555 pwm

[capwizard]

Yet Another Simple Pot-controlled 555 PWM Generator
https://www.instructables.com/id/Yet...-PWM-generato/

Picture
1)555 PWM circuits...We don't need to take a full picture of his original circuit values. See, what parts you have on hand and replace them directly.

2,3,4) it's really like the author said it's 95% adjustable duty cycle. The frequency 229Hz is for Samsung TV PWM 300Hz input signal.

5) eBay boot converter Jack up 18650 to 12v.

[PeteS in CA]

Re: 555 pwm

The 555 timer is not a PWM IC, though it can be used that way. IIRC, Boschert's OL50 and OL65 model power supplies used a 555 timer. These were late 1970s design bipolar transistor-based continuous flyback designs. These were discontinued in the early 1980s because their component count was ~110 components, and the new models ~90 components, i.e. the new models were much less expansive. The OL50 and OL65 were kind of finicky, too, IIRC. OTOH, the new design was scaled in different models from 25W or 30W up to 165W (185W, forced air cooled).

[eccerr0r]

Re: 555 pwm

Agree with PeteS, I don't like calling pretty much any use of a 555 as a "PWM." Rather I'd call it either a variable duty cycle oscillator (which the instructable describes, note that it cannot be modulated by a voltage unless you have a servo controlling the pot) or fixed on(or off) time oscillator that can be controlled by voltage.

Making a 555 into a true PWM circuit requires considerable more circuitry, at that point it might well not have the 555. You can build a true PWM with op amps, and using a similar 8-pin DIP LM358 is sufficient minus the drive strength. Yes, the 555's output transistor is one of its nice features...

[capwizard]

Re: 555 pwm

Thank you guys, I did use the tl494 to make a better PWM, this one is for Samsung TV power board as a led tester, 300Hz PWM brightness control sees if it control current (budm suggested) then I will have cc cv 350v 450mA power supply.

I'm trying to say is, the led tester needs to be cheap and simple, I designed specifically for Africans. (Similar to bug zapper led tester)

[budm]

Re: 555 pwm

That may be good enough for simulating the PWM dimming control, try it and see. It may need to have pull-up resistor if the PWM pin does not have one or one built-in into the IC PWM control pin.

[capwizard]

Re: 555 pwm

1,2)clockwise rotation from the bottom.

3) at 12:00 the current is maximum.

4,5) at the 5 crock current go down to a minimum.

I don't know why? when you put the potentiometer from minimum to maximum the current is from the minimum to a maximum at 12 clock position then go down. Maybe just like they(peters and eccer0r) said I need a better PWM, tomorrow I will try tl494.

The voltage will follow the current with some float. But the results for current limitation are quite satisfactory.

6) if it did not connect to the PWM signal, the tv backlight current almost double. so PWM just let tv gets half the power.

[budm]

Re: 555 pwm

1)clockwise rotation from the bottom.

2) at 12:00 the current is maximum.

3) at the 5 crock current go down to a minimum.

I do not understand the results.

[capwizard]

Re: 555 pwm

Suppose this is a volume control, The maximum volume is in the middle(12 o'clock direction), the volume (TV LED drive current) will gradually decrease from left turn or right turn from 12 o'clock direction.

this is 40" small tv, my conclusion, may be it is an analog signal input, what's why when it turn PWM to an analog signal will has this phenomenon.

--------------------------------------------------------

SmoothBacklight: a Hardware Mod for Improving Backlight PWM Frequency of Your TV
https://wiki.samygo.tv/index.php?tit...ncy_of_Your_TV


PWM driven LED backlight for LCDs - Focus LCDs
https://focuslcds.com/journals/pwm-d...ight-for-lcds/


For many applications a simple logic level FET (Field-Effect Transistor) type transistor is usually used for a driver. The FET needs to be switched through a resistor on the gate to limit gate current when switching. If current limiting is needed, then a resistor is required. Be sure to check the LCD display data sheet for appropriate back-light driving voltages and currents.

If using a micro controller, consideration should be made for connecting to an output pin that supports PWM (timer/counter) output if using hardware function for PWM.

