Hornby Pullmans: Power Consumption

[RFS]

I've been having some problems with DCC power consumption. I have a 5-amp Lenz DCC system, and a PSX circuit breaker. When running 3 or 4 trains I've noticed the PSX's overload LED flickering. There's no way that the trains I'm running should be stretching a 5-amp system. So thinking I had a short somewhere that wasn't enough to trip the breaker by itself, I changed the PSX trip point (via CV 49) to trip at 1.27 amps. It tripped immediately I switched on even with no trains running. I removed all locos from the track with no change. I have 11 Hornby Pullmans with lights and only when I removed most of these did the PSX reset. Tests showed that 4-5 Pullmans was enough to trip the CB and that I don't actually have a DCC wiring issue. With the PSX changed back to trip at 5 amps I can still make the overload light come on by placing one Pullman on the track and rocking it from side to side.

 

These Pullman coaches have 12-14 LEDs so does this mean each coach is consuming 200-250 milliamps of power? 11 coaches would thus be taking 2-3 amps of DCC power. I've switched off the lights for now by reversing the wheels in one bogie, and now the PSX LED no longer flickers when the layout is very busy.

 

Has anyone measured the current draw of these coaches, eg with an RRAmp meter? I'd be interested to know what this shows.

[spamcan61]

Just checked one for you, with 11 LEDs lit at 12VDC it's drawing 42 mA on my multimter.

[Edwin_m]

Sounds like you need to get that multimeter! If you have a 12Vdc power source you can measure yours on DC, though they may use a bit more on the higher voltage of DCC. Spancan's reading suggests you may have a "rogue" that is using more current than the others.

[Lady_Ava_Hay]

LED power consumption has to be measured in watts. There is a voltage drop across each one which seems to be between 2 and 5 volts and the current draw is usually rated at 20 milliamps. So by inference the wattage is no more than 100milliwatts so a coach consumes 1.5 watts max.

 

You have sixty watts to play with.

 

Say ten locos or 40 coaches but not both. A loco with five coaches is probably consuming 12 watts. Five all lit trains then.

 

I have erred on the cautious side though and my maths is based on technical data that I have gleaned on here, by running LED's on a power supply and from suppliers data of LED's I have bought.

[spamcan61]

I haven't seen the Hornby Pullmans operating under DCC, but at 12VDC the LEDs are pretty dim as you would expect at 4mA forward current, but then they're supposed to be table lamps not floodlights.

[Edwin_m]

The maximum rating may be 20mA but most small LEDs will light quite brightly on 5mA or less, which would put the total consumption close to that suggested by Spamcan. You could reduce it further by connecting groups of two or maybe three in series, but Hornby probably don't do this as it increases the voltage (running speed) needed to work the lights when on DC.

[Lady_Ava_Hay]

I haven't seen the Hornby Pullmans operating under DCC, but at 12VDC the LEDs are pretty dim as you would expect at 4mA forward current, but then they're supposed to be table lamps not floodlights.

 

They do seem brighter on DCC but I haven't ever reasoned it out before. Why only 4 milliamps? is that Hornby rating? Is there a little power board in the coach somewhere.

 

I have gone LED mad this weekend by fitting LED strips from Black Tech to a Scalescenes inspection pit. The instructions say any voltage between 9 and 22 volts including DCC bus and have a little separate power board with any polarity 'in' and observe polarity 'out' to the strips both marked -ve and +ve. Apart from that, wire them in parallel or series.

[Grovenor]

The Hornby coaches have a capacitor to reduce flicker, when you switch on the capacitors take a high current to charge up, hence switching on with 11 coaches on the track you exceed the current limit of your cut out. Put the coaches on the track one by one there will not be a problem, till it cuts out for some other reason and has to switch on again.

You can sort this out by putting some series resistance between the track pick up and the capacitor to reduce the charging current.about 100 ohms should be OK, just check that the lights are still bright enough afterwards.

(And I suspect that there is not an LED for each lamp, rather the light is piped to the lamps).

Regards

Keith

[spamcan61]

The Hornby coaches have a capacitor to reduce flicker, when you switch on the capacitors take a high current to charge up, hence switching on with 11 coaches on the track you exceed the current limit of your cut out. Put the coaches on the track one by one there will not be a problem, till it cuts out for some other reason and has to switch on again.

You can sort this out by putting some series resistance between the track pick up and the capacitor to reduce the charging current.about 100 ohms should be OK, just check that the lights are still bright enough afterwards.

(And I suspect that there is not an LED for each lamp, rather the light is piped to the lamps).

