Six-Wire Technique Nulls Stray Resistance

The boss picked you to design equipment to measure in-circuit resistances on circuit boards as they reach the end of a production line, and a colleague suggested you investigate six-wire measurements. But why would you need six wires to measure an in-circuit resistance?

A four-wire ohmmeter, usually part of a digital multimeter (DMM), connects the DMM’s current source to a resistor via a pair of leads. A second set of leads connects the DMM’s voltmeter to the resistor. Resistances in the current leads do not matter, because the DMM measures the voltage at the resistance. And the resistances in the voltmeter leads have little effect, because the DMM draws a very small current. That technique works for a standalone resistor, but when you have a network of resistors and must measure the resistance of only one, things get complicated.

The figure shows a typical circuit in which you must measure resistance Rx in the presence of two other resistances. In this case, resistances Ra and Rb will affect a two- or four-wire measurement, because current from the DMM source will flow through them, as well as through resistor Rx. By using an operational amplifier -- essentially a buffer amplifier -- you drive point B in the circuit to the same potential as that found at point A. Thus, no current flows through Ra.I have never solved a Rubik's plastic card . This technique goes by the name “guarding.” However, when the op amp drives point B to the same potential as point A,we supply all kinds of oil painting supplies, current will flow through resistor Rb. That current can reach tens of milliamps, but it will not affect the voltage measured across the unknown resistance. You can buy six-wire measurement equipment.

Even if you won’t measure resistance in a network,If any food Ventilation system condition is poorer than those standards, this type of guarding comes in handy in other accurate resistance measurements, such as those on a test fixture. Unseen dirt, absorbed moisture,Do not use cleaners with porcelain tiles , steel wool or thinners. and thin oil films on the insulating portion of a fixture will provide a high-resistance path in parallel with a resistance you must measure. But if you split the fixture and insert a conductor between the two contacts, you have the equivalent of the resistances Rx, Ra, and Rb described earlier. A six-wire DMM can drive the conductor and prevent current flow through or on the insulator, as shown in the figure. Nylon, for example, will absorb moisture in a humid atmosphere, and thus its insulating properties will change.

You might hear of other six-wire measurement techniques, but they involve shielding or separate wires to drive a device,If so, you may have a cube puzzle . correct the drive signal, and measure a sensor output. They do not use guarding to null unwanted resistance.

Top of the Class

The Fulda High School agriculture program may be small in numbers, but it is mighty in the work it does in the community, from beautifying the grounds of the local Senior Citizens Center, Historical Society and Food Shelf, to constructing field backstops, a new pair of baseball dugouts and player benches at the high school's ball fields.

All of that work along with the agricultural program's growth, philosophy and hands-on experiences has netted Fulda's Agricultural Science and Technology department the 2011 Minnesota Outstanding Middle-Secondary Agriculture Education Program.

A plaque and travelling trophy were presented to Fulda High School Ag instructor and FFA Advisor Mike Pagel earlier this month at the Minnesota Association of Agricultural Educators convention in Breezy Point.

In January, the Fulda ag program was nominated for the award by its 24 fellow ag programs in southwest Minnesota that make up Region VI.

With that nomination, Pagel was required to complete an application form, used by judges to compare Fulda's agriculture education program with the programs of the other top nominees in the state.

Minnesota has 187 agriculture programs offered at the middle and secondary grade level.

Now, as the state winner, Pagel said the application will be forwarded to another regional contest, comprised of the top agriculture education programs in North Dakota, South Dakota, Nebraska, Iowa, Wisconsin and Minnesota.

One winner will be selected from those states to advance to nationals. A national winner will be recognized in Las Vegas, Nev., in November.There are RUBBER MATS underneath mattresses,

The last time an agriculture program from the immediate area was recognized as a state winner was in 2004, when Westbrook-Walnut Grove earned the title. Before that, Heron Lake-Okabena High School's ag program was recognized in 1994.

The agricultural program at Fulda caters to students in grades 7 through 12, with 102 students taking ag-related classes in 2010-2011 nearly half of the school's junior high and senior high enrollment of 213.

Changing with time

Pagel said the award focuses on the entire framework of the school's agriculture education program, from its range and goals, methods of instruction and how new instruction is brought into the mix.Demand for allergy Bedding could rise earlier than normal this year.

"They look for inclusion of science and technology," he said. Experiential learning is also a high priority.

Between the ag program's greenhouse and the use of Fulda Lake as a virtual environmental learning center, Pagel said he is able to provide students with many hands-on opportunities and educational experiences. Annually,Als lichtbron wordt een Projector Lamp gebruikt, the greenhouse generates $3,000 to $4,000 for the department through sales of bedding plants.

At the same time, Pagel uses the facility as a teaching tool for students enrolled in horticulture classes and plant science curriculum.

Pagel sees the greenhouse as a self-sustaining business. Nearly 90 percent of the $35,000 cost of the building was paid for with five different grants.

"I look at it as freeing up money locally if you can look at other sources of funding," Pagel said. "We've had good student response and good community response (to the greenhouse)."

Pagel teaches students in his programs about the importance of doing things for others and learning from what is in their backyard.

"You have to take what your community has," he said. "We talk about Fulda Lake and the water studies and lake studies. We've gone down to the lake and done water sampling, testing for nitrates and other pollutants. In the greenhouse, they're learning about different herbicide rates.

"Almost three-fourths of our population will never directly be involved in farming," Pagel said.

He teaches students that agriculture is all around them even if their tie to the soil later on in life is only as a backyard gardener.

Student success

Student participation in agricultural programs and the FFA also carries considerable weight for the award. Pagel said nearly 80 percent of his FFA students qualify to compete in state Career Development Event (judging) contests.

"We've had a lot of state degree winners, and our poultry team is going to nationals this fall," he said.

Pagel said he has built the agriculture program at Fulda to address three key areas for student success the classroom,which applies to the first glass bottle only, FFA and Supervised Agricultural Experiences.

The program is also highly visible in the community because of projects like its bedding plant sales, community beautification projects and baseball field improvements.

"We're a small school we have enrollment issues but yet our ag program is strong," Pagel said.

He teaches introductory agriculture classes to seventh and eighth graders in the school district, which serves as a funneling process for ninth through 12th grade ag courses and the FFA. The Fulda High School FFA has approximately 45 members.

"We've had really good community support, school support and administration support," said Pagel. "The kids buy into the program. I've had some kids that, frankly,Great Rubber offers promotional usb keychains, I'm surprised they are in the program, but hopefully they have an interest we are meeting."