Digital Zeus™ HVAC Tool & Instruments Journal

Voltmeter Design

Most meter movements are sensitive devices. Some D’Arsonval† movements have full-scale deflection current ratings as little as 50 µA, with an (internal) wire resistance of less than 1000 Ω. This makes for a voltmeter with a full-scale rating of only 50 millivolts (50 µA X 1000 Ω)! In order to build voltmeters with practical (higher voltage) scales from such sensitive movements, we need to find some way to reduce the measured quantity of voltage down to a level the movement can handle.

Let’s start our example problems with a D’Arsonval Meter Movement‡ having a full-scale deflection rating of 1 mA and a coil resistance of 500 Ω:

Using Ohm’s Law (E=IR), we can determine how much voltage will drive this meter movement directly to full scale:

E = I R

 
E = (1 mA)(500 Ω)

 
E = 0.5 volts

 
If all we wanted was a meter that could measure 1/2 of a volt, the bare meter movement we have here would suffice. But to measure greater levels of voltage, something more is needed. To get an effective voltmeter meter range in excess of 1/2 volt, we’ll need to design a circuit allowing only a precise proportion of measured voltage to drop across the meter movement. This will extend the meter movement’s range to higher voltages. Correspondingly, we will need to re-label the scale on the meter face to indicate its new measurement range with this proportioning circuit connected.

But how do we create the necessary proportioning circuit? Well, if our intention is to allow this meter movement to measure a greater voltage than it does now, what we need is a voltage divider circuit to proportion the total measured voltage into a lesser fraction across the meter movement’s connection points. Knowing that voltage divider circuits are built from series resistances, we’ll connect a resistor in series with the meter movement (using the movement’s own internal resistance as the second resistance in the divider):

The series resistor is called a “multiplier” resistor because it multiplies the working range of the meter movement as it proportionately divides the measured voltage across it. Determining the required multiplier resistance value is an easy task if you’re familiar with series circuit analysis.

For example, let’s determine the necessary multiplier value to make this 1 mA, 500 Ω movement read exactly full-scale at an applied voltage of 10 volts. To do this, we first need to set up an E/I/R table for the two series components:

Knowing that the movement will be at full-scale with 1 mA of current going through it, and that we want this to happen at an applied (total series circuit) voltage of 10 volts, we can fill in the table as such:

There are a couple of ways to determine the resistance value of the multiplier. One way is to determine total circuit resistance using Ohm’s Law in the “total” column (R=E/I), then subtract the 500 Ω of the movement to arrive at the value for the multiplier:

Another way to figure the same value of resistance would be to determine voltage drop across the movement at full-scale deflection (E=IR), then subtract that voltage drop from the total to arrive at the voltage across the multiplier resistor. Finally, Ohm’s Law could be used again to determine resistance (R=E/I) for the multiplier:

Either way provides the same answer (9.5 kΩ), and one method could be used as verification for the other, to check accuracy of work.

With exactly 10 volts applied between the meter test leads (from some battery or precision power supply), there will be exactly 1 mA of current through the meter movement, as restricted by the “multiplier” resistor and the movement’s own internal resistance. Exactly 1/2 volt will be dropped across the resistance of the movement’s wire coil, and the needle will be pointing precisely at full-scale. Having re-labeled the scale to read from 0 to 10 V (instead of 0 to 1 mA), anyone viewing the scale will interpret its indication as ten volts. Please take note that the meter user does not have to be aware at all that the movement itself is actually measuring just a fraction of that ten volts from the external source. All that matters to the user is that the circuit as a whole functions to accurately display the total, applied voltage.

This is how practical electrical meters are designed and used: a sensitive meter movement is built to operate with as little voltage and current as possible for maximum sensitivity, then it is “fooled” by some sort of divider circuit built of precision resistors so that it indicates full-scale when a much larger voltage or current is impressed on the circuit as a whole. We have examined the design of a simple voltmeter here. Ammeters follow the same general rule, except that parallel-connected “shunt” resistors are used to create a current divider circuit as opposed to the series-connected voltage divider “multiplier” resistors used for voltmeter designs.

Generally, it is useful to have multiple ranges established for an electromechanical meter such as this, allowing it to read a broad range of voltages with a single movement mechanism. This is accomplished through the use of a multi-pole switch and several multiplier resistors, each one sized for a particular voltage range:

The five-position switch makes contact with only one resistor at a time. In the bottom (full clockwise) position, it makes contact with no resistor at all, providing an “off” setting. Each resistor is sized to provide a particular full-scale range for the voltmeter, all based on the particular rating of the meter movement (1 mA, 500 Ω). The end result is a voltmeter with four different full-scale ranges of measurement. Of course, in order to make this work sensibly, the meter movement’s scale must be equipped with labels appropriate for each range.

