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From: wolfgang@wsrcc.com (Wolfgang S. Rupprecht)
Newsgroups: sci.electronics
Subject: Re: solid state relays
Message-ID: <1991May14.135524.25014@wsrcc.com>
Date: 14 May 91 13:55:24 GMT
Article-I.D.: wsrcc.1991May14.135524.25014
References: <12500@qisoff.phx.mcd.mot.com>
Organization: Wolfgang S Rupprecht Computer Consulting, Washington DC.
Lines: 45

hbg6@citek.mcdphx.mot.com writes:
> I am building a controller for the evaporative cooler on my house. The
> blower motor has two speeds, selected by providing power to either
> the HIGH terminal, or the LOW terminal.

> I really want to use solid state relays to avoid the cost and 
> complexity of contactors and the associated drivers. The current
> spec. for the relays I have is in range but I have three concerns:

> 1 The SSR can easily handle the run current of the motor but is
> the starting current likely to kill it?

Yes.  Of course.  The more you beat on a device the faster it will
wear out.  While the SCR's (as any semiconductor) can tolerate a bit
of extra surge current, it does stress them.  Do this too often and
you break them.

> 2 Is the normal failure mode for a SSR open or shorted?

Both.  This is what fuses are made for.

> 3 What would happen if both the HIGH and LOW terminals on the
> motor are powered at the same time.

Lots of current would be drawn.  Smoke would be sure to follow.
Use a simple interlock between the two SCR's, and USE A FUSE.

> Bottom line; if the SSR fails closed and power is applied to
> both terminals am I going to burn up a $200 blower motor?

Not if you use a fuse.  ;-)

Oh one more thing.  You should really think about what the voltage on
the low speed SCR will look like when the high speed winding is
energized.  Hint: You will probably see quite a bit more than the
input voltage across the low-speed SCR.  Rate this SCR appropriately.

You will also want to use a good snubber network and a zero crossing
detector (for turn on).  This will reduce RF bursts that will lead to
radio interference and also an early destruction of the SCR's.

-wolfgang
-- 
Wolfgang Rupprecht    wolfgang@wsrcc.com (or) uunet!wsrcc!wolfgang
Snail Mail Address:   Box 6524, Alexandria, VA 22306-0524