I would like to take exception to your article about electrical (electronic) circuits as compared to hydraulic flow in the August 2007 issue of Hydraulics & Pneumatics.

1. In Figure 1 of the article with the OPEN switch, there is absolutely no potential applied to the motor from the battery. That is a fact. However, to the right of that with the valve open, there could still be a very slight potential on the hydraulic device. It may not be much, but could be a little. This is a difference in that if the flow of the valve is obstructed even slightly, the device could activate. In the lower portion of Figure 1, I agree that the circuits are the same, full electrical flow and full hydraulic flow.

Figure 1. Top, electrons flow when the switch is closed, whereas hydraulic fluid flows when the valve is open, second image. Even though the switch is connected in series, and the valve is connected in parallel; functionally, these are equivalent circuits.
Figure 2. An electrical circuit with no ground to mother Earth is analogous to a closed hydraulic circuit.
Figure 3. Unlike the hydraulic reservoir, ground can be located virtually anywhere in an electrical circuit.

2. To make Figure 1 more correct, you would need to change the switch in the upper left to parallel grounded and closed to make a similar circuit as upper right drawing.

3. Figures 2 & 3 are the ones that are really dangerous. Don’t believe me? Go to your truck and take a wire from the positive terminal of your battery and go to ground (engine block), then try to start truck, or even turn on the radio. Don’t really try that, it will explode your battery.

4. Figure 3, points “B” and “C” run to ground (Chassis) will cause a fire or battery explosion.

5. This article should be further explained and corrected for your readers so that someone doesn’t wind up hurting themselves.

6. These errors put the credibility of the book, advertised at the end of the article, in question for me.

7. There seem to be other misconceptions in the article, but, to be honest, I got bored with reading it. Sorry.

Petey Parish
chief estimator

Lakeland, Fla.
richard.parish@jacobs.com

Columnist Jack Johnson responds:

Thank you for taking the time to comment. I found your comments to be insightful as well as clarifying.

1. The circuits are equivalent from the standpoint of the motors. In both cases with them unpowered, they are able to free-wheel. When de-powered, both motors will coast to a stop, as opposed to dynamically braking. But when powered, the motors will turn. That is the intent of the circuits. They both serve to power and de-power the respective motors.

Some of the imperfections in the analogies occur because the electrical circuit uses a constant voltage source (battery) while the hydraulic circuit uses the familiar constant flow source (positive displacement pump). I pondered other circuit configurations, but in the end felt that it would only build in other complications, so I settled on the imperfect analogies.

Indeed, I agree with your observation, when the hydraulic bypass valve is open (flowing), there will be a small pressure felt by the motor, and the motor shaft may even rotate if there is no load on it. But bear in mind that the topic at hand is the nature of ground and what it means relative to the hydraulic reservoir; its not meant to be the way to properly control the speed or stopping of a hydraulic motor. Also, I wanted to give readers some background on why we “close” electrical switches to deliver current, but we “open” hydraulic valves to deliver flow. I think the text and accompanying figures provided that background.

2. If I understand what you propose, that puts a dead short across the battery, an undesirable situation. See my further responses, below.

3. Oh, but I do believe you! And I agree with you that if you were to simultaneously connect all three grounds as shown in Figure 3, there would be problems with short circuits, however, in the accompanying text, which I now quote:

“For example, the circuit of Figure 3 has three different ground points. Each represents an optional choice for ground location. Only one can be chosen, but it makes no difference which one.”

4. I agree, that if you connect from the positive terminal of the battery of a did not use the term chassis. I used the term ground, as in mother Earth, and I carefully made the distinction between common and mother Earth. You chose the term chassis. In other words, you missed the point, choosing to equate chassis with ground, and that is precisely the distinction that the article makes.

It is quite safe to make a connection between the positive battery terminal and mother Earth, as long as the vehicle is properly tired, giving the chassis electrical isolation from mother Earth. I urge you to re-read the text material on page 46, middle column, which takes up the subject of the difference between common (as in chassis) and mother Earth (ground).

5. Agreed. In retrospect, I wish that the qualifying statement in the text regarding only one ground being allowed had also been included in the caption of Figure 2. It would have helped to clarify the figure and the point I was making.

6. As for the credibility of the books, I will let them speak for themselves, and let those who have actually read them speak to their credibility. You should certainly not feel obligated to purchase them, but I would be pleased if you did. And I would be even more pleased to receive any feedback regarding them, good or bad. That’s how improvements are made.

7. I am sorry you were bored, however I would like to know what other misconceptions there were in the article. But, I will have to try to be more interesting in the future.

And don’t be sorry. Your comments have been helpful. They allow me to respond, and, I hope, get better in the future.