Regards

Keith

 

Thanks for the explanation as to why they (initially) draw more current on DCC than DC. I'm not sure if they're separate LEDs or a fibre optic, the latter would make more sense, although 40mA or so would be quite a lot of current for a bog standard LED

[DJH584]

And I suspect that there is not an LED for each lamp, rather the light is piped to the lamps.

Regards

Keith

 

You are absolutely correct, that is the case.

 

Regards

 

David

[RFS]

To eliminate any issue with my DCC bus I connected a couple of yards of track directly to the PSX output instead. 5 Hornby Pullmans tripped the PSX at 1.27 amps - a relief really that it wasn't my DCC wiring after all. I'm assuming they're not efficiently converting the DCC track voltage to DC. They do flicker on and off very quickly if there's a break in the current which suggests there's no capacitor - at least not on DCC.

 

Just wondering whether anyone as an RRAmp meter that could correctly measure the current draw on DCC.

[Gwiwer]

Very educational responses thank you. I run DC but with 13 Pullmans (11 "standard", one 12-wheeler and the observation car) the train alone draws so much juice that a locomotive stands little chance of moving it beyond a crawl from my 1.1A input. Performance is improved of course with a shorter formation and the table lamps are noticeably brighter the shorter the rake in use.

 

I can however cheat and input 2x1.1A simply by setting two points in a suitable way and turning two controllers to "On". Not necessarily good practice but it shifts the train!

 

Next question - how will the Brighton Belle be affected by these constraints?

[Nigelcliffe]

They do seem brighter on DCC but I haven't ever reasoned it out before. Why only 4 milliamps? is that Hornby rating? Is there a little power board in the coach somewhere.

 

 

I have no idea what is in the Hornby coach.

 

 

But, the "little power board" if used to limit the brightness could just be a resistor of appropriate value. Pick a smaller resistor and LEDs will be brighter, larger resistance and LEDs will be dimmer. A larger resistance will mean a lower current through the LED.

 

 

Why "brighter on DCC" -

On a DC layout, the track voltage is 0-12V DC, typically might be 8 or 9V when running.

On a DCC layout, track voltage may be around 15V (pseudo AC).

 

To deal with current reverse in DC, the lights will need either a rectifier, or two LEDs back-to-back, one for each direction of travel. This circuit neatly works for DCC as well.

 

If we assume it has a rectifier, that will drop about 1V. The LED will drop around 3V.

 

In DC running, at 9V, there is 1V in the rectifier, 3V in the LED, leaving 5V to drop elsewhere.

In DCC running, at 15V, there is 1V in the rectifier, 3V in the LED, leaving 11V to drop elsewhere.

 

Now the "drop elsewhere". The simplest solution is a resistor, this will also limit the current to the LED. If we use a 1Kohm resistor, the current through the resistor will be:

DC running V=IR, I = 5/1000 = 0.005A or 5mA.

DCC running I = 11/1000 = 0.011A or 11mA.

 

The current through the resistor is also the current flowing through the LED. With less than half the current flowing through the LEDs in DC running, they will be less bright than in DCC running.

 

 

 

- Nigel

[spamcan61]

OK this is probably beginning to read like an 'O' level physics paper, but I was sufficiently intrigued to go back in the shed this morning and have a look at the IV characteristic. So here's the current drawn at various DC voltages. Note the DC is supplied by a low noise regulated variable DC PSU I built from a plan in Everyday Electronics IIRC about a million years ago, so it is a proper flat DC not full wave rectified AC.

[spamcan61]

To eliminate any issue with my DCC bus I connected a couple of yards of track directly to the PSX output instead. 5 Hornby Pullmans tripped the PSX at 1.27 amps - a relief really that it wasn't my DCC wiring after all.

 

Did you add the coaches one at a time as suggested? that way we can see whether the problem is inrush current as a capacitor charges or something else.

[Gordon H]

OK this is probably beginning to read like an 'O' level physics paper, but I was sufficiently intrigued to go back in the shed this morning and have a look at the IV characteristic. So here's the current drawn at various DC voltages. Note the DC is supplied by a low noise regulated variable DC PSU I built from a plan in Everyday Electronics IIRC about a million years ago, so it is a proper flat DC not full wave rectified AC.

So, for every additional volt, the current increases by approx 4mA.

Thus, we can say that the resistance of the circuit must be 1V/4mA = 250 Ohms.

If this is so, at 12mA (the 6V figure) the resistor will drop 3V, leaving 3V Vf for the LED @12mA.

This seems to stack up reasonably well using the other voltage figures too.

[RFS]

Did you add the coaches one at a time as suggested? that way we can see whether the problem is inrush current as a capacitor charges or something else.

