Peak Inrush Current when Power Bank Starts Charging ?

[David-P]

I wondered if anyone knew what the peak inrush current is when an empty Power Bank starts charging - usually this occurs when the loco is first moved from cold and always causes a reset of the decoder for me. I'm assuming there is something on the decoder itself that can't handle that peak current. Once it's been charging for a few seconds all is OK.

I believe I have ruled out the supply to the track as I switched to a power supply capable of delivering 20A and I still had a decoder reset. I also run Live Steam, so all the tracks themselves are wired for 10A. Hence why I am suspicious of the decoder itself. I can live with it, but I'm just curious.

[96RAF]

This inrush stalling is why all profiles are being progressively converted to static charging, with a short lag after linking before the charging starts to stagger the inrush load on the track power source whether a PSU or a DCC controller.

[David-P]

That still makes the assumption that the track power source is the weak link, unable to supply sufficient inrush current. That's why I swapped over to a 20A supply - to remove the "weak" link.

[Fishmanoz]

The last thing I’d want on my layout would be 20 Amps flowing under fault (short circuit) conditions.

[LTSR_NSE]

@Daedalus - naturally all the power supplies sold (and therefore recommended) by Hornby contain short circuit & overload protection.

Power Supplies manufactured by 3rd parties (intended for other functions e.g. laptops) might or might not (if short circuit & overload protection are built into the device itself). Generic/compatible power supplies (that are deliberately manufactured to be as cheap as possible) are far less likely to include any form of protection circuitry and several have been found to breach safety regulations, creating a potential fire risk.

Obviously using 3rd party power supplies is not recommended and done entirely at the user’s risk. Unfortunately since the majority of general public are not qualified electricians/electronics engineers, it appears a too common attitude to place price above safety (of both user & delicate electronic components).

[96RAF]

One only has to dismantle a genuine Apple charger and compare with the innards of a cheap ‘identical’ knock-off to see the difference not only in poor design but also of quality to realise that the cheap one will fail on many fronts, usually by catching fire under overload. Plenty of examples on the internet with folk naive enough to blame Apple for it.

Tests have been done by Hornby on several laptop chargers with results previously reported. At risk of becoming monotonous I remind you that the onus is on the user of these alien devices to prove their safety protection or provide their own external safety device.

[David-P]

This thread has rather gone off topic - apologies for not being more specific. I was asking about the inrush current to the Power Bank; I only switched to the 20A power supply as a quick test - it's not what I normally use.

However, Hornby's change to charging Power Banks at startup does link nicely to short-circuit detection in the power supply. For a decent sized layout with (say) 20+ locos all charging up their Power Banks shortly after being switched on, isn't there a chance that the power supply will confuse this with a short circuit and shut down again? Which leads nicely back to the question of how big the inrush current is.

[96RAF]

No idea about the actual in-rush current value, probably in the order of a few mA, but the static charging is tempered by a short delay after the app finds the decoder before charging starts, so not all PBs will be grabbing current at the same time.

[David-P]

A few mA doesn't feel quite right. Please check my sums (approximate numbers only):

If the PB can power a 500mA motor for (say) 5s, but is charged at 10mA (one fiftieth the current), then it will take fifty times longer to charge up at that current, which is over four minutes. Maybe it just does take a very long time to reach full charge.

[96RAF]

The book says it charges in a couple of minutes. I have had one running sound and lights (no motor) for over a minute after turning power off, so it’s swings and roundabouts for current draw.

[Britannia Builder]

How does the static charging work when the app is not being used, ie on pure DCC operation? Do all the power banks start charging simultaneously as soon as the track power is switched on?

I don't think we can determine the peak inrush current without knowing more about the control circuitry. There is presumably a resistor in the charging circuit to limit the peak current. The famous manual says on p120 that the power bank takes approximately 2 minutes to charge. The individual supercapacitors are 1.0 Farad and 2.7 volts, so charging a single one at a constant current 'I' amps would take 1.0x2.7/I seconds, so for 120 seconds I=1.0*2.7/120, ie 22.5 milliamps. Three capacitors in series are presumably 1/3 of the capacitance and three times the voltage, so the resulting current would be the same (I think!).

