Reading Voltage Across Switches

Voltmeters read the difference in potential from one point to another. When a voltmeter indicates a reading of 120 volts, this means that one lead is 120 volts higher than the other lead. We often refer to this as voltage drop. I find that people often misinterpret voltage readings across switches. A switch is designed to either allow current flow, or stop it by opening the circuit. A voltage reading across a switch indicates that the switch is open (off). Let’s take a circuit with a light and a standard toggle switch controlling the light. With the switch off, you will read 0 volts at the light. If you read voltage across the light switch, you will read 120 volts with the switch off. The voltage is dropping across the switch. Now if you turn the switch on, the voltage reading across the switch will change to 0 volts. Checking the light, you now read 120 volts at the light. When current travels through a switch, there should be NO drop in voltage. When current travels through a load, there SHOULD be a voltage drop. A closed switch should have a resistance of close to 0 ohms, while a load should have a measurable resistance. Using Ohm’s law to calculate the expected voltage drop across a switch you would get 0 volts because 0 ohms times any amount of current would still be 0 volts. I have often seen students looking for the break in a circuit read voltage across a switch and declare that switch is OK because they got a reading. In fact, that switch is open because they got a reading. One point to keep in mind is that the voltage across all devices in the circuit should add up to the source voltage. If you are reading 120 volts across a switch in a circuit wit a source voltage of 120 volts, there is no voltage left for the load.

HFC Agreement

Recently on June 7, 2013, President Obama and Chinese President Xi Jinping met for talks in Rancho Mirage, California. One of the agreements to come out of the talks was an agreement for both countries to work together to reduce HFC production. The statement reads

"Regarding HFCs, the United States and China agreed to work together and with other countries through multilateral approaches that include using the expertise and institutions of the Montreal Protocol to phase down the production and consumption of HFCs, while continuing to include HFCs within the scope of UNFCCC and its Kyoto Protocol provisions for accounting and reporting of emissions."

Basically, the big news is that the two biggest producers and users of HFCs agreed with each other to work on reducing their use. Europe has already started their reduction of HFC use and their regulation of F-gasses. F-gasses are their catchy phrase for all fluorinated refrigerants. I believe the F part tells us what they think of them.  There is no need for immediate panic. Notice that the agreement basically says they believe that reducing HFC production and use is something good to do. There are not yet any targets, timetables, or details. However, it clearly lays out that HFC refrigerants will be with us for a far shorter period of time than the CFCs and HCFCs they replaced. You can read more details and background in this White House Press Realease.

Lights Out!

Last night my family and I were all engaged in our own individual electronic pursuits. All of us in close proximity, but worlds away in our own individual internet enabled abstraction. A thunderstorm cut off our power and our internet connection. So lacking the ability to communicate with our far flung internet friends, we were stuck in a room talking to with each other. So we talked about what was going on in our lives. What happened at church this past Sunday, what our friends were doing yesterday, details of the lives of people whose lives are intertwined with our own. Then my son got out his guitar and started singing. Nothing on you-tube compares to the joy of making music with your family. There are certainly better guitar players and singers on line, but they are not sitting in my living room. They don’t share my name and my blood. They don’t compare. I received an early father’s day gift when my connection to the outside world was severed. Preston Stanfield live in concert in my living room.

There is not an app for that.

Random Puzzle Assembly

Many students trying to troubleshoot HVACR systems have difficulty navigating the sheer amount of data one can collect. If you go about just collecting a lot of data, it does not take very long to confuse yourself. I often see people taking resistance, voltage, and current readings on just about every conceivable location on the system without having a clear idea why they are taking the readings. This is like trying to build a 1000 piece jigsaw puzzle by just randomly trying to fit pieces together without looking at the shapes or color patterns on the pieces. When building puzzles, it helps to have a plan. There are different plans – outside edges first, organizing pieces by color, or organizing pieces by shape, but having a plan improves your chances of success. Troubleshooting is the same way. You should have a plan to organize your data collection into something meaningful. You should know why you are taking a voltage, resistance, or current reading before taking the reading. Ideally, each measurement you make should eliminate an area of inquiry. A simple example is checking the power supply to the unit. If you read the correct incoming voltage, you can eliminate that as a source of the problem. But if you don’t read the correct voltage at the power supply, then there is no need to check anything inside the unit until you have solved the power supply problem. There are probably as many systems as there are technicians. It is not too important what your system is, so long as it is based on an understanding of how the equipment functions and proceeds logically. For a non-functioning component, I generally want to know if that component is receiving the correct voltage. If it is, then I need to take a closer look at the component. Otherwise, I need to check the circuit that supplies voltage to the component. Many technicians check the line voltage and control voltage first. This does not take long and covers a large number of problems. Another technique is to start at the thermostat and work towards the non-functioning component until you find a break in the circuit, or eventually reach the component. The key point is that each piece of data you collect should tell you something. This will allow you to take far fewer readings and isolate the problem much faster.

Are You Seeing Ghosts?

Have you ever taken a voltage reading with a digital meter and gotten something that really did not quite make sense? With high impedance digital meters, it is common to get readings where you don’t expect them. This can be very confusing for students. Volt-meters are basically electrical loads. Normally, for them to read anything, they have to be in a complete circuit. The impedance rating of a meter determines the amount of current required to operate the meter. The higher the impedance, the less current required to operate the meter and get a reading. High impedance meters are preferred for electronic work because they don’t add any load to the circuit they are testing. However, high impedance meters can behave differently from older analog meters. A phenomenon called capacitive coupling can cause high impedance meters to read voltages that low impedance meters don’t read. These voltages are sometimes referred to as ghost voltages. Unenergized wires and devices in close physical proximity to energized wires and devices are charged by the energized wires in their proximity. This is a static voltage which cannot move any appreciable current. Since high impedance digital meters really don’t require a current flow to operate, they can read the ghost voltage. To differentiate between a ghost voltage caused by capacitive coupling and a hard voltage that will move current, place a load across the voltage. When loaded, a ghost voltage disappears. Some digital meters actually have settings for both high and low impedance to allow technicians to distinguish between a ghost voltage and a hard voltage. Fluke makes an adapter, SV225,  that can be used with high impedance meters to effectively turn them into low impedance meters by placing a small load across the leads. Low impedance analog meters do require a small current to operate, so they typically will not read the ghost voltage built up by capacitive coupling.