 

I understand there is no current limiting in the Hornby circuitry so just placing one Pullman on the track is seen by the PSX as a large load. That's why just rocking a Pullman back and forth can make the overload light flash. The current consumption on DCC is likely to be higher than DC due to the need to convert the current to DC. It probably needs a DCC-compatible meter (such as the RRAmp) to accurately measure this. It's the Hornby circuitry that's taking the current rather than the LEDs themselves.

[spamcan61]

I understand there is no current limiting in the Hornby circuitry so just placing one Pullman on the track is seen by the PSX as a large load. That's why just rocking a Pullman back and forth can make the overload light flash. The current consumption on DCC is likely to be higher than DC due to the need to convert the current to DC. It probably needs a DCC-compatible meter (such as the RRAmp) to accurately measure this. It's the Hornby circuitry that's taking the current rather than the LEDs themselves.

 

Is there no way of reducing the sensitivity of the overcurrent protection on the PSX? the Pullman lights should only cause a high load for a fraction of a second as the capacitor in the lighting circuit charges, as Grovenor pointed out upthread. If a single coach is enought o cause problems then the protection circuit seems much too sensitive.

 

Is this the PSX in question?

 

http://www.dccspecialties.com/products/pdf/psx_hints.pdf

 

I'm a Duette guy so I don't do this DCC lecktrickery stuff

 

If so it seems odd that it is exhibiting this behaviour when the user manual specifically states

 

"This advance system solves the false overload problems that occurr in DCC Boosters and other circuit breakers that see the sound locomotive capacitors as a short circuit."

[Grovenor]

I stated near the beginning of this topic, that the pullmans have capacitiors on the input causing the high inrush current, many of you, apparently don't believe me, so why don't you take one apart and have a look for yourself, like I did. (Perhaps they are not all the same). hence all the waffle on what the circuit might perhaps be.

Anyway for the record here is the circuit diagram of my Hornby Pullman, add a resistor, between 10 and 100 ohms between one of the rails and the rectifier and you will solve the problem. No need even to take it apart as the resistor can be fitted to one of the bogies.

Resistor value corrected as post below. KN 8/6/11

Regards

Keith

[gordon s]

Just checked my Hornby Pullmans with a RRAmp meter and these were the results. I'm using the NCE 5A system. With nothing on the track the reading was 17.3v and 0.08A. I'm assuming the current draw is a PSX circuit breaker/auto reverse module. Adding one Hornby Pullman the current increased to 0.15A, two took it to 0.22A and adding a third gave a reading of 0.29A, which would suggest a current consumption of 0.07A per coach.

 

Hope that helps.

[RFS]

Just checked my Hornby Pullmans with a RRAmp meter and these were the results. I'm using the NCE 5A system. With nothing on the track the reading was 17.3v and 0.08A. I'm assuming the current draw is a PSX circuit breaker/auto reverse module. Adding one Hornby Pullman the current increased to 0.15A, two took it to 0.22A and adding a third gave a reading of 0.29A, which would suggest a current consumption of 0.07A per coach.

 

Hope that helps.

 

Thanks Gordon - that does help.

 

I'm assuming on my Lenz system, with the PSX and LDT RS8 occupancy detectors, that the consumption may be a bit more. Nevertheless 11 Pullmans works out at 0.8 amps, which is an awful lot just for some table lamps. I've decided they're pretty but not essential so will do without for now.

[Grovenor]

Managed to take the pullman a bit more apart and got a better look at the resistors and was able to put the meter on them as well, R1 and R2 in the diagram above are actually 470 ohms, not 200, otherwise the diagram is correct.

Regards

Keith

[spamcan61]

Managed to take the pullman a bit more apart and got a better look at the resistors and was able to put the meter on them as well, R1 and R2 in the diagram above are actually 470 ohms, not 200, otherwise the diagram is correct.

Regards

Keith

 

Thanks for your continuing investigations Keith, I hadn't quite managed to psyche myself up to take one to bits. I suppose 220uF is a pretty big capacitor to shunt the DCC signal really, although I'm not sure it's any bigger than the ones on sound decoders the PSX modules is compaptible with.

[Grovenor]

although I'm not sure it's any bigger than the ones on sound decoders the PSX modules is compaptible with.

I suspect its quite a bit bigger unless your sound decoder has a stay alive, and with coaches one may have rather more of them

Regards

(Now I have to put the pullman back together I will fit the extra resistor while I am at it, never bothered before as I only have the one so it never caused any trouble).

[two tone green]

And each monitored output section of a RS8 is rated at 3amps with a peak rating of 7 amps.