I suspect that the charging circuit will be constant voltage rather than constant current (or perhaps some more complicated regime), so the initial inrush current could be higher, perhaps substantially higher, but they must have designed it to not overload the decoder.

EDIT: the previous two posts were made while I was toiling over this calculation!

Regards, John

[96RAF]

@96RAF

Silly me I did not consider someone may try to use a fake obsolete 5V 5W Apple charger to run their model railway!

I consider PSUs and chargers to be different and you should check the CE Mark if any doubts of origin.

 

 

Sarcasm gets you no votes. Folk may use power supplies for any purpose and past experience has flagged up some horrors.

My post was to draw folks attention to the quality of knock-off kit and its usually catastrophic failure mode. These duds have all the correct markings anyhow but comply with none. If it’s cheap it is likely to be nasty.

 

 

[David-P]

Returning to first principles - thank you John (Britannia Builder) for pulling the pertinent information out of the manual.

If the total charging time is 120secs, then the time constant (RC) must be approximately 90s (time to 63%).

If RC = 90secs and C is 0.333F, then the series resistor is 90 / 0.333 = 270ohms (give or take).

Assuming a 15V supply, the maximum in-rush current is 15 / 270 = 55mA.

Got there in the end - an engineering approximation at least.

[Brew Man]

@96RAF

Silly me I did not consider someone may try to use a fake obsolete 5V 5W Apple charger to run their model railway!

I consider PSUs and chargers to be different and you should check the CE Mark if any doubts of origin.

Sarcasm gets you no votes. Folk may use power supplies for any purpose and past experience has flagged up some horrors.

My post was to draw folks attention to the quality of knock-off kit and its usually catastrophic failure mode. These duds have all the correct markings anyhow but comply with none. If it’s cheap it is likely to be nasty.

And I wouldn't trust the CE Mark either. I used to be A Quality Systems Manager for an NHS department and responsible for meeting Medical Device Directive compliance, and the number of CE Marks I've seen that don't even meet CE Mark logo compliance let alone its associated Standards requirements. Unscrupulous manufactures will slap a CE Mark on a device without a second thought.

 

 

[96RAF]

Returning to first principles - thank you John (Britannia Builder) for pulling the pertinent information out of the manual.

If the total charging time is 120secs, then the time constant (RC) must be approximately 90s (time to 63%).

If RC = 90secs and C is 0.333F, then the series resistor is 90 / 0.333 = 270ohms (give or take).

Assuming a 15V supply, the maximum in-rush current is 15 / 270 = 55mA.

Got there in the end - an engineering approximation at least.

 

 

Let me put you out of your misery. In lieu of not being allowed to post a schematic . . .

There is a transistor in the PB with the Emitter feeding the 3x caps positive end, which as stated are in series. The Base is fed by a 2.2K resistor and the Collector has 3x 330R resistors in parallel. The B and C are fed by the PB red lead.

Also in circuit is a zenner at the cap negative end and a normal discharge diode to the plug red.

Apply those clues to the circuits you can find on the internet and you are almost there.

The PB is linked from the decoder 12v line, normally when only the loco is moving but on a rolling basis this will be changed to static charging, presumably by reprogramming simple switching in the decoder MCU.

If it is any consolation some folk have reported hanging a normal 2-wire stay alive on the HM7K decoder and getting away with it.

 

 

[Britannia Builder]

Thanks Rob, that's very helpful.

So it looks as though the input resistance is 110 ohms (3 x 330 ohms in parallel) and so the initial inrush current is 12v/110 ohms = 110 mA?

Regards, John

[LTSR_NSE]

@Daedalus - nobody posting on this thread is suggesting that it is a good/sensible idea to use an incorrect/inappropriate electrical power source, supply, adapter, or charger - in fact we are all agreed on the exact opposite.

Most (if not all) posting here are also aware of the difference between source/supply/adapter/charger - however the internet & general public tend to use the words interchangeably (demonstrating their lack of awareness).

If you disagree with any of this, then I have obviously misunderstood your posts here. (Since all they communicate to me is your limited awareness of just how foolish people with no knowledge can be.)