Well-known 555 desulfator : correct duty cycle

Discussion in 'General Electronics Chat' started by Willen, Jul 27, 2017.

  1. Willen

    Thread Starter Member

    Nov 13, 2015
    187
    25
    As an experiment, I made a lead acid battery desulfator. Frequency is around 1.2KHz and 6% duty cycle. When I connected the device in a battery, the circuit is working. I can see the LED is glowing little and I can hear its switching (magnetic flux developed around L1 and L2) in my AM receiver. But the IRFZ44 (60V, 0.028 ohms, 50A) is getting VERY hot within few second. Small heatsink is also not efficient to prevent the heating. (I had modified some of the thing so I am in doubt.) I think it's not normal so I didn't tried one of my 2kg heatsink. :)

    Do I need to decrease duty cycle even more? Will it work for desulfation? Or any mistake around the circuitry?

    IMPORTANT: the LTspice simulation file attached below is not totally same as the circuit posted below, but PRETTY close. But the circuit is totally same as the desulfator I made.
     
    Last edited: Jul 27, 2017
  2. crutschow

    Expert

    Mar 14, 2008
    15,902
    4,243
    Are you certain the MOSFET is wired correctly to the proper pinouts?
     
  3. Willen

    Thread Starter Member

    Nov 13, 2015
    187
    25
    Hi, When I was operating at 6% duty cycle 1.2KHz it was extremely hot eve in small heatsink. Also two of the parallel 100uF around output was getting hot gradually. So I decreased the dutycycle to 1% and changed these two parallel capacitors with BULKY one rated 220uF 400V. Now all seems fine.

    But the 1% duty cycle brust can dissolve the sulfate back to the acid? Desulfator experienced needed.
     
  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    6,288
    1,004
    Looking at other working circuit diagrams, your fet is not being turned off fast enough, by putting a pnp transistor to drive the gate, also the inductors have different values

    Don't understand why you need an extra psu?
     
    Last edited: Jul 28, 2017
    cmartinez, atferrari and Willen like this.
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    6,889
    1,414
    With a 10V supply to the 555 the output voltage at pin 3 will be only about 8.8V. That may not be enough to switch your particular example of the IRFZ44 on fully.
     
    Willen likes this.
  6. Willen

    Thread Starter Member

    Nov 13, 2015
    187
    25
    Hi,
    -Why not enough? Due to the output voltage or current or impedance?
    -Won't a pullup resistor to the 10V from the gate work? If need PNP then the schematic below work?
    Curiosity from other post:
    Does the partially turned on MOSFET dissipate more heat/power than the saturated?
     
    Last edited: Jul 28, 2017
  7. dendad

    Member

    Feb 20, 2016
    353
    100
    If a FET is partially turned off (or on) it is really just a resistor and will dissipate like one.
    An ideal switched FET is either short or open so no power is dissipated. Unfortunately, in the real world, the FET cannot turn on to be a short, but can get pretty close if the gate drive is sufficient. Most of the power dissipated is in the switching on and switching off times as that cannot be instantaneous. The slower the switching (not the frequency, but the "slope" of the transition) the more power is dissipated.
     
    Willen likes this.
  8. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    6,288
    1,004
    Here is one Example....


    zappermk3.jpg.69eed0b63b3594e0cea3196af7bf8a2a.jpg
     
    Willen likes this.
  9. Willen

    Thread Starter Member

    Nov 13, 2015
    187
    25
    I learned two things:
    The 555 supply should be around 12V so that the output is enough to turn on the mosfet. And need to use 'pulldown-transistor' (maybe) to turn off at the speed of transistor's top rated speed. Pretty nice idea!
    Thank you for the sharing!
     
    Last edited: Jul 29, 2017
  10. ian field

    AAC Fanatic!

    Oct 27, 2012
    5,406
    961
    That PNP arrangement is common in high speed SMPSUs - but it shouldn't be necessary in that application. Use it by all means - but watch out for other causes of the hot MOSFET.

