DCC Point power problem

[Slohcin]

I am building my first layout using DCC. I am powering the points using a Dell power supply producing 19.5Vdc @ 3.4A.

I am using ESU Switchpilot decoders and Seep point motors.

When I check the power supply output, with nothing connected, it reads 18.3Vdc but when I connect it to the point power bus the voltage drops to 6.5Vdc. What is causing this volt drop? The three decoders are correctly connected so I am at a loss as to where the problem is, can somebody help please.

[smokebox]

This could be caused by the Dell power supply which I believe has a monitoring circuit built in which cuts the voltage.  It is, I think, basically a battery charger with a variable output and not designed to be used as a power supply.

 

I could be wrong though and stand to be corrected

[melmerby]

These supplies are designed for a specific purpose and may not work as "just a power supply"

 

They are switched mode and no doubt require certain load conditions to operate correctly.

 

Keith

[roundhouse]

Your best bet for a power supply is to buy a suitably rated plug in transformer pack from Maplins. They do them in various voltages and current draw. Some even have a selection of different type plugs you can use on the outputs depending on the type of power socket on the decoder although I normally change these.

 

Regards

 

Ian

[Slohcin]

I was told by Brian on this forum that a laptop supply would do the job, to quote:

"You may find it rather difficult to locate a 16 to 18 volt ac power supply cheaply with sufficient current output to allow reliable operation.

So I would suggest you look at using a dc power source and these can be obtained readily.

These are usually sold as Laptop PSUs. ebay is a good source of new switch mode PSUs offering 19 to 22 volts dc at anything up to 4.0 Amps. Here is an example... http://www.ebay.co.u...=item2562dc2d9d Note the mains lead is not included in the price but are easily available separately. Cut off the moulded dc plug and connect the two wires to the Switchpilot terminals Pwr A & B, or connect the PSU output to a supply pair of wires of suitable large wire size (32/0.2mm etc) and feed all switchpilots from these pair of wires if you have more than one."

So this is what I sourced, incidentally my second power supply, the first one 16Vac @ 1A was thought to be not up to the job.

The next time I spend money I want to be sure that I am buying the correct supply.

[Crosland]

I was told by Brian on this forum that a laptop supply would do the job, to quote:

It almost certainly will, posibly barring the odd funny with some manufacturers, perhaps but unlikely.

 

There are a few misconceptions to address:

 

These supplies are not just battery chargers, they will power a laptop quite happily with no battery fitted. The charging circuitry is in the laptop or the battery pack.

 

There is nothing about switch moded supplies in general that requires certain loads for them to work. Power supplies intended for internal mounting in a PC do require a minimum load but that's a cost saving for that application where there will always be a minimum load in normal (non-standby) operation.

 

So called "desk top" or "plug top" switch mode supplies are stable at zero load. Indeed, your supply gives its rated voltage with just the meter connected.

 

The thing to be aware of with switch mode supplies is that they wil cut out very abruptly and very quickly in the face of a short circuit. A transformer based "analogue" supply will fail more gracefully unless it uncludes a voltage reguialtor that has its own current or thermal trip.

 

I would strongly suggest you have a wiring fault somewhere.

 

Your 6.5V reading could actually be an average reading caused by the supply cutting out and cutting back in periodically. An oscilloscope would tell you if you have acces to one.

 

Disconnect everything from the point power bus and reconect one by one until you find the fault, or break it into sections and test each section.

 

Andrew

[Suzie]

I would expect what you have done to work, so next thing to try is to put a load on the power supply to verify that it is working OK. A H4 headlamp bulb with both filaments in series should be a good test. Measure the current with your meter.

[melmerby]

 

 

There are a few misconceptions to address:

There is nothing about switch moded supplies in general that requires certain loads for them to work. Power supplies intended for internal mounting in a PC do require a minimum load but that's a cost saving for that application where there will always be a minimum load in normal (non-standby) operation.

 

Andrew

Having worked in an industry where switch mode supplies were used and invariably designed for a specific purpose, I would say it is quite common to find ones that are extremely unstable without anything like their intended load, some require some current to flow through a sense resistor before fully starting up.

 

Another electronic supply which can cause problems are the 12v regulated transformers intended for low voltage halogen lamps.

I have two of these recovered from lighting systems. Both oscillate uncontrollably without a load.

 

EDIT PSUs designed for general use will be OK with no load however.

 

Keith

[Crosland]

Having worked in an industry where switch mode supplies were used and invariably designed for a specific purpose, I would say it is quite common to find ones that are extremely unstable without anything like their intended load.

I don't disagree. They were designed that way, just like PC power supplies. The point isthat there is nothing inherent in a switch mode supply that requires a minimum load for them to remain in spec.

