Newsgroups: sci.electronics
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!batcomputer!cornell!uw-beaver!milton!whit
From: whit@milton.u.washington.edu (John Whitmore)
Subject: Re: solid state relays
Message-ID: <1991May15.061140.9638@milton.u.washington.edu>
Organization: University of Washington, Seattle
References: <12500@qisoff.phx.mcd.mot.com> <1991May14.135524.25014@wsrcc.com>
Date: Wed, 15 May 1991 06:11:40 GMT

In article <1991May14.135524.25014@wsrcc.com> wolfgang@wsrcc.com (Wolfgang S. Rupprecht) writes:
>hbg6@citek.mcdphx.mot.com writes:

>> 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.

	Nope, I don't agree.  Unlike bipolar transistors, SCRs have
no 'hot-spot' failure mode (they're like MOSFETs in that regard),
so they are EXTREMELY tolerant of surges.  Motorola gives their
12A SCRs a 300A surge rating; your motor is unlikely to kill
'em.  Lightning strikes, however...

>> 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.  ;-)

	Well said.

	John Whitmore