    A 555 running from 10V SHOULD get the job done, but a logic level MOSFET would make sure.
     
  11. bertus

    Administrator

    Apr 5, 2008
    16,869
    2,912
    Willen likes this.
  12. Willen

    Thread Starter Member

    Nov 13, 2015
    187
    25
    Thak you for all of the suggestion and correction. I prepared the circuit and set 1.2KHz 1% dutycycle. And I applied it to a badly sulfated battery of around 10Ah 12V lead acid. There was pretty thick layer of white things around battery plates (some said it's sulfate). After a week of use of the desulfator continuously, now I am seeing just 10% white layer has been remained and 90% sulfated dissolved back to the acid. Before battery voltage even with direct charging would not go more than 12V, but now it has more than 13V. Maybe I need to leave the desulfator till another week. So maybe it's working and the totally expired battery will be recovered little bit.

    I realize that a fuse in series with the battery is VERY VERY important, which is not shown in the circuit mistakenly!!
     
    Dodgydave likes this.
  13. DC_Kid

    Distinguished Member

    Feb 25, 2008
    698
    11
    6% 1.2kHz is 0.05ms of on time per 0.833ms period. the gate charge of the FET is 67nC, so can the 555 move 67nC in 0.05ms?
    try running at 50% duty cycle, do they cool down?
    sounds like they are running most of their on time in the linear region.
    i also suspect you have ringing in the gate due to some reactance coupled with Qg. scope it and see.

    your FET has 110ns rise time which is way faster than 0.05ms, so maybe ringing in the gate is keeping that FET in linear too long, or the 555 is not moving the charge needed?
    see https://www.vishay.com/docs/73217/an608a.pdf

    and this gate ringing article
    http://application-notes.digchip.com/009/9-12721.pdf
     
    Last edited: Aug 12, 2017
  14. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    6,288
    1,004
    Have you tried putting a 22K preset in series with the D1 to increase the fet on time, this will saturate the inductors more giving a larger back emf pulse, this will alter the pulse ratio too, ideally a scope would help to improve this design, anyhow i hope it's working well for you...
     
    recklessrog likes this.
  15. recklessrog

    Active Member

    May 23, 2013
    504
    178
    D3 seems unnecessary as the IFRZ44 has a built in diode from source to drain which will be far superior than a 1N4007
     
  16. recklessrog

    Active Member

    May 23, 2013
    504
    178
    Somewhere I have a design for a desulphator made from an ATX power supply. It revived an old tractor battery that I believe is still going strong ten years on!!
    If I can locate it I will post details here. I've moved house twice since so may take me a while as I have about 3 tons of collected electronics and things I've made over the decades.
     
    Last edited: Aug 12, 2017
    Dodgydave likes this.
  17. DC_Kid

    Distinguished Member

    Feb 25, 2008
    698
    11
  18. recklessrog

    Active Member

    May 23, 2013
    504
    178
    From what I remember (back in about 1982), the modified atx psu I used, had the feed back voltage controlled from the 5 volt line and the output for the 12 volt was modified to be halfwave rectified with no smoothing, which was then connected to the battery. It was the high frequency that de-sulphated the battery but I can't remember if it was pulsed or not. It took over a couple of weeks to restore, but it was a big battery.
    I just wish I could think where I put it.
     
    Dodgydave likes this.
  19. Lumenosity

    Member

    Mar 1, 2017
    133
    18
    It's the high voltage that forces the reverse reaction AFAIK.....I've always heard the "pulsing" was a gimmick used to sell fancy and expensive gadgets....but not necessary.
     
  20. Willen

    Thread Starter Member

    Nov 13, 2015
    187
    25
    Hi,
    Your technical advice seems advanced. Being a home made DIY engineer (hobbyist) I cannot do as you are saying. :) But it's a new thing to study. Thank you.

    Hi
    I tried 6% dutycycle, the FET and the pump capacitor (across Gnd to Inductor) was VERY hot. Then I tried 2% and it still hot and then I tried 1%. And it was still hot so I used VERY BIG heatsink.
     
Loading...