PWM generator

hgmjr

Joined Jan 28, 2005
9,029
I don't think you will have any problem using the FZT705. Its switching speed is up to the task. The only penalty that you will pay for the use of a darlington device in place of a non-darlington device is the Vce saturation voltage for the darlington device is just under a volt for the small amount of collector current that the device will be switching. Your output will swing between +4V and 0V which should be adequate for the CMOS input you are driving. Vce saturation for the non-darlington device is more like 0.2V which would give you a output high level very close to the +5V power rail.

Just something to keep in mind as you go forward with you project.

Good Luck,
hgmjr
 

johnfoody

Joined Apr 26, 2005
11
Originally posted by hgmjr@May 11 2005, 03:22 PM
I don't think you will have any problem using the FZT705. Its switching speed is up to the task. The only penalty that you will pay for the use of a darlington device in place of a non-darlington device is the Vce saturation voltage for the darlington device is just under a volt for the small amount of collector current that the device will be switching. Your output will swing between +4V and 0V which should be adequate for the CMOS input you are driving. Vce saturation for the non-darlington device is more like 0.2V which would give you a output high level very close to the +5V power rail.

Just something to keep in mind as you go forward with you project.

Good Luck,
hgmjr
[post=7622]Quoted post[/post]​
Once again thaks very much for your help and I'll let you know how I get on.

john
 

hgmjr

Joined Jan 28, 2005
9,029
Originally posted by johnfoody@May 11 2005, 10:51 AM
Once again thaks very much for your help and I'll let you know how I get on.

john
[post=7623]Quoted post[/post]​
Would it be possible for you to divulge a bit of information about your PWM application?

hgmjr
 

johnfoody

Joined Apr 26, 2005
11
Originally posted by hgmjr@May 12 2005, 09:46 PM
Would it be possible for you to divulge a bit of information about your PWM application?

hgmjr
[post=7657]Quoted post[/post]​
Sorry, once again i've been busy the last few weeks. But i did find the time to etch that circuit and implement it with the resistor & diode approach. It works really well! i'll ask my supoervisor about how much i can divulge and get back to you, it's nothing too exciting.
Once again Thanks a million, it's exactly what i needed.

cheers john
 

hgmjr

Joined Jan 28, 2005
9,029
Thanks for the feedback.

Congratulations on the successful implementation of the design. I don't need to know the application's details. Knowing you were able to use the circuit to achieve the results you required is reward enough for me.

Good Luck,
hgmjr
 

Rowan

Joined Jun 1, 2005
12
Hi hgmjr,

Would the above PWM generator application be able to be modified to suit an analogue out of between 0-10vdc?

What I would like to do is use a low current analogue out to vary the brightness of LED modules from say 0 to 12v through a mosfet. (Would 1kHz be fast enough?)

Thanks in advance.

Regards,

Rowan
 

hgmjr

Joined Jan 28, 2005
9,029
Originally posted by Rowan@Jun 1 2005, 06:30 PM
Hi hgmjr,

Would the above PWM generator application be able to be modified to suit an analogue out of between 0-10vdc?

What I would like to do is use a low current analogue out to vary the brightness of LED modules from say 0 to 12v through a mosfet. (Would 1kHz be fast enough?)

Thanks in advance.

Regards,

Rowan
[post=8132]Quoted post[/post]​
Let me see if I am clear on your requirements as described in your post.

You have an analog control signal that varies between 0 to 10 that you want to use to control the dutycycle of a 1 Khz signal that swings between 0 volts and 12 Volts?

Is that a correct reading of your requirements?

I don't see any reason why these requirements could not be met with a few adjustments to the components in the circuit. I would be happy to assist you by revamping the circuit to meet these requirements if that is what you need.

hgmjr
 

Rowan

Joined Jun 1, 2005
12
Originally posted by hgmjr@Jun 2 2005, 11:17 AM
Let me see if I am clear on your requirements as described in your post.

You have an analog control signal that varies between 0 to 10 that you want to use to control the dutycycle of a 1 Khz signal that swings between 0 volts and 12 Volts?

