pwm need to increase frequency

A

amdx

Guest
I have built the PWM circuit located here,
http://www.cpemma.co.uk/pwm_erg.html
The triangle osc. frequency is about 150 hz.
I want to increase it to about 15 khz.
When I changed the capacitor, C1 and resistor, R1 the waveform
got very distrorted. The risetime of the waveforms was slow.
Are there adjustments that can be made to make the circuit work at 15 khz?
Or is the LM 724 to slow for the application?
Thanks, Mike
 
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:1qcpa4p0sn1m8d88aq4hm600g1q51n92gj@4ax.com...
On Wed, 20 Aug 2008 19:06:09 -0500, "amdx" <amdx@knology.net> wrote:

I have built the PWM circuit located here,
http://www.cpemma.co.uk/pwm_erg.html
The triangle osc. frequency is about 150 hz.
I want to increase it to about 15 khz.
When I changed the capacitor, C1 and resistor, R1 the waveform
got very distrorted. The risetime of the waveforms was slow.
Are there adjustments that can be made to make the circuit work at 15
khz?
Or is the LM 724 to slow for the application?
Thanks, Mike


Yup, LM324 is a pitifully slow opamp. And on some of them, all
sections screw up if one or more of them is driven open-loop, as two
of yours are.

John


Thanks John,
Click to expand...
Ok, so is there a direct replacement IC or do I need to start over.
Mike
 
<stratus46@yahoo.com> wrote in message
news:5eff6515-75a0-4c92-8e0e-fbc77c004c35@a8g2000prf.googlegroups.com...
amdx wrote:
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message
news:1qcpa4p0sn1m8d88aq4hm600g1q51n92gj@4ax.com...
On Wed, 20 Aug 2008 19:06:09 -0500, "amdx" <amdx@knology.net
wrote:

I have built the PWM circuit located here,
http://www.cpemma.co.uk/pwm_erg.html
The triangle osc. frequency is about 150 hz.
I want to increase it to about 15 khz.
When I changed the capacitor, C1 and resistor, R1 the waveform
got very distrorted. The risetime of the waveforms was slow.
Are there adjustments that can be made to make the circuit work
at 15
khz?
Or is the LM 724 to slow for the application?
Thanks, Mike


Yup, LM324 is a pitifully slow opamp. And on some of them, all
sections screw up if one or more of them is driven open-loop, as
two
of yours are.

John


Thanks John,
Ok, so is there a direct replacement IC or do I need to start over.
Mike
Click to expand...
I believe a TL 074 / TL 084 will drop in and be an OK starting point.

G˛

In comparing the unity-gain bandwidth I see the LM324 is 1 and the TL084
is 3.
I'm not sure that's enough, any changes I can make to the circuit to help? I
need to order the part:-(
Thanks, Mike
 
On Wed, 20 Aug 2008 19:06:09 -0500, "amdx" <amdx@knology.net> wrote:

I have built the PWM circuit located here,
http://www.cpemma.co.uk/pwm_erg.html
The triangle osc. frequency is about 150 hz.
I want to increase it to about 15 khz.
When I changed the capacitor, C1 and resistor, R1 the waveform
got very distrorted. The risetime of the waveforms was slow.
Are there adjustments that can be made to make the circuit work at 15 khz?
Or is the LM 724 to slow for the application?
Thanks, Mike
Click to expand...
Yup, LM324 is a pitifully slow opamp. And on some of them, all
sections screw up if one or more of them is driven open-loop, as two
of yours are.

John
 
amdx wrote:
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message
news:1qcpa4p0sn1m8d88aq4hm600g1q51n92gj@4ax.com...
On Wed, 20 Aug 2008 19:06:09 -0500, "amdx" <amdx@knology.net
wrote:

I have built the PWM circuit located here,
http://www.cpemma.co.uk/pwm_erg.html
The triangle osc. frequency is about 150 hz.
I want to increase it to about 15 khz.
When I changed the capacitor, C1 and resistor, R1 the waveform
got very distrorted. The risetime of the waveforms was slow.
Are there adjustments that can be made to make the circuit work
at 15
khz?
Or is the LM 724 to slow for the application?
Thanks, Mike


Yup, LM324 is a pitifully slow opamp. And on some of them, all
sections screw up if one or more of them is driven open-loop, as
two
of yours are.

John


Thanks John,
Ok, so is there a direct replacement IC or do I need to start over.
Mike
Click to expand...
I believe a TL 074 / TL 084 will drop in and be an OK starting point.

G˛
 
On Aug 21, 8:24 am, "amdx" <a...@knology.net> wrote:


 > >>The triangle osc. frequency is about 150 hz.
 > >>I want to increase it to about 15 khz.
 > >>When I changed the capacitor, C1 and resistor, R1 the waveform
 > >>got very distrorted. The risetime of the waveforms was slow.

I believe a TL 074 / TL 084 will drop in and be an OK starting point.


  In comparing the unity-gain bandwidth I see the LM324 is 1 and the TL084
is 3.
I'm not sure that's enough
Click to expand...
The problem is in the output drivers in the LM324; they are very good
at being power-stingy, but not suited (due to crossover distortion)
for
high-audio-frequency operation. The TL084 uses more quiescent current
because it has higher output stage bias current, and lower output
impedance.
You can also add pullup resistors to an LM324, aim for about 1 mA of
current and the outputs of the LM324 will stay in the low-output-
impedance
condition. That 1 mA of added current is triple the LM324's minimum
power
requirement, of course.
 
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