What provides an 18-137VDC power source on an Electric train? - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: April 18, 2018, 06:39:35 pm
content: Hello,We have a spec for 20W LED lamps to be powered from a train power source which is 18V to 137.5VDC.In fact, its wider than that, there is an emergency requirement of 4W LED power for times when this supply goes down to 8V. (-but that might be when the emergency battery pack switchs in?)Do you know what it is on the trains which is providing this DC power source? Is it a big battery bank?I believe they are electric trains. For example, it’s the trains that will be used in the HS2 project, soon to come in UK.Do you know what kind of source impedance this 18-137.5VDC power source will present to our 20W LED lamps?We don’t have room for much input capacitance in the LED lamps, so we are hoping there’s not much wiring inductance going back to the power source. Do you know what the wiring inductance is likely to be?Unfortunately, the client is not coming forward with much information on this for us.To me , it sounds liek a really "dirty" , "dont give a damn" power source which is probably more concerned with getting energy from regenerative braking into batterys...do you reckon?

author: dmills
author_rank: Super Contributor
timestamp: April 18, 2018, 08:34:44 pm
content: I would expect the 18V to be the emergency battery with the 130 odd volts being the output from whatever is producing the main lighting power, rail can be weird like that.I would expect the supply to be at least the length of a carriage away, and possibly the length of the train, so yea considerable inductance, also expect that sooner or later someone will couple two bits of train and plug the lighting power coupling in under load with the alternators or inverters running....Basically take the nastiest sort of automotive environment, add extra vibration and an EMC nightmare from arcing pantograph or third rail contacts, then have Bubba install it and connect bits of train together.Also watch out for fun like the railways fire and smoke safety standards, they tend to be a little ah, Different!Rail has a lot of standards all of its own, be prepared to do some reading.Regards, Dan.

author: ocset
author_rank: Super Contributor
timestamp: April 18, 2018, 08:41:21 pm
content: Thanks, and the inductance worries us, becuase we can only fit in 1uF of ceramic capacitance before each of the 6W buckboost converters that we have driving each 6W LED PCB...so we could start violating the Middlebrook criteria and getting input filter oscillations if the line inductance is high. I wonder if it could be more than 100uH?...and if so, wouldnt it have considerable capacitance (distributed) also...this would in some way ease the problem.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: April 18, 2018, 09:07:02 pm
content: Quote from: treez on April 18, 2018, 06:39:35 pmIf this is a contract-to-build situation, it depends on the structure of your contract, however, have you received your requirements in writing? If the deals is not already activated, or this is a bidding or concept run, I would write back into the contract with your own requirements regarding power supply and inductance. Also, YOUR requirements should also include that you need to see the circuit schematic of your power source, including harness runs. This way you can at least assess what you are up against.Just my 2 cents...

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 19, 2018, 08:17:24 am
content: Quote from: tpowell1830 on April 18, 2018, 09:07:02 pmAbsolutely.You need to give them a spec to approve, and if possible have a way to determine if it's been exceeded ( logging, fuse etc.)And of course have a clause that ensures you are not liable for any costs arising from failures due to exceeding your spec.

author: richard.cs
author_rank: Super Contributor
timestamp: April 19, 2018, 11:06:26 pm
content: I suspect it is for use across different trains with a range of nominal supply voltages. A lot of rail spec stuff is nominal +/- 40 %. I've had one where the customer wanted 14.4 to 154 V (40 % less than 24 V to 40 % more than 110 V). Yours sounds like the same range of nominal but only 25 % range either way.Usually it's derived from a lead acid battery that spends most of it's life on float charge.

author: Brumby
author_rank: Unknown
timestamp: April 20, 2018, 05:29:45 am
content: Railway electrics have been something I've not had any experience with - but I have heard some stories.One of them came from "Uncle Doug" - someone Dave has interviewed on several occasions. He tells a story of a project he did where an audio communications system had to be designed to operate through an inter-carriage connector - that had non standard pin assignments. (Apparently, this is not unusual.)In one scenario, the audio circuit of one carriage connected to the audio circuit of another and everything was fine - but in another, the audio circuit was connected to a power circuit (a couple of hundred volts, I think). The design brief was that the audio circuit had to survive this. Nice work, Doug.

author: james_s
author_rank: Super Contributor
timestamp: April 20, 2018, 06:10:33 am
content: Rail is a whole different world. You see things like incandescent lamps of several hundred Watts powered through a dropper resistor that dissipates as much power as the lamp. With the old steam trains there was often a separate steam turbine dynamo to power the lights but I don't know what more modern trains use. As recently as the 80s anyway there was a lot of *really* old tech still in use.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 20, 2018, 03:31:05 pm
content: Quote from: treez on April 18, 2018, 08:41:21 pmSounds like you are worried for a good reason - I bet it's almost guaranteed to blow in typical use conditions. If not in typical, then some day in worst case conditions.Just add the proper hi-ESR (C >> 1uF) damping capacitor, that's the bare minimum. Consider extra clamping as well. If you can afford slight loss of efficiency, consider some small series resistance as well.Don't sell something you can't make in reality. You just have to allocate space for the basic input filtration.What youcando is to get super clever and design some super-genius smaller-than-usual, yet still working input protection thing, but I think you don't have the skills or resources to do that. So just put the 100uF elcap, a bit of series resistance, a fuse and a TVS diode there, and you'll have at least something.