nuckolls.bob(at)aeroelect
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 Posted: Tue Feb 04, 2014 11:42 am Post subject: Off the shelf product update |
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A few weeks ago we were discussing several design
goals that would benefit from the incorporation
of solid state relays. At that time I cruised the
offerings on eBay and purchased this item
http://tinyurl.com/l2v72uf
[img]cid:.0[/img]
This is a DC/DC solid state relay rated for switching
up to 60 volts at 100 amps. I selected the 100 amp
rated device based on the notion that mos-fet transistors
rated for the greatest currents tend to have the lowest
On-Resistance ratings. I've used many 'oversized'
transistors in a task not because I needed some
'headroom' for reliability . . . but because I wanted
the lowest practical on-resistance which in turn
produces the simplest heat rejection problem.
When the first relay arrived, I biased the critter
up on a 20A constant current supply and it exhibited
a voltage drop on the order of 5 volts . . . obviously
unsuited to the task at any current level.
Wrote to the seller who was wanting to see
schematics on how I had wired it up . . . but who
didn't have a clue as to how it was supposed to
work. I finally lodged a complaint with eBay for
merchandise not as advertised.
The seller responded with an offer to replace it
and I received the second part directly from the
factory yesterday. This part does perform much
better . . . only 0.5 volts drop at 20A and probably
less than the advertised 1.0 volt drop at 100A.
I closed the 'gripe' with the eBay seller.
This discovery demonstrates the designers unwillingness
to 'bootstrap' the gate supplies to his power
FETs for the purpose of achieving the lowest possible
on-resistance . . . hence the lowest possible heat
dissipation.
This product is offered by many eBay sellers and
I've found a few bad-reviews . . . but for the
most part, these critters seem to operate as
advertised.
But we can do better . . .
I've got a 20A package I'm working with that is
in a plastic enclosure and requires NO heat-sink.
But it wont sell for $13 either.
I can recommend the product offered on eBay for
situations where the MINIMUM drop on the order
of 0.5 volts is not a stumbling point. At $13, its
a good value. I would not operate it at much more
than 20A for the same heat-rejection issues
we've been deliberating here on the list for
bus power steering diodes.
I'm going to throw the SSR in the goodies-bin
and move on . . . might have a place I can use
it on my truck someday.
Bob . . .
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