 

Andrew

[melmerby]

I don't disagree. They were designed that way, just like PC power supplies. The point isthat there is nothing inherent in a switch mode supply that requires a minimum load for them to remain in spec.

 

Andrew

Looks like you were posting a reply as I was editing my post!

I was just wondering if some of these laptop supplies could be unstable without any load, especially the cheap end of the market (i.e. not OEM)

 

Keith

[raymw]

^{In this application, the power supply, on start up, may well be trying to charge up capacitors and this will be seen as a short circuit. It then may go into some other 'safe' mode. Not sure even, if it was designed for lithium ion batteries, ni-cads or whatever, or even if it is just two power wires involved on the dc side. It is difficult giving specific replies, if the original specifications are unknown, and generic replies are only of use to those with understanding of the principles involved. Something about advice being worth what you pay for it also comes to mind.}

 

^{Best wishes,}

 

^Ray

[Crosland]

^{In this application, the power supply, on start up, may well be trying to charge up capacitors and this will be seen as a short circuit. It then may go into some other 'safe' mode. Not sure even, if it was designed for lithium ion batteries, ni-cads or whatever, or even if it is just two power wires involved on the dc side. It is difficult giving specific replies, if the original specifications are unknown, and generic replies are only of use to those with understanding of the principles involved. Something about advice being worth what you pay for it also comes to mind.}

 

^{Best wishes,}

 

^Ray

Specific reply (but still only what you paid for it):

 

The battery technology is irrelevant as the vast majority of laptop opwer supplies are designed to power the laptop. There is nothing magical about them. They are just power supplies. The battery charging and monitoring circuitry is inside the laptop or the battery pack.

 

Andrew

[Suzie]

You have to watch out for some Dell supplies with a big connector which have two outputs in the one cable, each providing half the current. Not sure how they work if you only load one or the other rail.

[Brian]

Hi

It's most likely you have a problem somewhere within your wiring or the decoder? Or you have an unusual PSU for some reason?

I use this style of laptop PSU to provide numerous supplies including a 19 volt one to power my MERG point Decoders.

I also use another 19 volt one to feed a CDU on another layout.

I even have one feeding 12 volts to my LEDs for indications.

All are used without any issues at all.

 

I don't offer help unless I'm sure and in this case I have several of this type of PSU.

[Slohcin]

I got a friend, who is an electronic engineer, to check one of the decoders. He says there is no problem with it.

So I connected the decoder just to my supply, with no other connections, and the voltage dropped again. I have come to the conclusion that Dell supplies are unsuitable for this application.

I am now going to source ANOTHER supply based on the recommendations of this forum, here's wishing me luck!

 

Can someone please point me in the right direction with supplier/type. There are so many laptop supplies, I do not want to get the wrong one again (not EBAY), thanks.

[raymw]

I'm not saying, don't want to get blamed...

[Suzie]

It sounds like you might have a faulty power supply. Do the load test and if it fails send it back for replacement.

[Phil S]

There are 2 branches to this problem: 1 is the nature (or possible nature) of the 'Dell' power supply designed for a specific model laptop

and the 2nd is the generic use of  a switched mode power supply when used with 1,2, or 3 or more ESU switchPilots, and then Seep/Peco motors...

 

1/ From my experience with Dell Laptop supplies, (for my Inspiron 9400) - and I've got through 3 or 4 -

a/ there is diagnostic communication at start up with the connected device .... and my Inspiron reports that it does not recognise the supply, if it is unhappy. however, I do not believe this stops the supply outputting the normal voltage...but it is possible that it may not supply full current.

(In the broadcast industry, with Triax cables, the real power is not connected until the cable connection is verified as correct for the attached camera).

 

b/ You may have unwittingly bought a fake device via Ebay .... one of the 'Dell' power supplies I bought was, I now believe, either a fake or reject or underrated for the model it was labelled as - it ran hotter than the others. It also failed after a few months. Unfortunately I disposed of it (and records of where it was supplied from) before hearing of the numeracy of these 'clones'.

 

2/ When used with the ESU switchpilot - a dc power supply is a valid option.   However, a switched mode supply is designed to be a well-controlled voltage under all ITS load conditions - which in this case is upto 3.4A.  This regulation is  not a requirement for the SwitchPilot which has its own regulator for low-voltage control logic internally.

 

I admit to still being puzzled - having re-read the Original and all the replies above  - that the output voltage falls to 6.5V - even without, apparently,  attempting to operate or connect the Seep Motor - I could understand this occuring when attempting to OPERATE the Coil - and I assume the switch is NOT SET TO CONTINUOUS OUTPUT - but to Pulse mode !!  

 

If the problem arose when operation WAS attempted (or the SwitchPilot was not in Pulse Mode)...

 

Most 'European' point motors use 'mechanical advantage' (ie levers and fulcrums) to reduce the Force [power] required to operate  the point, and therefore take currents which may well be under 1 amp.  This is well within the ESU SwitchPilot and the SMPS (if not a fake) rating.

