++++++++++++++++++ From: DolfinDon at aol.com Date: Wed, 2 Jan 2002 17:43:23 EST Subject: Re: [Elecraft] Resistance Readings with Digital Volt Meter To: rlewis at staffnet.com, elecraft at mailman.qth.net In a message dated 1/2/2002 3:40:20 PM Central Standard Time, rlewis at staffnet.com writes: Okay, I've got some questions. How does a "true DVM" differ from an "analog" DVM with a digital readout? How does a "true DVM" measure resistance without putting a voltage across the device being measured? ============================================== Hi There seams to be some confusion about meters here. First, all meters whether analog or digital read out are analog meters. Voltage current and resistance are all analog measurements. The digital meter converts the analog value to a digital display with an analog to digital converter and some form of numeric display. The so called analog meter is the type with a mechanical movement (Simpson 260) is really measuring the current flowing in the circuit made up of the external resistance an internal range resistor the meter movement and a battery. The resistance scale is usually non liner and the accuracy is not as good as the digital meter. Also on some ranges the battery may be 15 to 67 volts and that's why it may damage semiconductors. When measuring voltage the battery is out of the circuit, the range resistor is a different value and a liner scale is used on the meter. The digital meter measures voltage with an analog to digital converter. The range switch is designed so a constant resistance is always across the measure terminals (usually 10 megohms). To measure resistance the meter has a constant current circuit. This current is applied across the measurement terminals and the A to D measures the voltage drop. By selecting the right current the voltage reading is a liner relationship to the resistance. The constant current circuit will only apply 2 or 3 volts to the circuit and the current is limited to a small value. It requires .5 to .7 volts to turn on most semiconductors, so many meters have some ranges where the voltage applied to the circuit is below .5 volts. This way incircuit measurements can be made without diodes or transistors upsetting the readings. The answer to your question -- all meters apply some voltage to the circuit when measuring resistance it's just the amount of voltage and current that is different. The DVM is a safer choice. Don Brown KD5NDB K2 #1808,#2174,#2153 K1 #1000,#542 ++++++++++++++++++ Date: Wed, 02 Jan 2002 17:14:12 -0600 To: , From: "Timothy A. Raymer" Subject: Re: [Elecraft] Resistance Readings with Digital Volt Meter For a long time, there were Volt-Ohm-Meters (VOM) that were very common. The Simpson 260-8XL is a very common example of one of these that is still available. Those types of meters normally have an analog meter that displays the readings. The problem with them is that the meter itself participates in the amount of resistance that the circuit is presented with. Usually there will be an indication of this on the meter face, such as 20,000Ohm/V or something along those lines. So, this is usually a giveaway. There were also a few Volt Ohm meters with digital readouts that had circuitry to emulate Analog Volt Ohm meters. These were not real common, but show up occasionaly. The more expensive meters in days past were Vacuum Tube Volt Meters.(VTVM) The RCA WV-98C is an example that many of us may have seen in an electronics lab. They presented the measured circuit with a "self" impedence of several Megaohms, but were very expensive. And because they had a vacuum tube in them, they did not lend themselves to being very portable. Digital Multimeters came along, and gave us the best of both worlds: relatively inexpensive, essentially infinite input impedance, and for an additional cost, automatic ranging. And so it goes. To answer your question a little more directly: a digital Multimeter or DMM will put a voltage on a circuit to measure resistance. Typically they will have the positive voltage on the positive lead. If you know what to expect in terms of the reading, (100Kohm was our earlier example) the next largest scale on the meter should give you the most accurate reading. The difference is that in the older analog Volt Ohm meter (VOM) the meter movement itself can affect the measurement you are taking, due to its relatively low impedance. In essence, it can "load" the circuit under test. In addition, because the voltage on them tended to have the positive voltage on the black lead, this can cause strange readings in complex ciruits with diodes and other semiconductor devices in them because the voltage is opposite of what a Digital meter usually uses. An analog VOM with a digital readout would have the same loading characteristics as a VOM with a needle meter display, and would have the same 20,000 ohms/V rating on it as an analog VOM. If you look at the label for your meter, the odds are good that it says Digital Multimeter. As long as you select the appropriate scale for the testing you are doing, you should get comparable results to the manual. Tim Raymer 73 de KA0OUV At 16:29 01/02/2002 -0500, rlewis at staffnet.com wrote: >Okay, I've got some questions. How does a "true DVM" differ from an >"analog" DVM with a digital readout? How does a "true DVM" measure >resistance without putting a voltage across the device being measured? Timothy A. Raymer Missouri Department of Health and Senior Services +++++++++++++++++++ Reply-To: From: "Ron D' Eau Claire" To: , Subject: RE: [Elecraft] Resistance Readings with Digital Volt Meter Date: Wed, 2 Jan 2002 15:41:06 -0800 > Okay, I've got some questions. How does a "true