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Legacy Constant Current PWM Installation Instructions

Legacy PWM Body
PWM Body
LCD Controller
LCD Controller

Main Components

  1. PWM Body: The PWM body is enclosed in a black case with its switch and cooling fan. This is the actual PWM that does the work of controlling the power to an HHO cell.
  2. LCD Controller:The LCD Display/Controller module. This device shows you the important readings of your PWM, and also controls the PWM so that it maintains its constant current amperage as per your settings. This device is usually mounted in the passenger compartment of your vehicle so you can see at a glance what is going on with your HHO system, but can also be mounted in the engine compartment if you prefer.

If this is your first PWM install, it would not be a bad idea to quickly learn about the basics of a PWM and how it does it's job. I wrote an article for that purpose, and you can find it here: What Is A PWM? While its not absolutely necessary to understand how a PWM works in order to use one, I think you'll find it helpful.

Connection Instructions

We will cover the connections in 2 sections. First is the high current connections. These are the ones that connect the PWM to the battery, and the PWM to the HHO cell. We call them "high current" connections, but they are only carrying the amount of amperage you will set with the LCD Controller. We recommend using 10 gauge automotive primary wire for these connections. You can get wire of this type from any automotive store.

We will separately diagram and describe the low current connections. Low current connections are connections for such things as the control wire (to turn the PWM on and off), and the reservoir float switch wiring. These wires can be any size. We usually like to use 24 ga or 22 ga, but they don't have to be this big. It's just that any smaller and the wire is in danger of being damaged, just because it's so fragile. But the currents carried on these wires are very small, so small gauge wire can be used.

High Current Connections

PWM Circuit Board
PWM Circuit Board

First you'll separate the base of the PWM from the body to expose the circuit board inside. Then you'll be able to see the connection terminals, and circuit board markings that indicate the function of each connection. For the high current connections we will only be using 4 terminals. We will connect to these terminals using 10 gauge wire and #8 yellow crimp terminals.

We use 10 gauge cable for these high current wires for nearly all installations. But if you anticipate long cable runs or higher amperages, then use this chart to calculate the wire gauge you should use.

Important Note: "H-" on the circuit board, is not the same as ground (B-). Do not bond these 2 circuits or the PWM will not be able to control the current. Each needs its own cable.

PWM High Current Connections

When deciding where to locate your cell and your PWM, try to work things out such that the length of these high current cables is minimized. There is loss as the currents pass through the wire, and the longer the wire runs are, the more current is lost. You'll notice this loss as heat, and if you feel the wires, you'll notice they get warm after running for a while. To minimize this, try to locate the cell near the battery and the PWM body near both. It's not always possible to do this, depending on the arrangement of components in your engine compartment. So, you may have to separate things quite a bit to make them fit. Just consider this point of total line length as you plan the locations.

Be sure to screw these screws very tight, particularly on the high current terminals. Poor connections = high resistance = excessive heat. So, be sure to check each of the screws and be sure they are very tight. Further, the vibration of the car will cause any loose screws to eventually work themselves out. So, even the low current terminals or any unused terminal screws need to be tightened well so they don't come loose and cause a short circuit inside the PWM.

Once you have everything set up and you are testing your PWM, I recommend running it at the maximum amperage that you plan to use for a while, and then check the temperature of the cables near any connection points. Wherever you have a terminal attached to the high current cables, check to see if the wire is overheating. Poor connections to the terminal will show up by becoming very hot, even possibly melting the insulation on the cable. This is because there is too much resistance at a poorly connected terminal. Use this as a final check of your high current wires to ensure that all of your connections are properly done.

Low Current Connections

PWM Circuit Board Connections
Control Wire

In the above diagram, the "PWM Case Switch" is the switch built into the PWM Body. One side of the switch goes to the "Cntr" terminal on the circuit board. The other side goes to a source of 12 volts. We call this 12 volts a "control voltage". By this I mean that this 12 volts will activate the PWM's electronics and cause it to function. However, this voltage will not be passed to the HHO cell. It is a very low current circuit, the majority of which is powering the case fan. If the PWM doesn't receive 12 volts to the "Cntr" terminal, the PWM will not run.

Any 12 volt source will work for this function. However, we recommend getting this 12 volts from the output of your fuel pump relay. The fuel pump relay is only activated when the engine is actually running. Actually, it will also energize briefly when the key is first turned on to pressurize the fuel line, but this is very brief and for our purposes, can be ignored. Using the fuel pump relay as your control voltage source is the safest installation. If the engine ever stops running (and consuming the HHO), then the PWM will stop. So, there is no concern about making and accumulating HHO under your hood. It will always be consumed by the running engine.

Failing this, I recommend using a switched source of voltage. You will find a number of circuits that are energized when the key is in the "ACC", or "Accessory" position. This is not quite as safe as it means that if you are parked somewhere and turn on the ACC switch to listen to the radio, you will have to remember to turn off your HHO system. Otherwise it will cause HHO to accumulate under your hood. It's unlikely that enough would accumulate to cause an explosion. But it's not impossible. I really recommend going through the effort to locate your fuel pump relay and tap into the output of that circuit. Test with your meter that it only gives out 12V when the engine is running, and 0V when the engine is stopped, even if the ACC switch is on.

