Ringfield motor in class 85 loco

[ColinB]

Has anyone retro fitted a DCC decoder into a Hornby class 85 loco? I am wondering if I need to go for a high current version or is a normal Hornby decoder good enough? I know I should go and measure the current load, but given by experiences with Mainline locos ( I measured 400 mA, but the start up load must have been higher), I thought it would be easier to see if anyone else has done it. 

[Chrissaf]

I measured 400 mA, but the start up load must have been higher.

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That's why you measure the 'stall' current and not the normal current. The 'stall' current will be the worst case scenario.

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https://www.brian-lambert.co.uk/DCC_Page_1.html#Bookmark10

[ColinB]

Actually I think startup current is higher, but thank you for info. 

[Chrissaf]

At the instant of connecting power to a motor. The motor is in essence an inductor. It's impedance will be very low indeed and approximately equal to the DC resistance of the coil wire. But as the current starts to flow, a magnetic field around the motor coils starts to develop and the impedance rises as a consequence of this. This initial low impedance is extremely brief and the resultant instantaneous current draw should be well within the peak current rating of most decoders. The Hornby R8249 supports peaks of 1 Amp (motor plus functions).

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The 'stall' current is a better indicator of decoder long term suitability, because 'stall' currents can potentially last longer and be more sustained than brief start up current peaks. As such, the 'stall' current should be considered IMO as the 'worst case scenario' during normal running. Thus for a Hornby R8249, the measured 'stall' current should be less than 500mA.

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[jane1707819582]

Hi Colin.

You will be better with a higher current decoder .400 ma is sailing a little close to the wind.

[96RAF]

Note that even these 500mA decoders will take a short surge load of 1A during startup or overload conditions. See their spec sheet(s).

[ColinB]

I think we have chinese whispers here, there was no way I was going to use a 400mA decoder. I just asked if anyone knew of a suitable decoder that would drive a ringfield motor. I could go through all the process of measuring stall currents and running currents, but all I wanted to know if someone had some practicle experience of using a particular type and found it worked well. Yes, I know I can measure all the currents, but there is no substitution for information from someone who has done it. What I was saying was when I converted my Mainline locos, I used 6 pin DCC concepts decoders, and managed to fry two of them, replaced with right angle Bachmann ones and they worked perfectly. I read the DCC data that came with the decoder, and I was well within their published specifications for currents, so obviously there is something about a Mainline motor that a DCC concepts decoder doesn't like. When I get round to it I will ask them.

[ColinB]

Measured the stall current and it is 400 mAmps at 9v, so at 12 volts that should be less than 600 mAmps, so it looks like a normal Hornby DCC chip is OK.. I didn't realise, this must be a late ringfield model, it has pickups on all wheels. There is also lots of room inside. 

[Rog RJ]

I wouldn't risk an R8249.  It's rated at 500ma so you would be pushing it to the limits of its capabilities.  The 1amp peak that is quoted for this decoder is only for very short periods of time and you should avoid exceding 500ma as much as possible.

 

The Sapphire decoder woul be a better choice of Hornby decoder but there are also better suited decoders from other makers.  I would look for one with a normal current capacity of 750ma or more to give a margin of safety.

[ColinB]

Thank you for the advice, the Zimo ones are much higher current so I will probably go for one of those.

[Chrissaf]

If you re-read the link I gave in my earlier reply. It clearly states that the 'stall' current should be measured at the controller's full power setting. Subject to the DC Analogue controller used, this potentially could be more than 12 volts. Remember, you are selecting a decoder based upon 'worst case scenario' considerations. These very well may be worse than any condition that might be found in normal operation under DCC control. But it serves to give an adequate safety margin when selecting the decoder specification.

[ColinB]

It is not going to quadruple between 9v and 14 volts, but I will measure it. Looking at the web, it is as I thought it just follows ohms law. So measuring at 9 volts all you do is interpolate using the real voltage.

 

As long as you are using DC then the resistance is the same as the impedance at stall and can be used to calculate stall current. Torque is proportional to current. Current is equal to (applied Voltage - back EMF)/resistance. As back EMF is proportional to RPM then when the motor is stationary I=V/R