Old Hornby R912 shows 24v on multi meter?

[probedb]

So I've got my 80s HST set out to try and play with. I serviced the engine and bought some LEDs for it and they stopped working in one direction after not much use.

I've put a multi meter across the rails and it's registering 24V. I realise from reading it's not true DC but surely that's way too high?

Time for a new controller?

Thanks in advance

[AndyMac1707817969]

The HST will have nothing to do with it. What power source are you using? Most likely it’s not stable 12v DC which would explain the high DC value when measured with a multi-meter if you measured it with no load.

What size of resistor did you use in series with the LEDs? Should be the supply voltage  minus the LED voltage divided by the current drawn by the LEDs

[AndyMac1707817969]

I am guessing the R912 does use a thyristor to convert from AC to DC and thus the output needs to be measured under load i.e. something connected to it like an electric motor. If you just meter them open circuit using a multi-meter will read something around 21v DC not 12v DC as expect if it were stable DC.

[Chrissaf]

I concur with Andy, if this controller uses a Thyristor as its output controlling semi-conductor then you need a load on the output to take a reading.

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I also concur that the LED current limiting resistor values (assuming that they are even fitted) do not have high enough resistor values (ohms).

[probedb]

The guy who does them said they can take over 30V but the behaviour of the controller is a little strange as they're at peak brightness at lower power and now dim at higher voltages. Going to get a new controller anyway and see what happens. He said he'd replace them but I figured I'd try a controller that wasn't 35 years old first.

[Chrissaf]

...the behaviour of the controller is a little strange as they're at peak brightness at lower power and now dim at higher voltages.

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Controllers that use 'Thyristors' do not control voltage, they control current. So your concept that lower speeds on the controller equates to lower voltage is flawed. The Thyristor controllers are designed for controlling relatively high (compared to LEDs) current loads such as loco motors etc.

[Yelrow]

Being a simple guy, Do the H&m  Duettes use Thyristors, and what are they in laymans speak.

[Chrissaf]

Duettes used heavy duty wire wound rheostats as they were not an all electronic controller ... so no ... H&M Duettes, Clippers etc do not have Thyristors to control the output to the track.

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A Thyristor in layman's terms is a three legged semi-conductor where a small variation in the current fed into one of the legs, changes the amount of output current that the Thyristor can supply.

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Thus it is the current that is offered to the loco motor on the track that is controlled and not the voltage. To go fast, the loco motor needs more electrical power (current), so by using the Thyristor to closely control the current, you can control the loco speed. More current ... loco goes faster ... less current ... loco goes slower. The voltage therefore becomes a variable parameter that has very little impact on the motor control, which is why measuring the Thyristor controller voltage 'off load' does not give a meaningful voltage reading. It is the current that is controlled and the voltage is allowed to freely go up and down as it needs to.

[ColinB]

Thryristors are more of an AC device, I must admit I have not used them other than in "crowbar" protection of a power supply. I think they are used in switched mode power supplies but you wouldn't be using them in a model railway power supply. Generally you expect to see a transformer feeding into a bridge rectifier and either feeding an electronic circuit or in the case of H&M a wirewound rheostat. Incidentally, that is why I am always saying on this site, that nobody should use this type of DC power supply anywhere near anything electronic, so if your loco has a factory fitted DCC decoder but works on DC the unregulated voltage is enough to blow it up.

[Chrissaf]

...but you wouldn't be using them in a model railway power supply.

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I can confirm that the Chinese made Hornby R965 uses a Thyristor. However the earlier Margate made version of the R965 uses a Transistor instead. Thus there is no reason to believe that other fully electronic Hornby controllers from the same era [such as the R912] didn't potentially use Thyristors as well.

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The evidence for this statement is in two Internet resources. Firstly this one, which is taken from a web page reviewing and comparing a number of different branded controllers. Note that the R965 image shows "Made in China" embossed on it. This is significant .. see second Internet resource further below for why.

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If you want to see the original article in context, from which this extract was taken, then click here.

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Secondly, the earlier Margate version using a transistor statement comes from this YouTube video, where the Margate version transistor is uprated to improve higher current performance. The reference to the Chinese and Margate [England] versions being different is made from 2 minutes 30 seconds into the video.

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[ColinB]

That is weird, I took the one I got with the Pendulina set apart and even though it was fed from a separate supply, it still had a bridge rectifier in it. Admittedly this is a PWM controller. Looking at the videos on YouTube it looks like it uses the coil of the loco as an inductor, although it still has a bridge rectifier. I can only assume what the circuit is, as I said I am no expert on thyristors. By effectively "chopping up" the DC you get a more efficient power supply with less heat. I suppose that is why without a loco in the circuit, like open circuit you get high voltages. I assume they went to PWM as it is a better option and safer with DCC decoders running DC.