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From: hbg6@citek.mcdphx.mot.com
Newsgroups: sci.electronics
Subject: solid state relays
Message-ID: <12500@qisoff.phx.mcd.mot.com>
Date: 8 May 91 22:23:18 GMT
Sender: listen@qisoff.phx.mcd.mot.com
Reply-To: hbg6@citek.mcdphx.mot.com
Organization: Motorola Microcomputer Division, Tempe, Az.
Lines: 26

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?

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

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

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

Thanks,
John (everything over TTL is high voltage) Schuch

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John Schuch - Motorola Inc., Computer Systems Division (602)438-3008
All opinions expressed are mine and not Motorolas,        their loss.
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