 

For use with Peco® - standard and Seep® point motors, the SwitchPilot has an optional setting to allow the higher current for a short duration.

It is NOT a Capacitor-Discharge-Unit, but a perfectly adequate device for switching what I would call 'well designed' point motors - ie efficient ones.

 

If using it with the high-current 'direct-acting' (what I would call 'brute force and ignorance 8-) ) style of point motor, then a transformer, with its unregulated output, which is inherantly capable of short-term peak currents at the expense of a small voltage drop from the 'off-load voltage', is probably the better choice, IN THIS CASE. 

 

By comparison: I use 12Vac Ikea Wallwarts intended for their low-voltage lamps (20W) to operate Roco Point motors via Lenz or ESU decoders without problem.   However, 12Vdc into a SwitchPilot is insufficient to operate a Roco Uncoupler solenoid - let alone hold it up !  16Vac froma transfromer has no problem whatsover with the same task.    I use SMPS to supply my Roco DCC Units because they give an efficient regulated output.  In fact, EC regulations REQUIRE trains sets to use switched mode supplies now, in the aim of energy efficiency.  But horses for courses: There is no benefit in regulating a supply which then effectively strangles its capability to do the required job.

 

For Point-Motor solenoid operation, an ac supply, or specialised CDU is usually the better course, as the inductance of the coil is part of the design.

 

 

You  may find that the a 1.4x higher dc-voltage is required for the 'equivalent' operation compared to the 'same' ac voltage - because it will be the peak voltage of the ac (1.4x greater) that produces the greatest effect.   Staying within the voltage rating for the dc input may therefore restrict the power available making it less suited to Seep or Peco motors (using dc).

[Slohcin]

Phil,

Thanks for detailed reply.

The Dell supply is an original and was given to me. It has a triax cable (core, inner and outer screens), I used the core and inner screen as this gave me the 18.3V. I tried the core and outer screen, but this only gave me mV, so I cut this off, I assume this was correct.

I need to move on, either by getting the Dell supply to work or source another, but which one?

[Phil S]

For your application of a Seep and SwitchPilot - then find a transformer - you probably have one already that is suitable - approx 16Vac 3-4A rating is probably more than adequate.

 

When a  Switched Mode Power Supply is  recommended; as for example in my case powering my Roco units, or powering my 12V dc LED layout lighting -  rather than use a specific, possibly advanced model with inbuilt add-ons, I would use a generic 'after-market' product. 

[Note: Roco Sets now come with a switched mode power supply instead of their 16Vac 3.5A transformer

 

I do not have any circuit diagrams for the Dell power supply, and so cannot comment on the effect of cutting and not using the outer screen .... did you check how these were finally connected together in the Dell Power Plug ???  My experience with true - Triax® cables is that such action would prevent correct operation!   Personally I would not have cut the plug, but found a matching adapter socket [generic SMPS's usually have a range of plugs]

Without specific information and experience I doubt anyone on this group would advocate cutting power supply cables; even on the low voltage side.

 

My dubious 'Dell' Power Supply claimed to be original, and was marked as such - such is the case with fake goods. Other supplies I have bought from Ebay have been 'genuine' or worked perfectly - price appeared to be no guide !  I suspect it may have been the lower-power rated version, wrongly labelled..

[Suzie]

It sounds like you have the dual output type. Half the current is supplied on each of the outputs. You probably have to load up one of the outputs before the other will supply current. I have four of them lying around somewhere, but have not been bothered to tackle how to use them for anything useful! I might have to dig one out and have a play.

[Crosland]

Have you followed the suggestion of disconnecting everything and then connecting things up one by one to see what happens?

 

Andrew

[Slohcin]

As I said earlier, I connected one decoder to the PSU with no other connections ands the voltage still dropped to 6.3V.

I have given up with the Dell and put it down to experience.

Have got another supply on order, here's hoping.

[Brian]

I got a friend, who is an electronic engineer, to check one of the decoders. He says there is no problem with it.

So I connected the decoder just to my supply, with no other connections, and the voltage dropped again. I have come to the conclusion that Dell supplies are unsuitable for this application.

I am now going to source ANOTHER supply based on the recommendations of this forum, here's wishing me luck!

Can someone please point me in the right direction with supplier/type. There are so many laptop supplies, I do not want to get the wrong one again (not EBAY), thanks.

Who mentioned Dell power supplies? I didn't. My recommendation was for a 20 volt dc 3.25amp Advent PSU from an eBay supplier.

So did you order that one but got provided with something else or did you happen upon a Dell PSU.

[Slohcin]

The Dell supply was given to me.

I have bought a new supply, not Advent but similar output, from Amazon ( not sure about EBAY ).