Voltage from Variable PSU for track - What Level?

Pete,

Try loading the supply with something like a 12 volt 6 watt car sidelight bulb.when you take your voltage measurement. If your test meter has a very high internal resistance, there will be no load on the power supply, and if it is dependent on load to work properly, then you will get an almost flatline high voltage output as you describe.

Jim.

Could he also be measuring peak voltage rather than RMS?

RMS is only applicable to alternating current, it is the DC equivalent of the AC waveform. Peak and RMS are effectively the same thing in DC.

A multimeter measuring DC voltage measures the constant level (i.e. Peak) and when measuring AC will measure the RMS value.

For instance what we call 230V ac in the mains (or 240V for old school people) is actually about 330v peak.

Luke

If they're smoothed.

He said he got 17v and was expecting 12.

I don't have a sqrt key so I used 1.414 and got a result of 16.968.

So his meter is showing AC or DC unsmoothed peak for some reason. Perhaps he had it switched to AC by mistake.

Thanks guys for the suggestions. I need to sort around and find a suitable load so will get back on the question of whether it is functioning properly.

On the point about AC \ DC - The power supply is DC 12 Volt output as stated on the unit itself.

There is no AC output or output for peripherals etc. It's old, Hornby, English and simple.

As to whether the supply is simply poor half-wave rectification... I don't know.

The point about high internal resistance in the multi-meter is valid as it is digital so most likely..

I'll test under load tomorrow.. and let you know.

Pete

Its the nature of the beast, because this type of controller is just a plain old resistance type, it will measure high voltage when no (or almost none) current is being drawn. Measure on load & you will get an entirely different result.

For this reason the R915 is a poor quality controller. It has no regulation at all. There are many vastly superior controllers on the market. Try almost anything from Gaugemaster or consider DCC (and even DCC is supplied by Gaugemaster).

The comment about English is wrong, cos electricity rules are non discriminatory. ;-)

Two things are going on here;

firstly, the speed control is done purely by changing the resistance between the supply and the load, so without a load you will see full voltage all the time except when the knob is at minimum, where the output is effectively connected to 0V.

secondly, the supply is not a constant dc output, it is half wave rectified. That means that the meter is seeing pulses of half a cycle of the ac input and then nothing until the next positive part of the cycle comes along. This causes the meter to give a unexpected reading.

What you need to do is firstly give the output a load, this does not have to be large. and secondly, if you want an accurate voltage measurement, smooth the o/p with a capacitor so that you see the true dc level.

Regards Jeff

Define the true DC level. Even with a capacitor there will still be considerable ripple when under load.

MBQ

That depends how big the capacitor is and how much current you are drawing (and the setting of the controller, the resistance forming an RC filter with the capacitor), but a few milli-volts of ripple is insignificant in this context.

Regards Jeff

Good control depends on far more than the size of the capacitor (mine's bigger than yours, might impress from one point of view, but not another). Of course if getting rid of ripple is your aim, use a lead acid battery, no ripple on a model railway with one of them. ;-)

And if you do, make sure it's fused or you could be fried instead of rippled :-)

Jim.

I wasn't suggesting that you attach a capacitor when you use the controller for train control, merely as an example of how you could get a meter to read closer to the 'real' DC voltage.

Motors are not at all fussy about 1/2 wave rectified, un-smoothed DC, in fact there is a school of thought that it is actually advantageous in getting sticky motors to start.

Regards Jeff

Thanks folks for the suggestions and comments...

I've tested it now and it works as it should.

It's not a very good supply as it fluctuates like mad whilst a train is on the track but it will be useful none the less as a test unit...

I've learnt something new so cheers for help.

Pete