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I need to build an ammeter circuit for a model train layout. I want to use an analog ammeter (I just like the look of them) but I have to do some special stuff because DCC (the control system for the trains) is electrically weird. It's basically AC, except it sends data by flipping the polarity.

I found a how-to on making an ammeter for DCC, and it uses a DC ammeter gauge with a bridge rectifier. I have an old monitor power supply board (the backlight died, long story) that happens to have a bridge rectifier on it, so I'm thinking I'll use that instead of buying one.

My question is will having a bridge rectifier that is rated for more voltage than you need cause problems? I wouldn't think so, but I don't wish to smoke anything so I figured I'd ask.

Also, is there a way to measure the amperage consumed without a voltage drop on the track? A small one isn't really a big deal, but if I can remove any drop that'd be awesome.

Edit: My DCC controller is a Digitrax Zephyr Xtra, if anyone is wondering.

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    \$\begingroup\$ I would put the ammeter before the DCC output stage of the controller, it will just look like DC there. \$\endgroup\$ – Jasen Jan 25 '16 at 5:32
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    \$\begingroup\$ What do you mean before the DCC output stage? You mean between the power supply and the controller itself? \$\endgroup\$ – K4KFH Jan 25 '16 at 5:59
  • \$\begingroup\$ Yes. You might get a 'quiescent' / non-zero reading of the power used by the controller when the train is stationary but real electric trains draw power when stationary too to power air compressors, battery charging, lights, heaters, etc. \$\endgroup\$ – Transistor Jan 25 '16 at 7:16
  • \$\begingroup\$ Companies like LEM and others make clip on (or thread through) active DC current sensors that cause effectively zero voltage drop. They do cost a little but may be easy to implement. The volt drop across a bridge will be two diode drops and the drop in the ammeter. Not large but detectable. \$\endgroup\$ – KalleMP Jan 25 '16 at 7:50
  • \$\begingroup\$ @K4KFH i mean connecting the ammeter to the insides of the controller, somewhere where the power is still DC. \$\endgroup\$ – Jasen Jan 26 '16 at 8:38
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The voltage rating of the rectifier isn't important (so long as it is higher than the supply voltage) but its current rating might be. The Digitrax Zephyr Xtra can put out 3 Amps, so for safety your rectifier should also be rated for 3A or higher. Another possible problem is the voltage drop of about 1.5V, which could reduce the top speed of your engines and might affect other DCC devices.

Since your Digitrax Zephyr Xtra is powered by a separate 13.8V DC supply you could simply wire the ammeter into the power input lead, eliminating the need for a rectifier and reducing voltage drop to a minimum. The DCC controller will draw a small quiescent current. If this is unacceptable then you could use the meter's zero adjuster to null it out.

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  • \$\begingroup\$ Okay, that makes sense. So I could just make a little adapter (with a male-to-female DC plug cord) to go between the Zephyr wall wart output and the Zephyr itself, and the ammeter would be wired in to one of the two wires there? Also, do all analog ammeters have a zero adjuster, or will I need to specifically find one that does? \$\endgroup\$ – K4KFH Jan 25 '16 at 13:52
  • \$\begingroup\$ Male-female adapter plugs is a good idea. All good moving coil analog meters have a zero adjuster. \$\endgroup\$ – Bruce Abbott Jan 26 '16 at 0:29

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