Best way to switch between 2 moderately high DC voltages?

[scytheavatar]

I have a problem here: I need to design a switching system between a 48V battery pack and a 48V wallwart to a load of variable inductive, capacitive and resistive components. That can draw up to 15A. I have though of using an electromechanical relay, but I am struggling to find one that can meet my specifications without drawing too much power (preferably under 1W). Neither could I find a latching relay for my purpose. Is there any better solution other than electromechanical relays that I should consider?

 

[JimB]

The words:

48volts
wallwart
15amps

do not go together very well.

The voltage and current are individually rather high for a "wallwart" type power supply, but together, WOW, that must be some BIG wallwart.

Care to give a bit more information?

JimB

 

[mneary]

Breaking an inductive 15A load at 48VDC is going to draw a serious arc!

You sure come up with some really strange requirements for relays! And you seem to have a history of never coming back to answer questions about your application.

 

[qrper]

If you really had to, a mercury displacement relay would work just fine. You can get them in either NO or NC configuration. While I don't have the specs in front of me at this second, their drive current maybe a tad over the 1W you need.

I don't have the address for the company I have used in the past, but they're not overly expensive. If you're interested, I can dig up the info here in my files.

But I wonder why you would want to break that much current? Could you not turn off the load, then break the connection?

You did not mention if the source was AC or DC

and like JimB said... wallmart and 15 amps normally don't go together.

 

[Sceadwian]

Why do you need to actively switch from a 48v battery drawing 15 amps to a wall wart? Simple logic says you're trying to find a solution to a problem you're creating, turn the device OFF or put it in an otherwise low power state before you switch it and you don't have to worry about the switching requirements so much.

If you absolutely positively must keep the power source active during switching you need to explain exactly what you're trying to do better. 48 volts at 15 amps is 3/4's of a thousand watts. You don't generally want to switch that on and off via wipers if you can help it =) Do something like determining current zero cross and switch only then. You can use a much lower rated relay then.

 

[mneary]

If you need mercury displacement relays, I have some that were donated. (Yes, I'm serious.)

 

[Mikebits]

The voltage and current are individually rather high for a "wallwart" type power supply, but together, WOW, that must be some BIG wallwart.

Care to give a bit more information?

JimB

He must of meant Wall Tumor...

 

[qrper]

I re-read the original post and it's a 48 volt battery he want to switch. so there goes the zero crossing option.

That being said, I worked on the old general electric battery powered lawn tractors. The Elec-Traks. They ran on 36-volts and were controlled by a series of relays.

The largest one, the C-20 had a rated brake horsepower of 20hp. The first two speeds were controlled by one large contactor relay that switched in and out large resistors. The third speed was controlled by another contactor relay that shorted out the first one's contacts.

At higher speeds, (8 forward, 4 reverse) field weaking was used.

mercury displacement relays would have been better, but the constant bouncing (of the mercury) would have been a problem.

I don't know of any 48vdc golf carts, but a repair shop may have suitable high current dc rated contactors.

Although free mercury displacement relays is a deal, too!

mike

 

[Mike_2545]

How about a knife switch?

 

[mneary]

I suppose it would be too much to dream for the OP to return and answer our questions......

 

[scytheavatar]

Those are my specs, and yes it's a really large wallwart, don't question it. I know that it's going to draw some serious arcs, so I take it that there's no better option than some really expensive contactor relay?

 

[JimB]

Do you really mean a "wallwart", a small power supply which is completely contained within a mains plug body, or a mains driven power supply which is in some large box and has a cable with a mains plug on the end?
The second option would not exactly be described as a wallwart.

JimB

 

[Sceadwian]

Seeing as how you're going to get a power interruption during switching anyways why can't you use whatever control systems you have on the device to lower it's power draw during the switching period? Then you can use much cheaper relays. You could use solid state relays but you still have to worry about the inductive kickback so in the end the snubber circuitry is going to be likley as expensive as a relay rated for switching an active load at that current. A little more information about what is actually using the power would be helpful. "variable inductive, capacitive and resistive components" is not very descriptive. Also if you want low 'hold up power' see if you can find a latching relay, that how ever will add more to the cost.

 

[Ubergeek63]

a $250 to $600 power supply and FETs, of course the battery pack will be simularly unreasonable.

How long do you expect the batteries to last?