With such a meter design, each resistor value is determined by the same technique, using a known total voltage, movement full-scale deflection rating, and movement resistance. For a voltmeter with ranges of 1 volt, 10 volts, 100 volts, and 1000 volts, the multiplier resistances would be as follows:

Note the multiplier resistor values used for these ranges, and how odd they are. It is highly unlikely that a 999.5 kΩ precision resistor will ever be found in a parts bin, so voltmeter designers often opt for a variation of the above design which uses more common resistor values:

With each successively higher voltage range, more multiplier resistors are pressed into service by the selector switch, making their series resistances add for the necessary total. For example, with the range selector switch set to the 1000 volt position, we need a total multiplier resistance value of 999.5 kΩ. With this meter design, that’s exactly what we’ll get:

 
RTotal = R4 + R3 + R2 + R1

 
RTotal = 900 kΩ + 90 kΩ + 9 kΩ + 500 Ω

 
RTotal = 999.5 kΩ

 
The advantage, of course, is that the individual multiplier resistor values are more common (900k, 90k, 9k) than some of the odd values in the first design (999.5k, 99.5k, 9.5k). From the perspective of the meter user, however, there will be no discernible difference in function.

REVIEW:
Extended voltmeter ranges are created for sensitive meter movements by adding series “multiplier” resistors to the movement circuit, providing a precise voltage division ratio.

This document is being republished to the HVACPROTech.com Forums. Expressed consent for republication, distribution and expanding this article is hereby granted under the terms of the Design Science License Here:  http://www.mediafire.com/?l0xt4v2ssz2

‡D’Arsonval Meter Movement: http://www.engineersedge.com/instrumentation/electrical_meters_measurement/darsonval_movement.htm

ANNIE A-12 Full Manual: http://www.mediafire.com/?bjuyem4y9fz

ANNIE A-12X Full Versions of Screenshots in this Thread:  http://www.mediafire.com/?2gyyts2xlme 

The files above are in the Winzip and Winrar formats (respectively) that require you to decompress the folders after you have them downloaded on your system. Most operating systems are included with a compression/decompression (codec) that will decompress these files. If you do not refer to our HVAC PROTech® Technical Archives   HVAC FAQ’s Page and click the Utilities header, scroll down to Compression/Decompression Utilities. HVAC FAQ’s are located here: http://hvacprotech.org/faq%27s.html 

Choosing a Megger Insulation Tester

With over 30 models to choose from, selecting the proper Megger Insulation Tester can appear a bewildering task at first. Actually, all the process requires is a little organization. All Megger Insulation Testers perform essentially the same test in fundamentally the same manner, accurately and reliably. Refinements and added features, however, separate one model from another, in application and operator appeal.

Make a check list of important or essential features and specifications. This should automatically reduce the choices to a workable number, from which personal preferences can easily determine the final selection.

Test Voltage(s) - An electrician interested only in installation and proof testing may need only a single voltage. A repair or maintenance man, however, may want the diagnostic capabilities that derive from comparing tests at different voltages. Base your voltage requirement(s) on the rated voltage of the equipment to be tested, then decide if you want to test at rated, or perform stress tests at higher voltages. Do you want to carry out Step Voltage Tests?

Remember, pervasive insulation damage like moisture and oil soaks are revealed at any voltage, while mechanical damage like pinholes may require voltages high enough to arc an air gap in order to be detected. Test instrumentation commonly makes a quantum leap from 1 kV to 5 kV, so this may be your most critical voltage determination and most significant decision in selecting a tester.

Measurement Range - For an electrician or repairman interested only in proofing, infinity readings may be sufficient. For predictive maintenance, however, it is critical to be able to see the change in resistance between successive measurements, even though the actual values remain exceedingly high. Don’t limit your testing capabilities with a short-range model. The newest technologies permit resistance measurements to the tera ohm (T) range! Try to determine the insulation resistance values of your equipment when new, then select a tester that can actually measure to these values.

Power Source - The test is the same, irrespective of the power source. Not everyone believes you can get 1000 volts out of AA’s, but you can! Current limitation is the means. Batteries free the operator from the extra work of cranking, while hand-cranks relieve dependence on batteries and the possibility of human error. Remember, if you plan to do Polarization Index (PI) testing, you don’t want to crank for 10 minutes! Rechargeables are the most convenient, but throwaways are ready when you are without having to be charged over night.

Voltage Detection - Most models feature detection of unwanted voltage on the test item. Electricians may want an audible signal for rapid trouble-shooting that is not dependent on visually monitoring the display. Maintenance men for large equipment will want to be able to see high-voltage capacitive charges decay at the conclusion of a test.

Display - Digital or analog is largely a matter of preference, but newer models combine both capabilities in a single, convenient display.

W/kW Ranges - Generally referred to as “Continuity” and “Resistance” ranges, these are low-voltage, mid- to low-range functions that add greatly to the depth of testing capabilities that your tester offers. They can make the selective difference between several models that are similar in the more apparent functions, and should not be overlooked. Ohm (W) ranges can be used to verify integrity of circuits and connections, while kilohm (kW) ranges are useful in locating areas of insulation deterioration. The electrician will want an ê -range, the maintenance man will want kW, the repairman will want both!

Guard Terminal - This third terminal shunts the measurement function. It is useful in eliminating certain components of leakage from the measurement, and provides a valuable extra tool in analytical work. The electrician may not need it, but the maintenance man should, and the repairman will!

