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Everything posted by Kerrowman

  1. Hi Harry, That makes sense but in the case of the ignition coil HV systems, like in the link you gave, they can operate at a similar frequency of say 200-500Hz and still give very powerful sparks so is the construction of an ignition coil, and also a flyback transformer, different to such a degree that they avoid this pulse width issue? I'm inspired to build one or other of the attached, although the 555 based one has some useful duty and frequency adjustment. Maybe one of these will deliver 10+kV to my cell without significant losses? Julian
  2. Thanks Harry, I will certainly look into using ignition coils or perhaps a Tesla coil. If it's a question of impedance matching then surely I only have the option to up the secondary coil's impedance. Given that at the moment its resistance is about 1 Ohm, then does it make sense to add a 1 MOhm resistor in series with it to bring it more in line with the 1.6 MOhm resistance of the water cell? Or will that not work or introduce other problems? Julian
  3. It's interesting that you, in effect, got + and - 3.7kV across the output of the coil. That is certainly the sort of level I would expect, in which case I would expect more than the 3V I get when I attach my secondary to my load (water cell). Which brings me full circle to my earlier question, why am I getting only 3V across my water cell terminals when your simulation and your ignition coil experiments show much more? Perhaps I should build a Tesla coil and zap my water cell with that? As a square wave can be thought of a complex mix of different frequencies of sine waves (Fourier a
  4. Harry , I need to unpick what you have found. The highest output across the secondary of your coil was 159.8V, when I presume one would expect several kV? (I don't know what the turns ratio is in such a coil but ignition systems are designed to produce 5-15kV) Am I correct in saying that to get a higher secondary output one will need a bigger primary current - to store more energy in the primary coil? Also are you saying that the current in the primary is a function of the Gate frequency - perhaps a lower frequency will allow more current into the primary? What's DUT? Regards,
  5. Harry, that's very industrious of you to build a physical version of the circuit. Have you have substituted my upper 530 for a light bulb because it is just acting as a variable resistor? Also could you clarify if you got a high voltage on the secondary of the ignition coil? If you did, even with the small currents you measured in the primary, what does this all mean for my current design 'HV Supply 1B' shown a few posts back? Julian
  6. Hi Harry, I'm not clear where your two variable resistors are and in the attached Pic 1 I have put where I think one is. My supply is set at 50V and the gate pulses at 200Hz, 50% so 'hoping' to be able to get 750-1kV out at the secondary when connected to the electrolysis cell. As I come from a Physics background and not electronics I'm trying to understand why increasing the current in the primary will increase the voltage across the cell. Is the problem that the 530 is throttling both the voltage and the current and hence the multiplication by the secondary? From my limited ex
  7. Hi Harry, In your first reply at the start of this thread you suggested using a second MOSFET to control the current instead of a variable resistor, which is why I modified the circuit to include the IRF530. While it works to allow fine control of the current are you now suggesting I put a 50 Ohm resistor instead, based on your simulation? If so then this would surely need to be a 200W resistor (massive?) to be able to cope with 2A through it and I certainly don't have one of those - which is why I presume you suggested the FET in the first place. J
  8. Hi Harry, I inserted a 1.5 Ohm resistor into the Source line of the IRF840 as suggested (Pic 1) and calculated typical readings of the primary current of 30mA when the secondary was unloaded and about 160mA when the cell was connected (Pic 2), The value under load could have gone higher but not as high as 1A. This was with a square wave input of 200Hz from the pulse circuit and 12V from the PSU as before but with my bifilar coils in place (Pic 3). Once again the voltage across the cell drops to 3-4V under load and reads about 220V when the cell is not connected. I know to expect a vo
