Power supply LC ripple filter

[chad44]

I am trying to construct a simple 72 VDC power supply to charge the batteries on a diesel electric locomotive. I need about 5 amps. I used a standard wallbox lighting dimmer, connect to a bridge rectifier to go from 120VAC to variable DC. I thought I should have some filtering on the output to smooth the ripple. I was going to add an LC filter, but have no clue as to the correct values for the inductor or capacitor.

 

[Comcokid]

I'm a little concerned about providing additional information in reply to your post.

A standard light dimmer will not provide you with isolation from the AC mains. You could easily create a situation were someone stepping off the locomotive into a puddle of water could get shocked.

I recommend a isolation transformer as a minimum. Note - a variac or auto-transformer does not provide isolation either.

 

[Skogsgurra]

GO BUY A CHARGER!!!

Are you at all aware of what you are doing?

You risk yours and other's lives.

Connecting mains via a dimmer and rectifier to a battery will at best create a short circuit to ground and that's it.

If you have disconnected the battery from the locomotive, you may not blow a fuse. That means that both poles of the battery are connected to the mains. You will hurt/kill yourselves if you handle any blank metal in the circuit - including battery poles and clamps.

What is your name and address? I need it so that I can send an officer to your place. It is a crime that you are about to commit.

Gunnar Englund

http://www.gke.org

 

[chad44]

I agree, some others at our organization raised the same concern, so we will add an isolation transformer up front.

 

[chad44]

Fusing is already in place. A 2-pole knife switch on the locomotive disconnects the batteries from the locomotive and connects them to a plug on the side of the loco for connection of a charger. At the knife switch, there is a 10A fuse in each leg. Buying a charger is out of the question for the immediate future, due to cost (you can't buy a 72 VDC battery charger at Sears), which is why we are working on this temporary solution.

 

[Comcokid]

A quick google search (72 volt charger) found a 10 amp 72 Volt charger for around $210 . This is made to charge six 12 Volt batteries in series and should work on your application.

A little money for a lot of peace of mind.

http://www2.northerntool.com/product-1/340592.htm

 

[Skogsgurra]

OK,

I can see that you were warned by others as well - I will not send an officer.

Adding a transformer will add reactance in the circuit and that may be enough to "smooth" your current. You do not need to smooth it actually. No normal charger works with a pure DC current. It is always composed of chunks of current that flows when charging voltage is higher than battery voltage. It happens 100 or 120 times per second, depending on 50 or 60 Hz mains.

If you need to add a reactor, it shall not be connected to the DC side of the rectifier. It will get saturated at once and does not do any good. It shall be on the AC side to make the charger "compliant" i.e. give it a soft characteristic.

Gunnar Englund

http://www.gke.org

 

[chad44]

That charger on google will work fine. Thanks to everyone for the help

 

[Skogsgurra]

I was really surprised to find such a charger at that price. For a short moment I thought that it might be a simple "dimmer" type charger as per the OP. But at 36 pounds - barely portable - it does contain iron and copper enuff to be a safe thing.

Gunnar Englund

http://www.gke.org

 

[itsmoked]

Trains are a constant pain with that 72V. Then you have a (72V) ventilation motor burn out. And you need 72V lights not to mention 72V contactors...

Eventually you try to replace as much as you can.

 

[OperaHouse]

If you want to be cheap, try finding a 240V to 120V transformer from some old machine tool rated at about 1-2KVA. Connect the 220 windings to the 120V AC line and the 120V transformer windings to your bridge rectifier. That would give you pretty much the voltage you need for a charger with isolation. Pretty cheap solution. I recommend Anderson SB-50 connectors as a nice disconnect. I can find these used sometimes for only a couple bucks, one of my favorite connectors.