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Forum Index : Electronics : solid state switch

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brucedownunder2
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Joined: 14/09/2005
Location: Australia
Posts: 1548
Posted: 12:09am 19 Dec 2010
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I've been thinking (thats a worry),, anyhow I've got 3 big battery banks and I'd like to keep them separate and switch between them occasionally to the inverter. while that bank is being used ,I could be charging another..
I've previously used a very large rotary switch from an industrial switchboard..

Would anyone have a few clues on building a solid state multi pole ,say, 3 or 4 position switch I could make or buy ??

I've posted a post on the wanted section ,but no takers ,so far..

Merry Christmas

Bruce
Bushboy
 
VK4AYQ
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Joined: 02/12/2009
Location: Australia
Posts: 2539
Posted: 12:40am 19 Dec 2010
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Hi Bruce

The biggest DC electronic relay I have been able to get is 40 amps so would 8 to 10 of those for each pole times the number of battery banks, not very practical or cheap.

I have used old knife switches from old 3 phase motor starters with success but are a bit hard to find now due to the increase in scrap prices.

Another alternative I have been thinking of is using FET switches as there are quite a few for a $1 each rated at 50 + amps, low loss low price and low control current, just a suggestion.

All the best

Bob
Foolin Around
 
brucedownunder2
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Joined: 14/09/2005
Location: Australia
Posts: 1548
Posted: 12:49am 19 Dec 2010
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Thanks Bob ,,, yes, the solid state way would , I think ,be the way to go ..
One thing I have to be sure of ,is each battery bank has to be completly isolated before bringing the next bank onto line ..Not so much a no-break system ,just a break before make system as the change over occurs.

Thanks for the input,Bob.

Bruce
Bushboy
 
VK4AYQ
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Location: Australia
Posts: 2539
Posted: 01:14am 19 Dec 2010
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Hi Bruce

That could be built into the master switching with a led monitor to be sure the bank is off before selecting the next bank, A small rotary switch would handle the switching also a dpdt relay could be used but wouldn't give the dead position between banks. I liked the old knife switches but not much good for automating the process.

All the best

Bob
Foolin Around
 
Downwind

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Joined: 09/09/2009
Location: Australia
Posts: 2333
Posted: 01:48am 19 Dec 2010
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What voltage and whats the amperage you need to be able to handle. (2 very important bits of information)

Pete.
Sometimes it just works
 
brucedownunder2
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Joined: 14/09/2005
Location: Australia
Posts: 1548
Posted: 06:52am 19 Dec 2010
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Thanks Pete, thought about that on the way out this morning ,I knew I left out some important info ,,sorry ..

OK, it's going to switch a 48v batt. bank The amps should not go over ,say 40, . The inverter is 6Kw normal with a 16Kw peak momentarily.Power star n-psw 6000 W .

I'd be using maybe,50 to 75 mm square multistrand welding cable ,, as short a run as possible fused close to the solid state switch . Before or after the switch , I'm not sure ,yet.

I 'd be thinking of a small rotary switch with positions of ,say , 3-4 positions. This could activate the solid state switch and give indicators what is on-line , what is off.

Thanks so far

Bruce
Bushboy
 
Downwind

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Posts: 2333
Posted: 01:24pm 19 Dec 2010
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It all should be rather easy with the use of solid state relays, its just a matter of finding some that will handle the current you require (at the right price)

I will have a look around to see what i can find where for you, do you have a problem with buying out of China? (much cheaper)

Just one thing im not 100 % sure on with SS relays is if they are a bidirectional device on DC, and if this will effect what you want to do, Ie- power into the battery and also power out of the battery all via the relay, may not be possiable.

The other option is for a fully manual system and use battery isolator switches, this would mean switching one off and then the other on manually.
They are about $20.00 each at Jaycar.

