555 timer PWM

[rodrigopires]

Hi! I´m building a HHO dry cell and i need a power supply. I drew this schematics, adapted from one o found online. The cell will theoretically draw about 20A. Any recommendation?
I already tried this circuit on a breadboard, and it worked great powering a 36W 12v fan, low power. Used a IRFP250N just for testing but the final will have the IRFP064. About R1, using the IRFP250, i measured about 20uA with the 47 Ohm, thats good right? D3, how big does it has to be?

IC1 – NE555
M1 – IRFP064
R1 – 1K Ohm
R2 – 47 Ohm
Pot. – 50K Ohm
D1, D2 – 1N4148
D3 – STPS20H100
C1 – 0.1uF
C2 – 0.01uF
C3 – 470uF

[rdl]

Why is diode D3 there? Is an HHO dry cell inductive or does it serve some other purpose?

[Brumby]

After a quick bit of reading, I would like to venture some answers....

Who told you that is a power supply? All that is doing is PWMing your HHO.

The HHO is the load. Everything else is the power supply.

Basic information that I have gleaned:...

• HHO cells are just electrolysis cells - generating 2H₂ + O₂ from 2H₂0.
• There are 'Wet' cells and 'Dry' cells.  In wet cells, the power conductors extend inside a sealed space and connect to the electrolysis plates, and thus are in contact with the electrolyte and/or the generated gas.  In dry cells, the plates extend outside the sealed space and the power conductors are connected to the plates in free air.
• The purpose of HHO cells is to generate hydrogen and oxygen which is then fed into the fuel mix of the petrol engine of a motor vehicle.  The higher flame speed is supposed to increase combustion efficiency of the petrol.
• The pulsing of current appears to have two benefits - reducing the average current drain is one.  The other I think has to do with maintaining output - perhaps allowing gas bubbles a chance to dislodge from the plates, rather than build up.
• HHO cells are not claimed to be replacement fuel source - just a boost to petrol.

[rodrigopires]

Who told you that is a power supply? All that is doing is PWMing your HHO.

yes, sorry for my explanation... the real power supply is a HP ps-3701-1, this is just a modulator circuit. Since i never worked with one of these, i just wanted to make sure that it was correct before i start soldering...

Why is diode D3 there? Is an HHO dry cell inductive or does it serve some other purpose?

as i said, this circuit was adapted from one i found online, and it was for a induction motor. So since this is not an induction load, i don't need D3?

[rdl]

Why is diode D3 there? Is an HHO dry cell inductive or does it serve some other purpose?

as i said, this circuit was adapted from one i found online, and it was for a induction motor. So since this is not an induction load, i don't need D3?

That makes sense then. It's there to control what's called "inductive kickback" (google it). From what I've read in this thread, I don't think an HHO cell would be inductive. On the other hand, while the diode may not be needed, it probably isn't hurting anything either. A 1N4001 can handle 50 volts reverse and 30 amps surge current, use better if you think it's needed.

[Brumby]

as i said, this circuit was adapted from one i found online, and it was for a induction motor. So since this is not an induction load, i don't need D3?

No, you don't need D3 (for the reason you've given) - but leaving it there won't be a problem.

[TerminalJack505]

With that kind of current you might consider putting a MOSFET driver between the 555's output pin and the MOSFET gate.  You'll get less switching losses and better efficiency.

[rodrigopires]

With that kind of current you might consider putting a MOSFET driver between the 555's output pin and the MOSFET gate.  You'll get less switching losses and better efficiency.

Can you give me like a schematic? I've search online and found ones with optocouplers and others with PNP and NPN transistor... But how does that make a difference? Also how about R1, does it need to be higher? As i said, with the IRFP250N i got about 20-30uA 8V, is that good?

[TerminalJack505]

There are ICs dedicated to driving MOSFETs.  I was thinking about something like the MIC4422.

MOSFET drivers just hook up to the MOSFET's gate and will be able to switch it faster than the 555 so you will have less switching losses.  (Your MOSFET won't get as hot.)

[alsetalokin4017]

With that kind of current you might consider putting a MOSFET driver between the 555's output pin and the MOSFET gate.  You'll get less switching losses and better efficiency.

Can you give me like a schematic? I've search online and found ones with optocouplers and others with PNP and NPN transistor... But how does that make a difference?

You could use a basic simple non-inverting driver chip like MIC4422 (TC4422). The advantage is that this will work better to turn on the mosfet when the 555 timer pulse goes high. The timer itself can't supply a high current to fill up the mosfet's gate capacitance quickly. Using the driver chip allows a higher current to turn the mosfet on more cleanly and rapidly when it gets the pulse from the timer. At low frequencies this isn't really much of a problem but if your mosfet is heating or not giving good performance you might want to try using a driver chip between the 555 and the mosfet gate. There are also "totem pole" driver topologies that you can make out of a couple of transistors (pnp and npn) that will serve the same purpose but require a few more components than using a dedicated driver chip.

I would also consider using a separate power supply for the electrolyzer part of the circuit, rather than having it driven by the same connection as the 555.

Also how about R1, does it need to be higher?

