[GreenKeys] Solid State Motor Control Relay

[John Nagle]

> 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