How To Convert DC panel meter to read AC AMPs?

Look at a precision rectifier.

Firstly, you will need to use a bridge rectifier to convert the AC to DC. Then you will need to use a shunt in series with the AC line as a means of detecting a suitable voltage range to drive the meter.

Read Rod Elliot's article on meters and shunts.

Thanks for the references but building precision rectifiers seem a bit overkill. Are there any pre-built precision rectifiers that come on a single chip?

I really didn't want to make this such a big project that I would have to breadboard the supporting circuitry. I was hoping a simple diode would do the trick. That is why I tried a half phase, single diode test using 1N914/1N4148 silicon switching diodes. I made up a bridge rectifier (made of the same diodes) but this dropped the sensitive of the meter quite a bit. Using a standard 50V silicon bridge rectifier delivered almost no voltage to the meter! I need .9mA for full meter deflection.

Building two op amp circuits (one for each supply that I am measuring) and feeding the Op Amps their 5v source is more than I wanted to take on. The power supply is delivering 17V AC at about 5AMPs to my digital controllers.

thanks,

You're only getting about 1 volt peak across the sense resistor at maximum current, so you don't have much to work with. Schottky diodes or germanium diodes have lower voltage drops than standard silicon. Using schottky or germanium bridge with a double diode drop might still be low enough to give you a reading, but I don't see why you want to use a bridge. Wouldn't the extra drop affect the accuracy of your meter in the lower range? You're getting very little voltage from that sense resistor. Until there's enough current through your sense resistor to exceed the drop of your diode or diodes, the meter will read zero. The more diodes you use, the more inaccurate. If it was my railroad and I wanted the meter accurate I'd probably use a single shottky or germanium diode. You could also look into using a current transformer instead of a sense resistor. Theoretically at least, you could get a linear, full- scale reading on your 5 volt meter. The devil is in the details and I haven't used current transformers, so I'll have to defer to persons with more expertise on how to execute that.

If you can sacrifice some more voltage in the shunt here's a simple idea:

Put a pair or silicon back to back diodes in series with the shunt resistor and connect the meter to the shunt resistor and diode pair. The diodes have to be rated high enough to handle 1/2 the AC current that you will use - and there should be a fuse or some current limiting (or just use a pair of 20 amp diodes)

That will drop Point .6 volts AC and compensate for the diode drop in the meter's rectifier diode. You will have six tenths of a volt less going out to the train layout - if that's tolerable to you.

Likewise you could use two back to back pairs and compensate for a FWB rectifiers and waste 1.2 volts in the compensator diodes.

Another different and slightly more complicated approach might be to bias your meter rectifier on all the time with a small current returned to the opposite AC supply rail.

Your AC lead is connected to the shunt and diode anode, the load is connected to the other end of the shunt (same place the minus of the meter is connected). Your basic set up.

Add another diode and resistor connected in series from the junction of the meter and cathode of the meter rectifier and the opposite AC lead. The cathode of the compensation diode goes to the AC lead. Resistor value? That depends on how much current your meter needs but a watt or two of power dissipation should be plenty. Both diodes (meter rectifier and compensator diodes have to be able to handle the compensation current - so a 1N4148 might not be the best meter rectifier in that app)

I think that idea would work, but would welcome any criticism or critique, if some guru would like to lambaste me. The first idea would work the second idea doesn't lower the voltage out to the track so may be more appealing.

Basically you want a circuit to let you read a very low AC voltage with a DC meter. This is difficult to do. The original method was to use a current transformer, however today it is more usual to use a 'precision rectifier' circuit. See

You would, of course, be better to purchase a 5 A meter.

Way back, during the dawn of time (1930) they used copper oxide rectifiers for meters. Very low forward drop and very low (6 V) PIV. They were used for meters, battery chargers, and radio detection. Reasonably linear for meter applications since most of the meters were only good for a 5% accuracy.

I scoured the web looking for a supplier for them and didn't turn up any - but they still make moving coil AC meters that use them.

I have a few moving vane meters I got surplus. They are repair stock for an old X-ray machine and were made by G.E.. Iron vane meters can be very accurate and they don't care if the power is AC or DC. Mine have something like 15 turns of wire on the coil and were intended as current meters. I suspect all the bad press iron vane meters received is due to the cheap battery checker and charger meters that were produced with 20% accuracy.

Nice thing about old iron vane meters is you can take them apart and rewind the coils for different uses and change the scale markings.

On 1 Feb 2007 21:17:43 -0800, "ahwii" wrote:You should have directed this response to Jamie who was the one suggesting precision rectifiers.

According to this page

I just bought a few 1N5817 Schottky diodes on eBay (DigiKey way too expensive and my local electronics shop had none for the taking - at least that I found) and will give these a try instead of the silicon diodes I have been trying. I guess the small transformer might be the next step to try. Yeah, I need to keep the voltage drop across the shunt resistor pretty minimal as the DCC controller wants to have a pretty steady voltage supply and needs to be able to pull anywhere from 0 to 5.5AMPS.

thanks,

Jamie wrote in news:7twwh.1295$ snipped-for-privacy@newsfe05.lga:

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