Hi,
I am wondering how do you measure the amps on the tracks with the digital mutimeter? I can measure the voltage and that is 17.2 volts with engine running, but would like to see how many amps are being drawn by the engine while it is operating? Thanks.
laz57
Hi laz,
I am a LOOONG way from being Lionelsoni, but I believe that You need to connect your meter in line, rather than across the circuit. Transformer-Meter-Center Rail.
You can measure voltage, without any load(amps) by measuring between center and an outside rail, but to measure the amps you need the meter between the transformer and the load.
Your meter should have red and black lead inputs, try connecting the the Red lead to the transformer (Track connection, not common) and the Black lead to the track (Center Rail, not outside rail), without any paralell bypass wire, just through the meter only.
Doug
You need to have a resistor in series from your transformer to track. Then you will need to measure the voltage accross the resistor, and calculate the current using Ohm’s law…I=E/R…The R value of the resistor will generally be small, although the resistor has to have leads of sufficent size to carry the current. I am sure Bob Nelson will give you a value when he logs on; or a better way of doing things.
If you want to measure the current directly, you need a meter that measures AC amperes. Most cheaper meters don’t have this capability. If you have an AC ampere scale, make sure it will measure up to at least 10 amps. Then put the meter in series with either one of the leads from the transformer to the track. It doesn’t matter which one, and it doesn’t matter which way the meter is installed.
If you don’t have an AC amperes scale on your meter, you can put a resistor in series with either of the leads from the transformer to the track. The resistor should be about 0.1 or 0.2 ohms, and it needs to be a power resistor of at least 10 watts. The power dissipation is I^2*R (current squared times the resistance). 10 amps with a 0.1 ohm resistor is 10 watts. 10 squared is 100 times 0.1 is 10. The resistor is going to get hot, so put it on something that will not get damaged. You can use clip leads from Radio Shack to put this together.
Measure the voltage (AC) across the resistor. The current is the voltage divided by the resistance. So if you measure 1 volt, the current is 10 amps. A larger resistor will give a more accurate reading, but will also get a lot hotter, and will have a higher voltage drop and so may not allow the voltage on the track to get high enough.
Servoguy gives good advice. I do have a few additional comments, however:
Don’t put the resistor in series with the common, unless you’re not powering anything else that uses that common at the time you make the measurement. Otherwise you’ll measure the sum of all the currents being drawn from that transformer and any others sharing the common.
Unless you have a true-RMS meter, you can’t make a decent current measurement with a non-sinusoidal waveform. In other words, no CW80 nor “Powermax”.
Keep a jumper around the resistor until you’re ready to take the measurement, then read the meter quickly. As the resistor heats up, its resistance will rise and give you an exaggerated reading.
An inexpensive AC panel meter[$10] will give you + or [-] 2-3% accuracy for determining the amperage load. ALLElectronics has 0-15 RANGE amp meters for about $10.
Hey Laz,
Not positve (or negative) [(-D] but when at work when measuring current one puts the meter “in line” or “in series” with the load. Remember one is measuring the current being drawn “through” the line by the load. Hope this helps at least a bit.
Well I posted this and then saw that about 3 others posted in the time I was typing and too late to delete. Sorry to repeat what has allready been stated above![banghead]
Thanks for all the info. I guess if I measure in line I guess I’m ok?
laz57
Most inexpensive multimeters don’t have provisions to measure AC current but most will measure DC current. If this is the case with yours and you have at least a 0 - 5 amp DC scale you can still measure AC current by using a bridge rectifier. The rectifier needs to be able to handle at least 5 amps (10 would be better). If you plan on leaving the meter in the circuit the rectifier will dissipate quite a bit of heat so you might need to add a heat sink to the bridge rectifier. If it is just for a temporary reading then the heat issue won’t be a problem. There will be a voltage loss of about 1.2 volts due to the bridge rectifier but this shouldn’t be a problem with most transformers and engines. I also agree with Bob Nelson that if you are using an electronic transformer that alters the pure sine wave that readings will be innacurate.
Here is a diagram of a DC to AC ammeter circuit. The LOAD represents the track and like Bob says don’t place the bridge/ammeter in the common connection to the track if you are also using accessory voltages since you will then get a total current reading for everything that is powered by the transformer and not just what is on the track.
An easier way to measure AC current is with a clamp-on amp meter. A clamp-on meter has jaws that open and close that operate like a current transformer. With a clamp on meter one current carrying wire is placed within the clamp and current can be read without changes to the circuit.
Clamp-on meters are also useful for measuring transformer input current, again without breaking the circuit. There is a “Line splitter” adapter that allows plugging in a line operated device. It provides a “loop” for using the clamp-on meter. Mine also has test points for measuring line voltage. It is a separate device, usually listed with clamp-on meters. Quite useful and makes taking line measurements easy and safer.
I also have some clamp-on panel meters for permanent installation, but I haven’t seen them for sale for a while.
My favorite is an analog clamp-on amp meter. I can watch the needle for changing currents which can be more useful that the actual current readings. Digital meters sample the current and, so, aren’t as useful for observing changes to a circuit. Analog clamp-on meters are hard to find now, though. If you find a used one, snatch it up.
Last I looked Radio Shack had a digital clamp-on meter for about $20. Looked like a real bargain. Jameco (www.jameco.com) has six models starting at $49.95.
JK