Pete.
Sometimes it just works
 
brucedownunder2
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Joined: 14/09/2005
Location: Australia
Posts: 1548
Posted: 08:27pm 19 Dec 2010
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Thanks guys for your help ,,,maybe the big brute rotary switch will be the go ,,,also ,,maybe my not so big brute switche could have the gangs paralleled to handle the current??.

no problems with China

Bruce
Bushboy
 
RossW
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Joined: 25/02/2006
Location: Australia
Posts: 495
Posted: 12:25am 20 Dec 2010
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Bruce, as we discussed on IRC this morning... I think you're making it unnecessarily complex, expensive and sensitive.

You have 48V banks. Even if one is charged and one is discharged, the differential voltage is "relatively" small - a few volts most times, certainly nothing like the whole 48V.

A break-before-make changeover could be catastrophic to your inverter, it'd create large and nasty spikes.

What I did - and which I'm perfectly happy with - was to get some heavy duty battery isolators as used in boats, RVs etc, and use them. I have 3 strings, each of 500AH 48V, and used 3 of these switches. It allows me to connect 1, 2 or 3 banks (or potentially isolate everything, but I've never wanted that) - and the flexability to use any combination of banks.

I normally leave all 3 banks connected now, I'd only isolate one for testing or if working on it, but when I used to switch between banks, the relatively short period of time from closing one switch and opening another while changing banks, was only a few seconds (long enough to use the clamp meter to be sure the new bank was taking load).

The switches I have are rated for 500 amps - I think thats generous - but each has a 200 amp HRC fuse immediately adjacent to it (and in series with it) - so the switch should be pretty safe even in the unlikely event of very large voltage differences between banks causing lots of current to flow.

(click on images for larger view)
 

The switches were about $20 each.
 
GWatPE

Senior Member

Joined: 01/09/2006
Location: Australia
Posts: 2127
Posted: 01:01am 20 Dec 2010
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Hi Bruce,

I to use the same type of isolate switches as RossW.




I got mine from Repco.

I have 100A Fuses like these inline as well.




My batteries are connected in parallel, and the isolates are only used when servicing the batteries.

The problem with SSRelay isolates is that the mosfet outputs normally used for DC is the diode that is part of the mosfert. The relay only blocks current one way.

I would not opt for electronic switching for reasons similar to those that Ross has given. Most quality inverters have a specific shutdown procedure, and should not be just turned OFF AdHok.

Gordon.

become more energy aware
 
powerednut

Senior Member

Joined: 09/12/2009
Location: Australia
Posts: 221
Posted: 01:14am 20 Dec 2010
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hmm.. 48V * 40Amps = 1920 Watts - tad under rated for your inverter?

If you want the full power range your looking at 125 amps (334 amps, if you want to handle the momentary peaks).

A mosfet switch arrangement seems to me like it could work well.

To my electronics newbie eyes the STP60NF06L seems like a good choice (N-Chan mosfet (logic level), 60V 60A) at $1.83 each from digikey, though there are cheaper options available. For your maximum power rating i'd look at using 4 per battery bank - to keep the heat dissapation managable and to give you a nice margin for error.

An AOT460 (60V, 85A) might be a better choice still - heat would be roughly half that of the STP60NF06L, and its $1.75 each. The STP60NF06L is cheaper in the quantities you'd need (by 10c a unit).

*edited after reading the posts that were added while I was busy looking up mosfets*
the advice from the others seems to indicate this would be a bad idea. I'd like to point out that I know bugger all about electronics, and even less about big inverts, so following my suggestions probably isn't the best idea in the world.Edited by powerednut 2010-12-21
 
brucedownunder2
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Joined: 14/09/2005
Location: Australia
Posts: 1548
Posted: 01:26am 20 Dec 2010
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Thank You All,,,

I'll go with the repco switches and big fella fuses ....

Thanks and merry christmas
Bushboy
 
VK4AYQ
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Joined: 02/12/2009
Location: Australia
Posts: 2539
Posted: 03:40am 20 Dec 2010
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Hi Bruce

I use the breaker switches used for electric winches on four wheel drives they are rated at 200 amp as a breaker and can be switched manually as well, got them on fleabay for around $15 they will accept a short term overload on the inverter for a few seconds without dropping out. Part number HC-200F

All the best

Bob
Foolin Around
 
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