Important: Minimum Safety Circuitry For HHO Installs


HHO is safe to use provided you handle it properly. By this I mean that you must have all of your fittings sealed properly so that the HHO is getting into the engine. By making sure that all of the HHO you create is getting into the engine and getting burned, there is no HHO gathering in your engine compartment that could become hazardous. So the first step in safely handling HHO is checking all of your connections and making sure all of them are completely secure, and are not leaking.

However, another hidden hazard can occur when you are powering your HHO system incorrectly. The worst case of this we have seen was a guy wiring his HHO system directly to his car battery, with a switch in the engine compartment to turn the system on. One day he went to the movies with his girlfriend, and left the switch in the "on" position. As the system drew very little amperage, it continued making HHO which accumulated under the hood. When they got back to the car to drive home, he went to start his car and blew the hood off. No one was hurt, but as you can see, this could have been disastrous.

A better solution is to control your HHO system so that it can only run when the key is turned on. Some systems recommend this type of wiring. However, this is also not safe. Consider what would happen if you stopped by your daughter's school to pick her up after a sporting event. You are waiting for her while she gets dressed, and you are listening to your CD player. Well, to do this, you must turn on the ignition key, even though the engine is not running. And this means you will be making HHO that will be accumulating in your engine compartment while you are waiting.

The point here is that you must have a failsafe system for making sure that HHO is only being produced while the engine is actually running. This allows all of the HHO that is produced to be consumed and burned. Many manufacturers that supply HHO systems provide a relay that will not allow the system to operate until the relay is activated. However, if you are purchasing a system in parts, or building your own system, you will have to provide circuitry for this for yourself. This document is designed to show you how to create your own circuit to handle this function.

The Basic Circuit

The essence of the circuit you need is shown below. As you can see this circuit is installed between your battery and your HHO Cell. If you are using a PWM, then the output of this circuit would go to your PWM. It includes a circuit breaker and a relay. The circuit breaker is required so that if your cell tries to draw too many amps, the breaker will open the circuit and prevent the system from being damaged. Then we see the relay. The relay will not allow power to the HHO system until it is energized by 12 volts. If the 12 volt control voltage is ever removed, then the relay falls into the "off" position, and the cell cannot operate.

Power Control Module

Circuit breakers come in a few different varieties, which differ in how they are reset once they have blown. The most basic type is a manual reset, which requires that you press a "reset" button to get the breaker to turn back on. These can be a little tiresome, so there is a breaker with an automatic reset. These breakers are designed to heat up as the current rises and when the current gets too high, they trip. But after they cool down again, they will reset themselves. We recommend using a 3rd type of breaker, called a "modified reset" breaker. With this type, it will auto reset, but only if the voltage coming in to it is cycled off, then back on again. We like this type of breaker because if your system is drawing too much current, causing the breaker to trip, then you need to fix something before continuing to run the system. This way the system will stay off until you are able to stop and check out what's wrong.

Wire Sizes

Notice the wires going from the fuel pump relay to the our HHO system relay, and the wire from our HHO relay to ground. This circuit will carry very little current or amperage. The controlling circuit for an automotive relay will draw less than 150 milliamps (.15 amps), which is a very tiny current. Therefore you can use just about any size wire you want for this part of the circuit. A 24 gauge wire would be plenty, which can carry an amp or more. You may want to use a bit larger wire just so it will be less fragile in your engine compartment.

The wires that are on the current path between your battery and your HHO cell will carry much more current and must be sized appropriately. Notice the red lines in the diagram above. These lines represent the wires that must be heavy gauge wire. The exact size will depend on the circuit breaker you use. Your wire must be able to carry the full current that your breaker will carry without tripping, even if your system is designed to draw much less current. The reason is that if anything goes wrong causing your cell to draw more current, it could be drawing as much as the circuit breaker will allow, and must be large enough to carry it. For a 25 or 30 Amp breaker you must use 10 gauge wire. For a 20 Amp breaker you can use 12 gauge, and for a 15 Amp breaker you can use 14 gauge wire. Note, that these gauges are for stranded copper wire, and for best results, you want to use wire that is specifically made for automotive use.

The Fuel Pump Relay

Notice that we are controlling this circuit from the Fuel Pump Relay. Automobiles have already had to solve the of fuel delivery when the engine isn't running. It's vital, for instance, that the fuel pump be shut down if the car is in an accident. The way this is achieved is to shut down the power to the fuel pump when the engine stops running. All but the oldest cars today have an electric fuel pump. This pump is controlled by the Fuel Pump Relay which is designed to only be on when the engine is actually running. There are 3 ways that I know of that this is achieved:

1) The car's computer controls the relay directly, turning it off if the engine ever stops running,

2) Oil Pressure Switch (or Oil Pressure Sending Unit): In this case there is a switch that will provide 12 volts when the engine is running, but if the engine stops, then the switch falls into the "off" position, which turns off the relay and the fuel pump. In this case the car's computer must first energize the fuel pump relay long enough for the car to get started, then it shuts off it's circuit and the Oil Pressure Sending Unit continues to energize the circuit.