  9. Thanks. I will see what current is flowing in the primary and report back.
  10. Hi Harry, The cell measure 1.164 M Ohms and a capacitance measurement using the same meter showed 1.058mF. That last reading is surprising since I did a calculation last year based on plate area, separation and the dielectric of water which came to 5.18nF so either the meter reading is unreliable in that context or the distilled water surrounding the cell is contributing in some way. With the resistance measurement that would suggest that the average current flow through the cell was 50/1.164 E6 = 42uA where 50 is a typical rounded voltage peak I'm getting on the scope. I'm unclear h
  11. Hi Harry, I tried using a 0.22uF, 2kV capacitor across the secondary windings and Pic 1 shows the result without it and Pic 2 with the capacitor. Pic 3 is with no capacitor but instead with a diode across the secondary windings, as shown in the circuit in Pic 5 (but without the bifilar coil that I have yet to add in). Pic 4 is with the load attached, the load being a simple cylindrical electrolysis cell in distilled water. It seems that with the load attached, and only about 50mA being drawn from the Variable PSU, the voltage across the secondary windings and cell drops dra
  12. Harry, I will set it up with the load in the next day or so and also add a 1uF, 2kV capacitor across the secondary coil and report back. I'd be interested to see what the simulations shows. Thanks Julian
  13. Hi Harry, I have now built the circuit using an additional FET to regulate the current as shown in pic 1 and it does give me fine control. I’ve used a diode in two different orientations, and also without it to, see the effect on the output but in all cases I get a waveform on the transformer secondary (no load) as in pics 2-4 which shows + and - voltage swings instead of just a + as in your simulation, which is what I need. Pic 2 has no diode, Pic 3 has cathode towards the IRF530 Source and Pic 4 has the diode cathode towards the IRF840 Drain (i.e. reversed). What do
  14. Thanks for doing that. The sharp pulses are fine for what I want which is to feed an electrolyser. You used an IRF530 and, as I have quite a few, I hope the IRF840 will work just as well. Thank you J
  15. I'm putting in gated square wave pulse where the HF part is 100-500Hz and the gating frequency 10-20Hz. I've tried to measure the primary and secondary inductances and got a reading of 720mH for the secondary but just a resistance reading of 155 Ohms for the primary. Based on the turns ratio I'm guessing the primary has an inductance of around 5mH? That would be great if you can quickly simulate the circuit. 😀
  16. Thanks Harry, That Rheostat is far too big to incorporate and your idea of a MOSFET (or BJT?) to control the current is far more elegant and practical. I have revised my circuit in the attached so would that work ok? Jules
  17. I have a circuit (see pic) that includes the primary winding of a 1:20 self wound transformer. Without some form of additional resistance this circuit will draw too much current from the power supply and damage the FET I’m using so I plan to put a rheostat inline with the primary winding to control the maximum current flowing in it. Given the basic equation for power expended in a resistor of P=I^2R, I’m trying to work out the maximum power rating for the rheostat. If I set the input voltage to 50V and set a 100R rheostat to 50Ohms then my current should be 1.0A. In th
  18. Hi Harry, The good point you raise about voltage headroom is presumably why 18V is supplied by the transformer in the full power supply circuit shown on that first link. However, the note nearby says that it has not been built so is it a bit of a gamble that it will work as indicated? The FET approach to dropping volts will work for the time periods I’m using for certain tests but for long term running I will likely need something more efficient. For the FET approach, in Spice simulation the FDS6699S mosfet works we
  19. Hi Harry, As suggested above I have drawn my own version of the power supply on the first link you gave using 5 LM338s. I could try and simulate this in LTSpice but that will take me a while. Any thoughts on whether it would do the job - probably better than the Buck converter? Jules
  20. Hi Harry, I have used this buck converter before but its never supplied even 20A consistently. In the first link you gave it talks about using the LM338 in parallel, as in the attached. Can I not stack 3 of them in parallel to do the job? If so I will assemble a circuit and run it by you? Thanks
  21. Thanks but my particular need is to take 12V, 25A max and make adjustable from 6-12V and a max current of 25A so would the attached from the previous link be suitable? I find the text doesn't make easy reading and is hard to understand. J
  22. Hi Harry, Thanks for the suggestions. The voltage regulator seems a good idea but I don't need the whole power supply but just an add on set up to adjust down the 12V nominal output I already have. Would I need then item 4 in the list? If so how is the voltage energy dissipated and how efficient is it overall? The motor controller uses a PWM approach it seems (12kHz) and I already have a PWM in my setup. Jules
  23. Hi all, I have a PWM power supply that has a nominal output voltage of 12V and a current capacity of 25A continuous. While there is about 0.5V adjustment on the PSU itself, I need to be able to adjust this output to supply 7-12V so I need to drop a few volts after the output. I will lose 1-2V due to a current flow of 10A or more but still need some further adjustment. I attach my suggestion based on an approach I used with a lower powered supply that used an MJE3055 and so am asking: 1. Would this arrangement work with the high power MOSFET (acting as a sort of valve like on a hos
  24. I've just run a Spice simulation and it seems, surprisingly, that the resonant frequency between points A & D is unaffected by having L3, the secondary of a transformer, in place. I had set the inductance of that coil at 3mH and being in parallel I thought it would have some effect but apparently not. While it's true that L3 would not be driven directly by the voltage source, but via induction from a primary winding, I doubt that would change this situation. Jules
  25. I'm assuming that resonance will still occur but, due to the extra inductance of the transformer secondary, it will be at a lower frequency (see attached). I will mock it up in LTSpice and see what happens. If that's ok then I will put some components together in the new year. The purpose of resonance is not to determine fluid level, which can be done with a simple float detector, but rather to make the most of the energy input in building up a high voltage across the electrodes. This method of electrolysis relies of the breakdown of the water dielectric while at the same time limiti
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