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I use a small adaptor based on a circuit published around 1995 on NCE yahoo groups, to measure the voltage using a DC meter, analogue or digital, or a DMM.  I use a 20V DC analogue meter.  Works a treat but a digital readout would be better for visibility at a distance.

Basically stores averaged near peak voltage in a capacitor.  Consists of two transistors, three capacitors and three resistors.  Claimed to be accurate to within 100mV, probably due to the necessary charge and bleed resistors plus transistor biases.

Should cost about 5 to 10 quid to make.

Pity the topic wandered in the realms of expensive equipment and daft views of DCC/AC wave forms rather than just address the original simple question and application.

Edited by NinOz

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7 hours ago, NinOz said:

I use a small adaptor based on a circuit published around 1995 on NCE yahoo groups, to measure the voltage using a DC meter, analogue or digital, or a DMM.  I use a 20V DC analogue meter.  Works a treat but a digital readout would be better for visibility at a distance.

Basically stores averaged near peak voltage in a capacitor.  Consists of two transistors, three capacitors and three resistors.  Claimed to be accurate to within 100mV, probably due to the necessary charge and bleed resistors plus transistor biases.

Should cost about 5 to 10 quid to make.

Pity the topic wandered in the realms of expensive equipment and daft views of DCC/AC wave forms rather than just address the original simple question and application.

Excellent suggestion. For an even simpler option that should give a consistant error of 0.7V too low on the dc voltage setting is a simple peak detector.  This consists of a diode in series with a non-polarised capacitor - measure the voltage across the capacitor. Cost from about 10p + vero board and some leads. Although not super accurate it should be consistent, and much better than the ac setting of a typical home digital multi meter (dmm). An analogue multi-meter will give a lower voltage reading due to its lower input resistance.

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14 minutes ago, H2O said:

Excellent suggestion. For an even simpler option that should give a consistant error of 0.7V too low on the dc voltage setting is a simple peak detector.  This consists of a diode in series with a non-polarised capacitor - measure the voltage across the capacitor. Cost from about 10p + vero board and some leads. Although not super accurate it should be consistent, and much better than the ac setting of a typical home digital multi meter (dmm). An analogue multi-meter will give a lower voltage reading due to its lower input resistance.

Even better just a bridge rectifier without a capacitor feeding a DC voltmeter.

It will read about 90% of the true RMS value.

4x 1N4148 diodes to make the bridge.

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23 minutes ago, melmerby said:

Even better just a bridge rectifier without a capacitor feeding a DC voltmeter.

It will read about 90% of the true RMS value.

4x 1N4148 diodes to make the bridge.

 

Fit the rectifier diodes on a bit of PCB offcut, add a cheap three digit DC voltmeter display (numerous on sale for under £2), a couple of bent brass tabs (etched kit offcuts) at the end to push onto rails, and there's an under £5 track voltage tester.   

 

Addition of a couple of LEDs (with resistors) before the rectifier will mean it can distinguish DC from DCC.  On DC, one LED will light, indicating direction of voltage (which rail positive).  On DCC, both light. 

 

 

 

Nigel

Edited by Nigelcliffe
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15 hours ago, Dagworth said:

 

No, that's the DC vs. DCC argument. Not quite what I had in mind, but still good.

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9 hours ago, NinOz said:

stores averaged near peak voltage in a capacitor. 

 

??? DCC is a square wave so no need to average or detect peaks or do any RMS calculation. As already said, a simple bridge rectifier is more than adequate.

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4 hours ago, melmerby said:

Even better just a bridge rectifier without a capacitor feeding a DC voltmeter.

It will read about 90% of the true RMS value.

4x 1N4148 diodes to make the bridge.

A full bridge rectifier will have the disadvantage of an error of approximately 1.4V.  This is 90% at a track voltage of 14V but may be easier to think of as a drop of 1.4V.
If a full bridge rectifier is used a capacitor will still be useful as without it the voltage measured after rectification will fluctuate a little, especially if the system is under load (which is when some faults show up). The ideal dcc voltage will switch cleanly, the actual signal will not due to the layout being a complex load.
A full bridge rectifier will have the advantage of it not being important which way round the circuit is connected to the track. This can be solved if using a single diode by simply switchIng the dmm leads around.
For accuracy the transistor circuit above may be best but most expensive, followed by the single diode and capacitor solution which is cheap and then the full bridge circuit which is more expensive and less accurate (due to the 1.4V drop).
As with everything, each solution has its own advantages. Take your pick...

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Again we stand to go off into the realms of nit-picking.

What are we trying to achieve with measuring the voltage, what was the OP trying to do?  Just trouble shooting wiring problems or trying to fix booster/supply circuit problems, DCC signal quality?  I assume the former for the OP.

My NCE adaptor just monitors voltage at the booster outputs, really not doing much except noting shorts and indicating all is fine at the booster.

For checking wiring quality I use my little DMM on AC setting usually.  I don't care what the voltage or what the measurement bias really is as I am looking for any significant resistances in the supply to that point.  Measure the voltage unloaded, drop a load on that point and measure the voltage, look at the voltage sag, if significant trace any anomalous results, fix.

Doesn't matter if the bias is 0.1V, 0.7V, 1.4V or x.xxV as long as it is consistent.

 

"??? DCC is a square wave so no need to average or detect peaks or do any RMS calculation. As already said, a simple bridge rectifier is more than adequate."  :huh:

The circuit I use averages due to balance between built in rate of charge and discharge of the capacitor on each +/- pulse.  Nothing in my post demands that averaging, RMS or any such is required.  It is just how the circuit works.

Some people could start an argument about anything even if not said or implied.:aggressive_mini:

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On 06/07/2020 at 02:01, NinOz said:

Some people could start an argument about anything even if not said or implied

I hope the poster sees the irony of his somewhat argumentative post?

 

Andi

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By the way mention was made of using scopes, unless you have , an isolated scope, battery powered scope , differential probes, or setup to use two inputs in differential mode , don’t connect a standard scope probe across a DCC track , as the scope probe  gnd  is at mains ground and will short out the DCC track. 

 

 

Edited by Junctionmad
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47 minutes ago, Junctionmad said:

Nice.  Built in DCC decoder , what scope is it 

 

I asked that and Melmerby answered it in the second last post at the bottom of page 4.

It's a PicoScope:

 

https://www.picotech.com/products/oscilloscope

 

It's worth going and having a look at that post.

 

 

Kev. 

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3 hours ago, Junctionmad said:

By the way mention was made of using scopes, unless you have , an isolated scope, battery powered scope , differential probes, or setup to use two inputs in differential mode , don’t connect a standard scope probe across a DCC track , as the scope probe  gnd  is at mains ground and will short out the DCC track. 

 

 

I use my Picoscope with the laptop. It's USB powered and current consumption is quite low.

 

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