3) Mass Air Flow Sensor: Some vehicles have a switch incorporated into the Mass Air Flow sensor. In this case instead of oil pressure, the flow of air into the engine is used to keep a switch on that powers the fuel pump relay. In all other respects, this type of system is the same as the Oil Pressure Switch.

This may be too much detail on how the Fuel Pump Relay circuit works, but this point is so important I wanted you to have a better understanding of it. I am not familiar with every make/model of car, and can't say for sure that all vehicles have a fuel pump relay that will shut down when the engine is off as described here. It is vital that you test this function for yourself. To test, you'll need to find the wire that goes between the fuel pump relay and the fuel pump. Put a voltmeter on this wire and measure it with reference to ground. Then turn on your ignition key. Usually the relay will come on for a second or two in order to charge the fuel system for starting, but then it will shut off again. You will see 12 volts on your meter when the system is on, and 0 volts when it is off.

A more thorough test would be to run your engine, and then make it die without turning off the ignition key. You should see 12 volts while the engine is running, and when it dies, your meter should drop to 0 volts after the engine revolutions slow down. How do you make your engine die? Well one way is to cut the fuel pump relay wire to your fuel pump. Since you are going to tap into this circuit anyway, you can do a final test to make sure it is behaving correctly before trusting your life to it. The wire on the near side of the cut should still read 12 volts until the engine winds down, and then it will drop to 0 volts.

Another Solution

If the your fuel pump relay turns out to be an unworkable control source, there is another idea you can use. They make a vacuum switch that will switch on only when it's plugged into a vacuum line. You can get these from DigiKey for $20 (part number 480-2057-ND). You would cut one of your vacuum lines and insert the switch into the line. Then you would use the switch to control your relay. Note that the switch can't handle the current that goes through your electrolyzer, so you'll still need to use the automotive relay described above. But this will give you a way to control that relay so it is only on when the engine is actually running.

The FuelSaver-MPG Power Control Module

We are currently in final prototyping of a unit that will provide this function that you will be able to purchase from our store. It will have a couple of more bells and whistles. The following diagram shows what it's circuit board consists of. The Fuel Pump Relay is not part of the circuit board, but is shown in the diagram for information purposes to show where it would connect.

Power Control Circuit

The terminal block is used to make connections to the circuit board for low current carrying wires only. It will only take a maximum size of 16 gauge wire. For now we'll just cover the new items. The ground terminal is where you will bring in a ground wire. The only reason this board needs ground is so that the relay will operate. The relay requires 12 volts to activate it, but it also has to have a ground connection.

Next notice the 2 wires labeled "Amps +" and Amps -". These terminals lead to a shunt. A shunt is a very high current, but low resistance value resistor. It has a very precise resistance. As current increases through the shunt, it will develop a voltage differential from one side of the shunt to the other. This shunt is designed so that for each amp of current flowing, the voltage difference will me 1 millivolt. For instance if 25 amps are flowing, then there will be 25 millivolts across the shunt. By connecting a volt meter to the 2 bottom terminals on the block, you will be able to read your amp draw very precisely.

Adding a Switch to the Circuit

I've been showing the circuitry in bits and pieces to make it easier for folks who aren't used to reading schematic diagrams, and to keep explanations better compartmented. But this diagram shows what comes inside the box with one of our purchased units, and the switch represents the switch we provide to turn your system on and off. Notice how the voltage from the Fuel Pump Relay must go through the switch, and only if the switch is "on" will the current pass to the relay and turn the system on. Also note, that all of these switch wires are low current, and small wire gauges can be used.

Power Control Circuit 2

This final diagram shows how to wire a 2nd switch. We designed our unit so that you will have a switch on the Power Control Unit itself, but can also have a remote switch in your passenger compartment. To add the remote switch wire it as follows:

Power Control Circuit 3

The 2 switches are wired in series, which means that both switches must be on for the system to operate. We feel this is the most useful arrangement.


The only components that are vital for safety are the ones shown in the top diagram, above. You must make sure your HHO cell manufacturer has included these items, or you must provide them yourself. This is vital for your safety and for the safety of those who ride in your car. The rest of the diagrams are to show you how we are implementing this circuit in our professionally produced switching circuit. I hope it helps you improve the safety and functionality of your system.

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