A little help would be appreciated. Blew the fuse on a cheap multimeter when trying to do something besides the continuity/ohms test. Thanks Radio Shack for replacement, no questions asked. But, there's a reason I got the meter. To what do I touch the probes to do a volts or amps reading? I'd like to know these values at several places around the layout and with/without engines running. I do DCC but can easily switch to a DC power pak for any test. Believe that values for DC will satisfy my curiosity, but would expect the tests would be the same for either. *Well, are they?) Practically, I'd like to know the amp draw for different engines under different circumstances; the volts at different points; total amps w/many engines and local amps at different places. I'm trying to figure when my Digi-Zephyr will run out of juice.
You hook the meter to the circuit differently to measure voltage then you do to measure current. The below information is from a document I wrote awhile ago trying to explain how to use a meter. If you're measuring DCC set the meter for AC, if using an analog power pack set the meter for DC. Meter basics: Measuring voltage is accomplished by connecting the meter across the circuit you want to measure. To measure the voltage at a spot on the track set the meter to either DC volts or AC volts and connect one meter lead to each rail. If you want to measure voltage across a component connect one lead to each side of the component. Measuring current requires placing the meter in series with the circuit. This requires creating an open in the circuit and then connecting the meter across the open. This results in the circuit’s current flowing through the meter. Because the current flows through the meter there is some danger involved. If you connect a meter to a circuit with a higher current flow then the meter is set to or designed for you will either blow an internal fuse or damage the meter. If you set a meter to read current and connect it in the manner used to measure voltage, you will again blow a fuse or damage the meter. Measuring resistance involves connecting the leads across whatever you are trying to measure the resistance of. The main thing to remember is resistance is measured in un-powered circuits. When set to measure resistance (the omega symbol on the meter) the meter will actually supply a small calibrated voltage to the circuit being measured. If you hook a meter set to measure resistance to a circuit with power turned on you can damage the meter. A note on measuring DC: Digital meters tend to automatically handle polarity. If you hook the positive lead to a positive voltage and the negative lead to a negative voltage the meter will display a positive reading. If you hook the meter up backwards it will display a negative value. Analog meters don’t do this. If you hook up an analog meter properly the needle will swing to the right and you’ll get a reading. If you hook an analog meter up backwards the needle pegs itself to the left side of its scale; if this happens reverse the leads.
biggrin: Thanks, Mark. Think I got it. Only issue is that to do current I can't just touch the probes to the rails, wires, etc. Believe I would need to link the probes to the circuit path after breaking the circuit. Any chance that just touching the probes to the line as if I were doing ohms but setting to measure current would give something like 1/2 the current reading (there being 2 paths for the current?) arghh: I'm hesitant to experiment because I blew the meter once and would be embarassed to do it again. (My Radio Shack status would fall!) laugh:
Nope, no chance at all. But a very good chance of eliminating your RS status entirely. See the last line of Mark's para on current - connecting for Ohms and Volts are similar and doing either with the meter set to current (A, mA) is high risk. To measure current in a DC type circuit (I'm being careful here) at our level requires 'something' to be placed in the circuit through which the current flows. Usually it's a multimeter lashed in or a built in meter wired permanently. I believe there are devices (Hall effect?) that can get round this but they are a bit special and probably relatively expensive.
FYI, a multimeter can't accurately measure DCC track voltage. It's a square wave AC voltage, and a typical multimeter is designed to measure AC sine wave voltages, and present the estimated RMS voltage. To see what's going on with DCC track voltage, you need an oscilloscope or some specialized instrument.
Thanks guys. Understand that I'd need to patch the probes into the circuit for amps and run the juice through the meter. Hope that a simple AC volts check would give an imprecise but consistant reading for DCC volts. I don't need to pin a perfect measurement, just want to confirm that I'm getting same DCC volts everywhere.
It should be consistent and probably within10-20% correct IIRC. To check track volts (DC or DCC) you need a load to show up any poor connections or too long/thin feeds. Pulling about an Amp should do it. I'd consider connecting a 15 Ohm resistor across your meter leads, so when you put them on the track the load is applied at the same time and place. The volts reading will then be realistic for a moderately hefty train at that point. Important points - The resistor will be dissipating about 15 Watts and will get hot, so a) get something like a 15W or 20W rated resistor (sometimes called power resistors), and don't hold the probes on the track too long.
If you have a true RMS meter, then it should report the correct voltage. Of course they are expensive and most people do not have one. When using a typical meter set on AC, it should be consistent, though off a little. The RMS voltage of a sine wave is equal to the average voltage times 1.11. What most meters actually do is measure the average voltage, but they are calibrated to show the RMS voltage of a sine wave, so they show the average multiplied by 1.11. The RMS voltage of a square wave is equal to the average voltage. What this means is you can take the voltage reported by the meter and divide by 1.11 and you will be very close to the actual voltage.
Something else I meant to mention. Notice that the multimeter probes have to be plugged into different places to measure current instead of voltage. I've blown more than one fuse forgetting this little detail.
