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From: Fred Bloggs
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Subject: Re: Relay controlling electric doorstrike - kickback protection needed?
Date: Sun, 06 Oct 2002 20:46:13 GMT
NNTP-Posting-Date: Sun, 06 Oct 2002 13:46:13 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
> I'm looking into control an electric doorstrike using a device made by a
> company called B&B Electronics called the 232DRIO:
> According to the website (and the PDF manual there), this device has two
> relay outputs, so I am thinking of using the relay to close the loop
> between a 16VAC transformer and the doorstrike. Here's the page on the
> doorstrike and transformer:
> I spoke to (supposedly) and engineer at B&B today, and was asking if the
> 232DRIO is used with something like a doorstrike (i.e., an
> electro-mechanical device), whether I needed to add anything to protect
> the relay against kickback, or whether the 232DRIO has protection
> already built in. She couldn't tell me, so I asked if she could tell me
> specifically which relay(s) were used in the device so I could look up
> the specs myself.
> Unfortunately, I don't think the person I spoke to knew much, as she
> kept telling me that there was no relay on the board, and that the relay
> was "on the chip" (presumably the microcontroller) [Note that the
> 232DRIO is spec'ed to handle 120VAC].
> I was wondering if anyone has had any experience with the 232DRIO? If
> so, can you provide info about which relay is used?
> If not, I was wondering, in general, if I should add any additional
> circuitry between the relay and the doorstrike to protect the relay?
> Thanks in advance!
You are wise to consider protection for this application. I assume you
are going with the 16VAC version of the door strike power supply. The
nominal volt-ampere characteristics indicate a 64 ohm reactive load so
this will be 170mH in series with about 7 ohms resistive. Because this
application will see frequent activation, relay contact protection is a
must. In this case, I suggest a series combination of low cost varistor
and flameproof metal oxide resistor across the relay contacts, as well
as a 0.01uF wired directly across the transformer secondary. In addition
to protecting the contacts, there is always the typical wiring
insulation rating of 150V to consider as well. If you go with the
Panasonic D-series ZNR available from DIGI-Key, in particular the
ERZV07D330, in series with a 2 Watt 100 ohm flameproof metal oxide
resistor, you will limit the peak kickback voltage to just a little more
than 100V in the absolute worst case extreme of 30% high line in
coincidence with contact opening at exactly current peak. The resistor
will ensure that turn off will be short and overdamped- less than 1msec-
and this will place your operating point at well under the stress limits
giving over 10^6 activations on the "Impulse Derating" chart of the ZNR.
Connect the parts using gas tight uninsulated crimp connections to
eyelet for the relay screw terminals- do not wrap the leads directly
onto the terminals. The finished circuit will look like this and cost
under a dollar.
View in a fixed-width font such as Courier.
| | |
| o COM |
| / |
| / ---
| / /
+-----+ o NO / ERZV07D330
| | | ---
) | | |
) |0.01U | |
XFMR ) --+-- | /
SECNDRY ) ----- | 100
16VAC ) | | /
) | | \ 2W
| | | |
| | DOOR STRIKE |
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