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Discussion Starter #1
Since the EVtech list appears to be down, I'll post this here.

----- Original Message -----
From: "Al" <[email protected]>
To: <[email protected]>
Sent: Sunday, March 13, 2011 1:08 AM
Subject: solid state relays (again)


> It's been pretty quiet on the list so I thought I'd start some discussion.
> I am going to try a battery monitor/management system similar to Lee
> Hart's battery balancer.
> I am going to use solid state relays (mosfets) to switch between
> batteries.
>
> First problem is mosfet voltage rating. Higher voltages have higher on
> resistance and cost more. Instead of having all the "relays" connected to
> a common buss, I could break the pack into manageable pieces like 12 6V, 6
> 12V or 24 3V. I am planning on maximum 10A balancing current. Also using
> bidirectional TVS across each "relay". How much headroom should I have on
> the mosfets and TVS's? Should I double the worst case voltage and current?
> Should the TVS be rated just below the mosfet rating?
>
> With SS relays, I need to be darn sure that only the proper relays are
> energized, even small "glitches" could turn them on. Would an IC decoder
> be the best coice for this? I checked on Mouser and most of this "old
> school" logic is getting rare. The 4028 1 of 10 decoder that Lee uses
> looks like my best bet. Do they make a such a thing as a 1 of 16 decoder?
>
> BTW I am using photovoltaic drivers for the mosfets. These things are
> great, no need for an isolated gate drive. You can have a separate supply
> for the LED side that if killed would disable all relays.
>
> Check out these fuses that I will have on each line right at the battery
> terminals. They are rated 500V AC and DC! even in the higher amp range.
> They also have excellent interrupt ratings.
>
> http://www.littelfuse.com/data/en/Data_Sheets/Littelfuse_Fuse_505.pdf
>
> Enough for now, thanks for any comments.
>
> Al

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70 Posts
Discussion Starter #2
Al Bigg wrote:
> I am going to try a battery monitor/management system similar to Lee
> Hart's battery balancer. I am going to use solid state relays (mosfets) to switch between
> batteries.

>> First problem is mosfet voltage rating. Higher voltages have higher on
>> resistance and cost more. Instead of having all the "relays" connected to
>> a common buss, I could break the pack into manageable pieces like 12 6V, 6
>> 12V or 24 3V.

That's possible, but I think it will still require that any given relay
has to be able to withstand the peak worst-case pack voltage in its
"off" state. They don't switch on and off instantly; the first one off
will see the full voltage, even when you have several in series.

> Also using bidirectional TVS across each "relay". How much headroom
> should I have on the mosfets and TVS's? Should I double the worst case voltage and current?
> Should the TVS be rated just below the mosfet rating?

With solid-state devices, a 2:1 safety margin is about the minimum you
can use to insure reasonably long life and reliability (like a 400v 20a
part in a 200v 10a circuit). 3:1 is better.

>> With SS relays, I need to be darn sure that only the proper relays are
>> energized, even small "glitches" could turn them on. Would an IC decoder
>> be the best choice for this? I checked on Mouser and most of this "old
>> school" logic is getting rare. The 4028 1 of 10 decoder that Lee uses
>> looks like my best bet. Do they make a such a thing as a 1 of 16 decoder?

Look at my circuit carefully. It has evolved to make is as near to
impossible as I can to turn on two relays at once. There are several
layers of protection -- even if one fails, the others back it up.

For example, the 4028 can't normally turn two outputs on. But it can
fail with 2 or more outputs stuck low. So my relay drivers are current
limited so they can't drive two relays on at once, even if the 4028
commands it.

> Do they make a such a thing as a 1 of 16 decoder?

Sure; the 4067, 4514 or 74HC154, for example. But if you use a 1-of-16
decoder, you have to be sure there's a way to turn them *all* off.

>> BTW I am using photovoltaic drivers for the mosfets. These things are
>> great, no need for an isolated gate drive. You can have a separate supply
>> for the LED side that if killed would disable all relays.

They are cool, but expensive. Also, note they are exceptionally slow.
They take as long (or longer) than a mechanical relay to switch on/off.
Since they gradually change states, you need to be sure they aren't
carrying current when they switch. For example, if one switches 100v at
10a, then it will see 50v x 5a = 250 watts for about 1 msec each time it
switches!

>> Check out these fuses that I will have on each line right at the battery
>> terminals. They are rated 500V AC and DC!
>> http://www.littelfuse.com/data/en/Data_Sheets/Littelfuse_Fuse_505.pdf

Yes, these are the same ones I'm using. They do indeed clear safely at
high DC currents. (You are going to test this feature as you design your
relay boards :O ).

Warning: These fuses have a sealed ceramic body and are filled with dry
sand to quench the arc. Littelfuse outsourced them to China. The Chinese
(bless their heart) used wet sand. Guess what happens when such a fuse
gets hot from soldering it in, or due to high current when it tries to
blow)? The water turns to steam, and KABOOM!

If you get a batch, and the end blows off any of them during soldering,
reject the whole batch, and send them back! And, mark them "defective"
so the distributor doesn't just put them back on the shelf to sell to
the next sucker!

This is precisely why you want safety components that someone *other*
than the manufacturer has tested. Littelfuse should have discovered this
problem; but they didn't.
--
Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
leeahart earthlink.net | That's how the light gets in -- Leonard Cohen

_______________________________________________
| REPLYING: address your message to [email protected] only.
| Multiple-address or CCed messages may be rejected.
| UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
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