DCC with multimeter The below meters show about 13.6 VAC when checking the track voltage using my NCE Power Cab. http://www.harborfreight.com/7-function-digital-multimeter-90899.html I have three of these meters and pretty much agree with the readings on my expensive meter and 'Oscope. readings. This is not rocket science. I was a NASA contractor for 14 years. I used the best equipment. A difference of maybe 0.1 or 0.2 volts is not a problem. Just look for a trend. In a pinch, I put thin layer of aluminum foil around the fuse if no fuses but I know what I am doing. I then hit RS for a fuse pack. Gues how much to replace a $200.00 multimeter if misused? Guess how much the replace a mulitmer if misued in the link I provided? The manual that came with your meter has instructions. There are many Internet links on using a multimeter and You Tube no doubt has a lot of videos about using a multimeter. Rich
I do not use these meters to read DCC current. I use the below link idea with these meters for DCC current. Two Integrated circuits convert the DCC to DC. http://home.cogeco.ca/~rpaisley4/DCCammeter10.html I have verifiied the readings and a NCE 5 amp Power Pro trips right at 4.9 amps. A Ramp meter is good for those who do not build their own circuits. Rich
I can't believe that you are suggesting that a blown fuse can be wrapped with aluminum foil to a newbie looking for advice. I know what I'm doing too, and I would NEVER do that. Typical DMMs measure AC by rectifying it, integrating the signal, and then scaling the results to approximate the RMS (NOT the average value, but close) voltage ASSUMING that it's measuring a sine wave. Meters with "true RMS" circuits do a better job, but can still be fooled. But it would not be a good idea to trust the DMM reading of DCC track voltage. However you can make measurements at different points and look for differences (e.g. voltage drops), but I wouldn't trust the absolute measurement. If you do want to know the absolute DCC voltage on the tracks, either use an oscilloscope or this simple circuit: Connect the track rails to the AC inputs of a bridge rectifier (any bridge will do). Hook up a small capacitor across the "+" and "-" outputs of the bridge. This capacitor should be 1-50 microfarads, at 50 Volts or higher. Make sure that you hook up the "+" side of the cap to the "+" side of the bridge. Now just set your meter for DC Volts and measure the voltage across the capacitor (i.e. the "+" and "-" of the bridge). This will work universally and give you a very accurate measurement. -Mark
To be clear, I was not saying that the meter shows the average voltage, it tries to show the RMS voltage by multiplying(scaling) the average by 1.11, which is correct for a sine wave but not for a square wave. This method will be off also because of the voltage drop of the diodes. Depending on the rectifier, it will be off around 1.5 volts.
I did not tell the newbie to wrap the fuse in foil. I just said what I do. I give people here the benefit of the doubt and figure they have common sense. The last I knew, everyone comes with common sense by default. But I have been wrong before. Rich
What I'm trying to say is that there is a multitude of ways that different meters will try to display AC Volts, and none of them are designed to handle an asymmetric square wave, i.e. the DCC track power. Some may even freak out, others may just be "off" a fixed amount. You are correct about the voltage drop. I figure (in my own way) that any other DCC device will also rectify the DCC signal with its own diodes, and then you end up with the same "net" DC value anyway. -Mark
Yes, people have common sense, and I would have hoped that that common sense extended to "NASA experts" bragging about their unsafe practices to people that may or may not know better. -Mark
My Radio Shack meter has 2 settings for amps----Where you plug the probes in there is 3 holes one is for amp readings up to 300milliamps and the other is 10amps UNPROTECTED witch means if you exceed the 10 amps you WILL TAKE OUT THE METER. and the 3rd hole is common ground My suggestion/point is if you think you amp draw you are checking is over 300milliamps, change/ swap your probes to the higher setting (holes) or run the risk of blowing the 300milliamp fuse located inside the tester . When I take amp readings I also have a 10amp in line fuse on my prob lead to prevent a epic disaster. One other thing I always turn the dial to read voltage, or off when I'm finished with my meter to prevent a mishap ( Plugging a meter into a circuit and THINK you reading voltage but in reality your in amp mode you just shorted out you meter and watched it smoke cause in amp mode it was a direct short across the circuit. Alan C.
You can also read the manual FIRST. I know, it is not a Male thing. The foil part was because of my model railroading experience. It is not rocket science. There is a difference. I like to do that for the Ranters I see in many forums. Provides fodder for them. Rich
arghhid read the manual first; didn't trust it in re amps, plus was hoping that I could "bridge" across the bus for amps getting a proxy reading. confused:Also confused about getting amps w/wo load and at what points to "probe" the wires. Mostly was helped by the discussion here. smile:Thanks again to those patient contributors who indeed helped me. cool:BTW, learned from another site that to get Zephyr's voltage at any point one must test the A track/wire or B track/wire separately by probing the test point and the common post on the Zephyr. Sum of Va, Vb equals total voltage. Should be very small variance Va, Vb or something's not right. Good news is there was no significant Vdrop at the points most concerning me. Had a V8 looked to see an unexpected gradient across the area; leveled the platforms--fixed.
Based on this thread and a friend bugging me, I went out and purchased a multimeter, got a reasonable deal on it as it has been discontinued with its newer brother in with auto ranging, mine doesn't. Still debating whether to take it back and get the newer one, I don't know whether I will use it that much but having said that, I really like it and have used it already. But to the suggestion to read the manual - right. The first budget analogue meter I bought (which I found the day after I bought my new one) didn't come with instructions, the instructions for the new one are not geared for the newbie, so reading the instructions isn't always the way to go. Instead as some one here suggested, hit youtube, which I did and watched a number of videos until I could approach my new meter with a mild inkling of an understanding. Not all multimeter manuals are created equally. Here is one, but you can link to others through this one: http://www.youtube.com/watch?v=bF3OyQ3HwfU&feature=related Heck I'll throw this one in as well, two videos for the price of one. Unfortunately this video is done by a chap who may understand multimeters but he knows d... about how to video decently, the images will shake in his hands and be out of focus, easily solved if he had just left the meter on the table: http://www.youtube.com/watch?v=n08-OzE7CkE