Float Switch or Level Sensor

If you use a float switch in your HHO system reservoir, you will likely want to be alerted when the float switch is activated. The "L" terminal on the circuit board is for the "Level" switch. Our circuit is designed so that the switch is activated when the terminal is connected to ground, not to voltage. So, as you see in the diagram, go through your float switch to ground. Connecting 12 volts to this terminal will damage your LCD Controller.

Your level switch may be normally open or normally closed. In other words the switch may connect to ground when the level sensor is activated, or it may be connected normally, and will break the circuit when the level switch is triggered. Either type will work. The controller can be set up for either type of switch in software. Controller software settings will be covered separately.

LCD Controller Connection

The LCD Controller is connected to the PWM Body with a Cat 5 computer cable. While you are initially setting up your HHO system, it is handy to have the display near your system so you can easily see what is going on when you first start things up. Later, when you have everything working properly, you will want to run the Cat5 cable into your passenger compartment and permanently mount the display where you can see it.

Connections to Your Cell

Different Cell Configurations

Most installations will be simple. One cell with 2 connection points, and 5 neutral plates between them. Connect HHO+ and HHO- to those 2 plates. In this case, all you need is the diagram above, "PWM High Current Connections". This section concerns cell configurations that aren't that simple. Such things as multiple cells, a cell with more than 7 plates and 24 volt system installs all need further clarification of how to connect the cell to the PWM.

It has been found that the best cell configuration for 12 volt systems is to have 5 neutral plates between the positive and negative terminals, creating 6 gaps. This arrangement causes the voltage to be split up evenly such that there are 2 volts between each plate. Adding more than 5 neutral plates makes the voltage too weak to pass the current, and less neutral plates makes the cell run too hot and therefore less efficiently. Both of these conditions cause there to be less gains from using the HHO system. So, to understand the basic goal of the following configurations, realize we want 5 neutrals, making 6 gaps, between each pair of plates connected to 12V power.

If you have more than one cell then you can extend one cable out to where your cells are, and then split the cable at the cells so that each cell gets both positive and negative connections. Multiple cells are connected in parallel. The diagram below shows both series and parallel connections in order to illustrate the difference.

Parallel and Series Connections
Parallel and Series Connections

Each of the cells in the above diagram are assumed to be 7 plate cells, each with 2 connections to the outside plates, and 5 neutral plates between them. As stated, for 12 volt systems, connect them in parallel. That way, each cell gets 12 volts and the resulting plate to plate voltage will be 2 volts, which is what we are trying to achieve.

If your electrical system is 24 volts, you would connect them in series as you see in the diagram. Connecting them in series causes the cells to "split" the voltage. If you measure from ground to the wire between the cells, you'll read 12 volts. Similarly if you measure from 24 volts to the center of the cells, you'll also read 12 volts. And if you measure between any 2 adjacent plates, you'll find the voltage to be approximately 2 volts. This is what we want for maximum efficiency.

Important Note: If your system is 24 volts, the legacy PWM can still work, but you must read the section at the bottom of this document about 24 volt systems. Failure to do so BEFORE you connect up your PWM can result in the PWM being damaged permanently. This is for 24 volt electrical systems only.

Multi-stack cells: A stack is defined as 2 hot plates (plates that are connected to power from the PWM), with 5 neutral plates between them. A 2-stack cell, or Double Stack Cell can be thought of as 2 adjacent cells. The center hot plate is common to both "cells" on either side of it. 12 volt systems would connect to the double stack cell in parallel, and 24 volt systems should connect in series. See the diagrams below. For 24 volt installs, once again I'll point out that you must see the important note about 24 volt systems below.

Double Stack Cell Connected to 12V
Double Stack Cell Connected to 12V
Double Stack Cell Connected to 24V
Double Stack Cell Connected to 24V

Important Notes For 24 Volt Systems

The Legacy CCPWM can control 24 volts on it's high current terminals. That part is no problem. But it's control circuitry cannot. If you are fitting this PWM to a 24 volt system, then failure to follow the instructions in this section can cause permanent damage to your PWM. If you have a 12 volt system, this section doesn't apply to you.

The PWM control wire must receive 12 volts. The control wire is the wire that connects to the PWM Case Switch, which then connects to the circuit board at the "Cntr" terminal. See the diagram "PWM Low Current Connections" above. This circuit draws negligible power - about .35 amps, most of which is used by the case fan. To be clear, the PWM can control 24 volts on its heavy gauge wires. It is perfectly OK to connect 24 volts to the "B+" terminal. But it must be controlled with 12 volts at the "Cntr" terminal. Using 24 volts on the "Cntr" terminal will irreparably damage the LCD Controller.

You can supply the 12 volts to the control wire using a DC to DC converter. Low current DC converters are pretty cheap and can be found on Amazon and Ebay. You'll want a 24 volt DC to 12V DC (or 13.8V is also common and OK to use). You'll want one that can carry an amp, but more is OK. Failing this, you might want to consider using any of our later PWM models. All of our other PWMs will use 12 or 24 volts natively at both the Battery terminal and the control terminal.


This document took you through everything you need to know about connecting your PWM to your system. The next document you'll want to read is how to program the controller. That can be found here: The LCD Controller

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