555 driving high power fet (simple)

Discussion in 'The Projects Forum' started by sveindaniel, Feb 14, 2011.

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  1. sveindaniel

    Thread Starter New Member

    Feb 13, 2011
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    Hello

    I am designing a simple, pulsed, low current, DC supply for a batch water ionizer (electrolysis). For making of drinking water.
    I want the pulses to be under 10µs in duration and about 50-70% duty cycle. And so I have configured a 555 circuit to drive a fet that is connected in series with the electrodes.
    The current will be low, under 30mA.

    Heres the link to the fets datasheet: https://www1.elfa.se/data1/wwwroot/assets/datasheets/ddIRF710_data_en.pdf

    In the circuit design I substituted the water tank and electrodes with a resistor, because MultiSim don't have water tank compontents.

    My question is if this circuit will work?

    Here is the simplyfied circuit. Keep in mind that this circuit is only for learning how to drive a high power fet. It is not a representation for actual circuit, which will have safety in place.
    [​IMG]

    Other relevant notes
    - When I measured the current and voltage of another setup with the same water tank and electrodes, but with only rectified 230VAC, I measured 290VDC and about 29mA. This was a continous DC voltage, not pulsed (without the 555 and fet).
    So we are talking about low currents.
    - It is key that the DC pulses has short rise and fall times, therefore the fet.


    It is important for me to keep the circuit simple. I realize that the 555 timer doesn't utilize the FETs switching potential, because this fet has rise and fall times of less than 10ns, and the 555 IC is much slower, so if someone know of a faster simple driver, I am all ears.

    I am no expert in high power fets, so my circuit for driving it might be completely wrong. That's why I am posting here.

    I appreciate any thoughts.

    Kind regards
    Svein Daniel
     
    Last edited: Feb 14, 2011
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    You definitely have to use an isolating transformer before you try to do anything with mains voltage.
    I think that 400V Vds is not enough for such high voltage, you should use a transistor with at least 600V capability to be on the safe side.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    2,536
    Circuits without a transformer is a death trap. We keep getting threads like this, and they keep getting closed. Even with a 1:1 transformer you will have around 320VDC on that sucker. It is still a death trap.

    It looks a lot like a HHO circuit. We get a lot of that here.

    You can electrolyze water with 12VDC, you don't need the extremes you have sketched up. Scale it down, a lot.

    A little potassium hydroxide will increase the conductivity of the water, which is all you really want. It is current, not voltage, that does the trick.

    A 555 can switch faster than the datasheet says, it just can't oscillate faster than 1Mhz.
     
    Last edited: Feb 14, 2011
  4. sveindaniel

    Thread Starter New Member

    Feb 13, 2011
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    OK. This is just a simplified circuit to understand how to drive the fet. It is not to demonstrate the actual setup.

    Thank you, I will look into a higher voltage rated fet.

    I understand the dangers of mains voltage, and the electrodes will not be available for touch. And the final circuit will have current limiting safety in place.

    But I don't understand how this will reach 640VDC ? The maximum theoretical is about 320VDC.
    This is just for ionizing drinking water, not for high power application like producing HHO or something like that.

    Also the electrodes has to be platinum plated, which is very expensive. If I use large electrodes, which cost a lot, I can use low voltage. But the spacing between them has to be very small. These devices cost a lot and need precision.

    On the other hand, if I use higher voltage, the spacing is less important and the electrodes can be small and cheap. The electrodes in my batch ionizer is spaced maybe about 20 cm from each other, so the current is low (20-30mA). But the water is ionized fast.
    Using 30VDC would take 8 hours to prepare 2-4 litres of drinking water. Using 12VDC isn't realistic with my batch ionizer, the spacing between the electrodes is too big. Using 300VDC it takes half an hour or so.
     
  5. John P

    AAC Fanatic!

    Oct 14, 2008
    1,634
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    Your diagram shows the 15V supply as having a ground symbol that's separate from the rest of the circuit. That won't work: it has to connect to the low side of the FET.

    If the voltage driving the ionizer has to be that high, then it has to be. It would be safer with an isolation transformer, because at least then it would be floating relative to ground, though it would still have a "local ground" to worry about. Running the circuit off a ground fault interrupter outlet would be a very good idea, but I'm not sure even that would make you totally safe.

    The 10K resistor (Rd) on the FET gate serves the function of making sure the FET is off if the 555 gets disconnected, so it's a useful feature. The 100 ohm resistor (Rg) will slow the circuit down. If speed is important, can you live with a smaller resistor there? Also, if your frequency is high then you need to consider power dissipation in the resistor. That's based on gate capacitance and the frequency of current pulses--how much charge flows through there, how often.
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    Sorry we do not support direct mains fed circuits.

    Bertus
     
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