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PWM help

Discussion in 'General Electronics Discussion' started by LectricCircuit, May 17, 2017.

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  1. LectricCircuit

    LectricCircuit

    33
    2
    Apr 7, 2017
    so I made a PWM circuit using a 555 timer. It's to add to my unregulated mosfet mod I use to vape with. I used a 0.1uf ceramic capacitor a 0.01uf ceramic capacitor a 555 timer ic a 1k ohm resistor a 10k potentiometer and 2 diodes (1n4148). These are the exact specs that the schematic calls for that I got off the Internet. I finished it and installed it into my dual series 18650 mosfet mod. Fully charged the batteries are at 8.4 volts. In theory when I turn the pot up to 100 there should be no pulse and it should just be direct voltage from battery and when I turn the pot down the pulse width of 0v should increase and the pulse width of 8.4 volts should decrease. Being that it's switching on and off at a very high speed it will show on a multimeter that the voltage is decreasing as I turn the pot down towards zero. Even though that's not what's really going on. If the pot is turned to 25 this means that in one cycle the mosfet should be off for 75% of the time and on for 25% of the time. The problem I'm having is that For some reason when I have the pot turned up to 100, the output voltage is only at about 3.34 volts even though my multimeter reads the batteries at 8.4 volts. Everything is working. It adjusts the voltage up and down. With pot at zero it's reading 0.64 volts. So I know it's doing its job as far as adjusting the voltage but I have no clue why it would only be 3.4 volts when the pot is turned to 100. At 100 the pot should be completely open allowing the full voltage of whatever the batteries charged to. Thought maybe the resistor was too high so I added another 1k resistor in parallel with the other 1k resistor which would bring the resistance down to 500 ohms but surprisingly the output voltage at 100% went down about 0.2v. Please help. I have the diagram I used attached to this thread. I do not have a scope to test it so I have to work based off logic and theory.
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,639
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    Nov 17, 2011
    Please learn how to draw schematic diagrams that are universally understood. For example like this one found here.

    It is cumbersome to find out which connections goes where from a quasi-realistic image like yours (or a breadboard image often seen here, alas).
    From such a general schematic it is quickly to be seen what function a pin of any component has whereas from an image like yours one has to look up the datasheet to find the funtion of the relevant pin(s).

    Thank you.

    With respect to you question: A 555 based PWM controler cannot be controled between 0% and 100%. There will always be some dead time, otherwise the 555 wil not oscillate at all. So 5% to 95% could possibly be expected. For analyzing the behavior of the circuit an oscilloscope would be a great tool. Lacking that, a soundcard may be 'abused'. Of course you'll have to scale the measured voltages to the input range of the soundcard by adding e.g. a resistive voltage divider.
     
  3. Audioguru

    Audioguru

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    Sep 24, 2016
    I suspect that since you did not say the battery voltage when the PWM circuit was bypassed and the battery was powering the heater directly, then the batteries, battery holder, switches and wiring are heating and throwing away most of the voltage charge.

    Measure the voltage at the heater when the PWM circuit is bypassed.
     
  4. Audioguru

    Audioguru

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    Sep 24, 2016
    Maybe your Mosfet is a cheap fake one from ebay?
    Did you read the datasheet of the 555? Its output can not go higher than 1.2V less than its power supply voltage which should be fine for a real IRFB3813 if the supply voltage stays above about 6V.

    The heater draws a much higher current when cold so where is the supply bypass capacitor that will provide it?
     
  5. LectricCircuit

    LectricCircuit

    33
    2
    Apr 7, 2017
    The diagram attached isn't my design. It's a diagram I found online. I personally do know how to draw a schematic and read schematics but as I didn't know how to make a PWM circuit prior to this, I had to use someone else's design. Also I know that it wouldn't technically be 100% of the battery voltage after going through the PWM circuit but I didn't know the exact specs so I generalized that statement. So thank you for informing me of the actual specs. I will draw up a schematic based of the circuit I built from the diagram and attach it to this thread. I'm fairly new to working with electronics and such but I know the basics. Like I said I don't have a scope to test it but what do you mean by soundcard?
     
  6. LectricCircuit

    LectricCircuit

    33
    2
    Apr 7, 2017
    After disconnecting the PWM circuit completely I tested the voltage at the heating element and it's reading at 4.12v. What in the world could cause that much of a voltage drop? I didn't use the mosfet in the diagram. Instead I used the irlb3034pbf which is the most popular fet among unregulated vaporizers. I don't see how there could be such a significant voltage drop from wiring or battery sled. Also what did u mean when asking where the supply bypass capacitor? I just wired it up as the diagram showed it. Should a add a capacitor somewhere else? And if so where and how many microfarads?
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    9,639
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    Nov 17, 2011
    The audio interface of a PC/laptop (used to be a separate card, hence the name which has stuck even though this is today integrated onto the motherboard in most cases).
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    9,639
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    Nov 17, 2011
    What is the battery voltage (measured) in this circuit when the load is on? Could it be the battery voltage drops from too high a load current?
     
  9. Audioguru

    Audioguru

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    Sep 24, 2016
    I listed everything that reduces the voltage. The battery is #1 especially if it is a cheap Chinese no-name-brand thing bought on ebay.
     
  10. LectricCircuit

    LectricCircuit

    33
    2
    Apr 7, 2017
    Oh ya I know what a sound card is lol. I figured there was a different kind of instrument in electronics called a sound card or something like that. I have on old sound card from my desktop. How could I use that to test this circuit though?
     
  11. LectricCircuit

    LectricCircuit

    33
    2
    Apr 7, 2017
    I have a lot of batteries and of them there are 4 different brands. Brands that I've used in many other mods with no problems whatsoever.
     
  12. LectricCircuit

    LectricCircuit

    33
    2
    Apr 7, 2017
    not 100% sure what you're asking but if I take the atomizer off so there's no coil attached it measures roughly 4.5v at the 510 connector and if I attach the atomizer and test the voltage at the positive leg and negative leg of the coil while firing the mod it measures the same. Just with vape coming off of it lol.
     
  13. Audioguru

    Audioguru

    2,725
    614
    Sep 24, 2016
    If you measure the battery voltage when the heating element measures only 4.12V then it will show you if the battery can supply the high current without dropping its voltage. If the battery voltage is reasonable then measure the voltages of other parts of the circuit to see what is reducing the voltage.

    Measure the voltage at each battery cell, not at the battery connections at the battery holder.
     
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