Is that a correct reading of your requirements?

I don't see any reason why these requirements could not be met with a few adjustments to the components in the circuit.  I would be happy to assist you by revamping the circuit to meet these requirements if that is what you need.

hgmjr
[post=8134]Quoted post[/post]​
That is correct, although I am not too sure on the dutycycle frequency, whatever will make sure the LED's won't appear as though they're just flashing quickly.

I made a circuit using a 555 timer, with a .0047uF cap and a 1MOhm pot which varies the duty cycle, and from the output of the 555 timer through a power mosfet the LED brightness goes from 'off' to 'full' (as though i was varying brightness by adjusting their invput voltage from 0 to 12v, the led modules have internal resistors), but I have no idea what the frequencies would actually be, but i'll start with 1 Khz and go from there :)

The LED modules draw around 1A.

Any help would be much appreciated! I'm sorry if I was a bit unclear in my original, and probably this post too. Thanks for the quick reply :)

Regards,

Rowan
 

hgmjr

Joined Jan 28, 2005
9,029
Rowan,

Your original posting was pretty clear, I just wanted to make sure I was interpreting the information correctly. Long distance problem solving is by nature full of inefficiencies. Depending on the complexity of the problem it can take several clarifying posts to get the details straight.

One thing I need that has not been mentioned yet is the power supply voltage that is available to power the PWM. Obviously, you have a +12V power source to drive the bank of LEDs. Is this circuit to be powered from the +12 volt supply? If not, what power source are you planning to use. This PWM circuit should not take more than 50 milliamps to run.

As for the frequency, 1 KHz should be fine to start with. This frequency is definitely high enough that you will not see any flicker from the LEDs. We can alway adjust the frequency to a more appropriate one by tweaking a couple of the component values.

What is your time frame for completing this project?

hgmjr
 

Rowan

Joined Jun 1, 2005
12
hgmjr,

The PWM circuit can run off the 12v supply.

I also just measured the output of the 555 timer and found that the frequency range was between 30Hz (~40% duty cycle, which is where the LED's apear almost to be off) to 190Hz (~99%, full brightness).. not too sure on those duty cycle readings though... but yes, 1kHz will be fine! (unless its better to run at lower frequencies? anything ~200Hz would be great then).

There's plenty of time for this project, this is an 'on the side' job :)


Thanks again for your prompt replies!

Regards,
Rowan
 

hgmjr

Joined Jan 28, 2005
9,029
Hi Rowan,

I need another important piece of information if I may.

Can you tell me whether your LED module is referenced to ground or referenced to the +12V supply?

The answer to this question will allow me to select the correct mosfet either n-channel or p-channel.

Thanks,
hgmjr.
 

Rowan

Joined Jun 1, 2005
12
Hi hgmjr,

With the 555 timer circuit I was using a n-channel mosfet with the LED's referenced to the 12v rail, ie;


---------------------------12v
|
|+
LED (w/internal resistor)
|-
|
drain
|
MOSFET------------gate--------------from 555 timer output.
|
source
|
-------------------------0V



I think thats right, was using a IRF520 power mosfet. Thanks :)

Regards,

Rowan
 

hgmjr

Joined Jan 28, 2005
9,029
The IRF520 appears to be a very good choice for this application. I will proceed with the design using the IRF520 as the LED module driver.

Thanks,
hgmjr
 

hgmjr

Joined Jan 28, 2005
9,029
Hi Rowan,

Here is the first cut at the PWM you requested.

[attachmentid=695]

Let me know if you have any questions about the circuit design.

hgmjr
 

Rowan

Joined Jun 1, 2005
12
Awesome, thanks so much for your work! I'll let you know how it goes, still waiting on the arrival of the 2903.


Regards,

Rowan

:D
 

hgmjr

Joined Jan 28, 2005
9,029
I went over the design with a fine tooth comb. However, I am not infallible so don't hesitate to let me know if anything looks amiss.

hgmjr
 
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