DVM" differ from > an "analog" DVM with a digital readout? How does a "true DVM" > measure resistance without putting a voltage across the device > being measured? Never heard of a "true DVM", there are "true rms ac voltmeters", but I suspect this refers to a digital multimeter or "DMM" compared to an analog "Multimeter" or "Volt-Ohm-Milliammeter" (VOM). The difference is that the DMM circuit does not require as much power to operate the metering circuit, so it takes less current from the circuit under test to measure voltage or current and applies less voltage to the circuit to measure resistance. In an analog VOM or "multimeter", the circuit whose voltage is being measured provides the power to move the meter pointer. In ANY circuit being tested, measuring the voltage or current alters the voltage or current to some extent. The "quality" of an "analog" VOM was largely measured by the sensitivity of the meter movement. A good VOM meter movement may require only 50 microamperes to deflect it to full scale. Cheaper meters may require as much as a milliampere to produce full scale deflection. That's current that must come from the circuit under test. In many situations the amount of power taken from the circuit to measure the voltage or current is insignificant, but not always so. Very high impedance circuits are one example. Even a very sensitive VOM with a 50 microampere movement will draw enough current to change the voltage in such a circuit so that any measurement made is essentially meaningless. This problem was overcome by the advent of the "vacuum tube voltmeter" or VTVM. In a VTVM a vacuum tube provides the power to operate the meter. The meter is controlled by applying the voltage from the circuit under test to the grid of a vacuum tube, which has a VERY high impedance and requires only the tiniest amount of current to control the meter. Now one can make accurate measurements of extremely high impedance circuits. The current required from the circuit under test by a VTVM is almost entirely that needed by the voltage dividing resistors used to set the voltage scales. These can be very large resistors. Most VTVM's have input resistance of 10 or 11 megohms for d-c measurements. Then came the "Digital Multimeter" or DMM. It is a digital version of the VTVM, using a field effect transistor at the input. The FET also has a very large input impedance and most DMM's have a 10 megohm input resistance, similar to VTVMs. Today, almost all multimeters you see with an analog movement will be a VOM, not a VTVM, and they require a lot more current from the circuit under test than a Digital Multimeter. Measuring resistance, in every meter that I have used, digital or analog, involves applying a voltage to the circuit and measuring the resultant current. The meter or readout actually measures the current, but it is calibrated in Ohms. In older VOM's, it was common to use a 22 or 45 volt battery in the ohmmeter circuit. That meant that you might apply 22 to 45 volts to the circuit under test and the meter might supply several milliamperes at that voltage. That was no problem for a tube circuit designed to work at several hundred volts, but it is deadly to modern solid state gear. VTVM's were not quite so bad, typically using 1.5 volts for the ohmmeter functions. But that can be dangerous to some solid state circuits. Modern DMM's use much less voltage and current. My 'bench' DMM delivers at the most 0.3 volt to the circuit being tested and never allows more than 1 mA to flow in the circuit being tested for resistance. That the maximum values, and it is usually much less. Those levels are safe for any solid state circuits. The process is the same. A known voltage is applied to the circuit, the resulting current flowing is measured, and the result is displayed as ohms on the readout. However, even a few tenths of a volt is sufficient to cause some solid state junctions to conduct, somewhat. That conduction will cause the resistance re ported to be something less than infinite. That's why the Elecraft manuals say that circuits where you'd expect the resistance to be in the range of many megohms usually say to check for a resistance of >100K ohms. Different DMM's use slightly different voltages and so they will report substantially different resistance across such circuits. Ron AC7AC K2 # 1289 ++++++++++++++++ Reply-To: "Don Wilhelm" From: "Don Wilhelm" To: , Subject: Re: [Elecraft] DMM suggestion Date: Sun, 17 Mar 2002 14:02:51 -0500 Dan, For DC measurements, it makes no difference which type of meter is used. For AC measurements - on a true sine wave signal there should be no difference in the two types of meters. Either type of meter makes assumptions about the waveshape and really can't be trusted unless you know that the waveshape is the same as that which the meter reading is calibrated for. In difficult situations and unknown waveshapes, I prefer to read the peak value and at the same time observe the waveshape is so I can determine anything I want from there - but that requires a 'scope to determine both at the same time. Bottom line for use of any test equipment - KNOW YOUR EQUIPMENT!!! That means to know its capabilities AND its limitations - if you follow that rule, you won't go wrong with any meter, but there will be times you have to say "I still don't know" because the meter is not capable of showing you the right thing. Or stated another way - 'match the tool to the task at hand'. 73, Don W3FPR ----- Original Message ----- > I just learned in Troubleshooting class, that it is better for your DMM > to measure Voltage RMS (root mean square or True Voltage) instead of AVG > (average). +++++++++++++++++++