get a few
microseconds
of unspecified behavior at each rising edge. If there's any
capacitance
on the output (as most regulator datasheets recommend), the reverse
bias at shutdown is also a problem (clamp diode from output to input
is the usual recommendation). One could use a linear regulator
from steady 24V with a logic-disable input, but that wastes the
high-current driver you already have.
 
"Jonas" <jonas.adiator@hotmail.com> wrote in message news:2904ab5d-51b6-4720-9579-f9a6516094c4@m26g2000yqb.googlegroups.com...
Hello

HAs anyone ever tried to use a linear regulator to regulate a PWM
signal?

My situation:
Input signal, 24V PWM 400Hz
I want to have 12V PWM signal 400Hz
I was thinking of using a linear regulator (type LM317) and use the
24V PWM as input and get 12V PWM on the output, does anyone know of
any problems related to this (lifelength, EMC etc).

/Jonas
Click to expand...
 
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