Controlling the port pins directly can be done through a timing function, such as a hardware interrupt or an operating system timing facility (ie: callback). The advantage of this is the PWM signal (to the driver) which can be connected to any output on the processor/computer. The disadvantage is that this uses more processing time (CPU overhead); although, in practice, it is minimal.

[budm]

Re: 555 pwm

The position of the pot will depend on what kind of pot you are using, are you using audio taper pot or linear taper pot.

[capwizard]

Re: 555 pwm

I thought it's because I used audio balance pot? finally, I change another one still the same so it is that 555 circuit has a problem.

I change to TL494, everything seems to be okay and budm's theory is correct, PWM can change the current pretty smoothly.

Picture
1) I adjusted LED backlights current to 10 ma

2) I adjusted current to 100 ma

3) I adjusted current to Maxima

I have a question.....TL494 has two sets of PWM. what can I do about the other one?

[eccerr0r]

Re: 555 pwm

TL494 only has one PWM, though it has two error amplifiers. It's up to you what you want to do with it, could be used for current limiting when used as a voltage regulator.

[capwizard]

Re: 555 pwm

but-but-but tl494 pin number 8 and 11 emitter terminal of BJT output one and two? So it did has two outputs?

Do I have another option for SMD 6 pins mini PWM IC?

[budm]

Re: 555 pwm

I thought it's because I used audio balance pot? finally, I change another one still the same so it is that 555 circuit has a problem.

I change to TL494, everything seems to be okay and budm's theory is correct, PWM can change the current pretty smoothly.

Picture
1) I adjusted LED backlights current to 10 ma

2) I adjusted current to 100 ma

3) I adjusted current to Maxima

I have a question.....TL494 has two sets of PWM. what can I do about the other one?

1) Is the new pot is Linear pot or not.
2) No matter what pot you use, when you rotate the pot are you seeing the pulse width changes?

[capwizard]

Re: 555 pwm

1) it is 10 turns linear potentiometer.

2) yes both are changing the duty cycle from 0 to 100% on my oscilloscope.

use PWM change p-DIM is good enough for current control. (If I want 76 mA there will be no problem to adjust.) Do we have an easy way to change the voltage because all the Samsung power supply designs are all different?

[budm]

Re: 555 pwm

So it works with the 555 PWM then?

[eccerr0r]

Re: 555 pwm

but-but-but tl494 pin number 8 and 11 emitter terminal of BJT output one and two? So it did has two outputs?

Do I have another option for SMD 6 pins mini PWM IC?

It has configurable output, you can tie the two outputs in parallel for more drive strength or have the two outputs complementary to each other if you want to run a half bridge output.

There are some 6-pin SMD SMPS ICs out there...but I don't know any off the top of my head.

[capwizard]

Re: 555 pwm

So it works with the 555 PWM then?

Yes, I think that circuit is no good when I clockwise the potentiometer, current went up then went down. I wanted to try another circuit then I thought about you taught me to use pull up resistor but I used 100-ohm resistor and 555 timer burn out.

[capwizard]

Re: 555 pwm

It has configurable output, you can tie the two outputs in parallel for more drive strength or have the two outputs complementary to each other if you want to run a half bridge output.

There are some 6-pin SMD SMPS ICs out there...but I don't know any off the top of my head.

But I thought the output waveforms are 90-degree reverse

[budm]

Re: 555 pwm

Yes, I think that circuit is no good when I clockwise the potentiometer, current went up then went down. I wanted to try another circuit then I thought about you taught me to use pull up resistor but I used 100-ohm resistor and 555 timer burn out.

100 Ohms with 5V supply for the 555 worst case will be 5V/100 Ohms =50mA. Pull-up can be 1 ~ 5K Ohms.
555 can source or sink up to 200mA. https://cdn.badcaps-static.com/pdfs/...e1308f3f05.pdf
Something was not hook up right.
BTW: https://www.badcaps.net/forum/attach...4&d=1551108646
Where is 0V line ref on that screen? scope set to DC?

[capwizard]

Re: 555 pwm

1) I was there praying with analog input so, I crank It Up to almost 18 volts.

2) yes, 4I always use DC input.