Price - Megger Insulation Testers range in price from a few hundred to several thousand dollars. Regardless of your budget, there is a model that will fit, and offer a surprising range of features as well.

“Extra” Features - Remember, Megger Insulation Testers are not arrayed from “good” to “better” tests; all perform accurate and reliable measurements. Rather, additional capabilities and greater flexibility are achieved from one model to the next. The newest versions offer such additional features as pre-programmed standard tests (Polarization Index, Step Voltage, Dielectric Discharge), calculation and storage of results, downloading to computer or printer, timed tests, measurement of leakage current and capacitance, and “burn” mode. These may not be necessary, but they certainly are convenient. If it fits your budget, go for it!

Insulation Testing / Affording the Best

The Megger name has traditionally been associated with the highest quality in Insulation Testers. But high quality need not impose a high price. Through years of leadership in the testing field, Megger has rightly become recognized as the highest standard in test instrumentation. But that fact need not be interpreted by those with tight budgets or limited applications as putting Megger quality beyond their reach.

Remember, there is no such thing as a “better” insulation test…not as long as a Megger instrument is employed. All of our models perform a reliable, high-quality measurement to rigorously specified standards. The more advanced models provide greater capabilities…more features, more flexibility…added to the basic concept that has defined Megger for a century: the performance of the best possible insulation measurement.

Megger Continuity Testing

Most models of Megger Insulation Testers have “continuity” ranges. It is not uncommon, however, for this function to be regarded much like the low gears on an automatic transmission; that is to say, by disuse. This practice isn’t too hard to rationalize. After all, insulation testers are thought of as high-voltage units capable of measuring enormous resistances. The opposite ends of the resistance and voltage spectra are easily overlooked, inadvertently consigned to the province of multimeters.

That “W ” symbol next to the selector switch indicates a low-voltage, low-resistance test. Specifications vary somewhat with the model, but in general (and for all of the new hand-helds) a fixed 5 V dc measures from 0.01 to 99.9 W . If nothing else, this convenient function frees the operator from obtaining, carrying, locating, and employing a second piece of instrumentation. You don’t have to pause to hook up a multimeter when a simple click of the switch on the insulation tester will do the job.

There’s more. A continuity range neatly complements the already considerable capabilities of the insulation resistance ranges, giving the tester an added dimensional quality. If the insulation test pegs to the low end, assessment of the problem doesn’t have to stop there. Unexpectedly low resistance? Your Megger Insulation Tester doesn’t just tell you it’s low; click to the continuity range, and you can determine how low.

Insulation looking just fine? What about proper circuit connections? Wherever there’s insulation, there’s also a current path, and this needs to be checked and maintained just as well as the insulation. The continuity range can determine if connections are tight, or indeed, if they have been made to the correct circuits! Old connections can be checked for corrosion, or for assurance that they haven’t loosened. New connections can be checked for a good weld or solder joint. The 200 mA current produced by the newest models is especially useful in this manner, making the Megger Insulation Tester not just the equivalent of a multimeter, but critically superior. The robust test current stresses bad joints and reveals weaknesses in marginal electrical connections that will pass the few milliamps produced by common multimeters with deceptive ease. In short, the continuity function not only determines that current flow is established across connections, but that it is also not present between isolated elements.

The electrician can perform his full function with a single instrument, assuring that isolated surfaces are properly insulated and that wires have not been skinned or damaged, and that connections have been made between the correct elements and are tight. Models with buzzers are particularly advantageous in permitting concentration on the work, not on the display. Maintenance men installing motors and other equipment can put the unit through its specifications test, then check for good connections. Motor repairmen can test that the insulation has been brought back up to spec, then check that all the proper reconnections have been made, in order and firmly. Technicians and design engineers can trouble shoot boards and circuits quickly and easily. Verify plate-through on sandwich boards with the touch of a probe. Continuity buzzers speed the process while applying a quantified pass/fail criterion of 5 W . Even heavy-duty applications can be enhanced with judicious use of the continuity function. Utilities can help characterize a cable fault by determining the amount of resistance, an important step in deciding on the proper locating equipment and procedure to reduce trial and error.

Even the simplest Megger Insulation Tester can provide a surprising wealth of added applications once all of its functions are fully utilized.

Removing a Cordless Drill Chuck

A lot of cordless drills require the removal of the chuck for repairs, especially when repairing the gearbox or torque selector collar. This little guide should help. Please remember that these instructions only work for threaded-on chucks and not taper-fit chucks (almost all cordless drills have threaded chuck spindles). Use caution and common sense.

Remove the chuck screw

In many drills there is a small screw deep inside the jaws of the chuck. This screw helps hold the chuck onto the spindle during operation, preventing it from spinning off. Check to see if you have a screw inside your chuck’s jaws. Depending on the brand and model of your drill, you may have to use an allen key, torx driver, or a flat-head screwdriver to remove the screw. The most important thing to remember is this: the chuck screw is reverse-thread. You will have to spin the screw clockwise to remove it. This reverse threading helps to act against the torque of the drill and keep the screw tightly connected.

Read the Rest of this Article:  http://hvacprotech.forumwise.com/hvacprotech-thread4933.html