[GreenKeys] Solid State Motor Control Relay

[ad7i]

Thank you John.  This is very interesting.  I'm always amazed when high
voltage circuit boards are routed out in such a fashion that the designer
*didn't* take advantage of the maximum creepage and clearance possible.  It
costs nothing to do that, and yet they so often don't.

Paul, ad7i


On Mon, Dec 28, 2020 at 11:36 AM John Nagle <nagle at animats.com> wrote:

>
> > To: greenkeys Keys<greenkeys at mailman.qth.net>
> > Subject: [GreenKeys]  Solid State Motor Control Relay
>
> > One comparison I meant to make in the post below is this:
> >
> > Many older TU designs, W2PAT, W2JAV, various versions of the TT/L all
> used vacuum tubes to
> > do a lot of 'switching'.  Tubes could stand a lot of brief, excess
> voltage spikes and survive.
> > Solid State devices like SSRs, SCRs, and the like, are often 'one zap'
> devices then they die.
> > They cannot repair themselves, and some spikes can put them out of
> service.  Once a junction
> > is breached, the device is dead or nearly so
> >
> > If the 28KSR motor is a 1/20 hp device, the ASR motor is a 1/12 hp
> device, and its inrush
> > will be even higher than the 9A mentioned in several earlier e-mails.
> If you have a reasonable
> > choice between a 15A and a 25A relay, take the 25A device.  Call it
> overkill or being 'super-
> > conservative' or whatever, but for a small cost difference, 'go big'.
>
> For a Teletype motor, a 15A solid state relay is more than enough. If
> it's a a real 15A solid state relay.
>
> If you get it from Digi-Key directly, and it's from Crydom or Potter
> and Brumfield or Allen-Bradley or Panasonic, it should work.
> If you get it on eBay or Amazon it's probably counterfeit and way, way
> overrated as to current.
>
> Most Fotek solid state relays on eBay are counterfeit. See
>
> https://youtu.be/DxEhxjvifyY
>
> for a teardown of a fake. If it says "Fotek" and "Made in China",
> it's fake. The real ones are made in Taiwan and look like this:
>
> https://www.fotek.com.tw/en-gb/product-category/143
>
> They also have honest ratings. A real Fotek 10A unit can take a 10A
> load forever, and a 135A surge, so the upstream circuit
> breaker will trip if it's shorted. And it has overheat protection
> and will shut down if overloaded.
>
> Here's one from China, from the actual manufacturer.
>
>
> https://www.alibaba.com/product-detail/1PCS-SSR-10-DA-DC-Control_1600149635502.html
>
> They actually provide a table which shows how much they
> exaggerate the ratings. They're over-rated by 2X for
> a resistive load, and 6x for an inductive load.
> "For a motor with rated current of 15A and the motor
> is inductive load, the formula is 15A x 6 = 90A, so
> you should choose to buy 100A solid state relay".
>
> That's the problem.
>
> I used to get Crydom made in USA solid state relays in surplus
> stores. They work fine at their rated load.
>
> If you want a cheap one from China, buy it direct from an actual
> manufacturer on Alibaba, after reading the data sheet.
>
>                                 John Nagle
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