R1 is one of the components that sets the frequency of the circuit and to a lesser extent the duty cycle. Higher value here means lower frequency. I would leave R1 at 1k and change the frequency (if needed) by changing the value of the capacitor C1. I have also found that a 10k potentiometer works well, where you have 50k.

As i said, with the IRFP250N i got about 20-30uA 8V, is that good?

I'm afraid I don't know what you mean here. Do you have your electrolyzer cell connected? What are you using for a power supply? How are you measuring this current and voltage? What potentiometer setting? There isn't enough information to say whether or not your numbers are "good". Certainly a current of 30 uA isn't going to do much electrolysis, but the average current through your cell will depend on duty cycle setting, frequency, mosfet performance, etc.

[rodrigopires]

You could use a basic simple non-inverting driver chip like MIC4422 (TC4422).

I can only find TC4426 on my supplier, works as well right?

R1 is one of the components that sets the frequency of the circuit and to a lesser extent the duty cycle. Higher value here means lower frequency. I would leave R1 at 1k and change the frequency (if needed) by changing the value of the capacitor C1.

I'm afraid I don't know what you mean here. Do you have your electrolyzer cell connected? What are you using for a power supply? How are you measuring this current and voltage? What potentiometer setting? There isn't enough information to say whether or not your numbers are "good". Certainly a current of 30 uA isn't going to do much electrolysis, but the average current through your cell will depend on duty cycle setting, frequency, mosfet performance, etc.

Sorry... I meant R2, the current limiting resistor on the gate. With the 47 Ohm resistor i got about 20-30uA from 0 to 50K on the pot. I wanted to know if that was the "normal", btw i took this measurement with a 2A load. Also if i use the gate driver IC i still need R2?

I have also found that a 10k potentiometer works well, where you have 50k.

What is the difference between the 50K and a 10K? Don´t do i get more "range" with the 50K?

[alsetalokin4017]

You could use a basic simple non-inverting driver chip like MIC4422 (TC4422).

I can only find TC4426 on my supplier, works as well right?

TC4426 is a high-speed, inverting, dual mosfet driver. It has two channels to drive two mosfets, and is rated at 1.5 amps peak output current. You could use this driver, but since it is inverting your PWM control will work "backwards"; that is, when you have the smallest pulse width from the timer chip you will have the greatest ON time of the mosfet. This may or may not be a problem. When I breadboarded the circuit I tried both non-inverting and inverting simultaneously (using the 555 output directly to drive a N-ch mosfet and a PNP transistor at the same time, each with its own load) and for me, just testing with LED loads, the inverted gave a more pleasing dimming effect.

R1 is one of the components that sets the frequency of the circuit and to a lesser extent the duty cycle. Higher value here means lower frequency. I would leave R1 at 1k and change the frequency (if needed) by changing the value of the capacitor C1.

I'm afraid I don't know what you mean here. Do you have your electrolyzer cell connected? What are you using for a power supply? How are you measuring this current and voltage? What potentiometer setting? There isn't enough information to say whether or not your numbers are "good". Certainly a current of 30 uA isn't going to do much electrolysis, but the average current through your cell will depend on duty cycle setting, frequency, mosfet performance, etc.

Sorry... I meant R2, the current limiting resistor on the gate. With the 47 Ohm resistor i got about 20-30uA from 0 to 50K on the pot. I wanted to know if that was the "normal", btw i took this measurement with a 2A load. Also if i use the gate driver IC i still need R2?

I still don't know how or where you are measuring that 20-30 uA current. Remember that the  mosfet does not function in the same way that a BJT transistor does; it is not controlled by base-emitter current but rather by gate charge. When you turn on a mosfet's drain-source channel you are actually charging up its gate capacitance. Once the gate is charged there is essentially no current flowing through the gate to the other pins. To turn the mosfet off again the gate must be discharged; this discharge path can be provided by a high value resistor between gate and source, or by the driver chip itself pulling its gate output low.

It is common to use a small resistor (like 10 - 100 ohms) in series with the mosfet Gate. The resistor is there to help protect the mosfet from turn-on and turn-off voltage spikes. The value is a compromise between protecting the mosfet and slowing it down too much. With noninductive loads you use lower value of this resistor. For your purposes, unless the hydrolysis cell and wiring is unusually inductive, I should think you can use 10 ohms without problems.

I have also found that a 10k potentiometer works well, where you have 50k.

What is the difference between the 50K and a 10K? Don´t do i get more "range" with the 50K?

Slightly.  Using the cap values you have and 12 v supply, with a 10k pot you'll get a frequency of about 1 kHz and a PWM range of from about 9 percent HI to 99 percent Hi.
With a 50k pot you'll get a frequency of about 215-230 Hz and a PWM range from about 2 percent HI to 99 percent HI.  So the pot value is a tradeoff between what frequency you need and the PWM adjustment range.
You can also change the 0.1 uF capacitor value to get different frequency ranges. With a 33nF cap and 50k pot you'll get 700-735 Hz as your base frequency, etc.

[rodrigopires]

So is this final? Anything to add? I only used one channel here, should i use both with the same input and connect the outputs together?