Date: Sat, 16 Aug 2008 23:20:54 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Relay polarity
In-Reply-To: <BLU136-W1925BF1F049DBDCD8C7B2CDB6F0@phx.gbl>
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At 09:46 PM 8/16/2008 -0400, Edward Duntz wrote:
>I've noticed a difference in the wiring diagram that comes with the
>Bosch WR1 relay
WR1 is a kit, not a relay as such. It uses an SPST relay, with no
87a (NC) contact.
>The website http://www.the12volt.com/relays/relays.asp says that it
>doesn't matter which way the polarity is for 85 and 85 as long as
>there isn't a diode in the coil.
That is correct. However, 86 is the "official" input terminal and 85
the "official" ground.
> How do I know if there is a diode in my relay (Bosch or other wise)?
It may be stated in accompanying material.
There may be a diagram printed or molded on the relay housing.
By getting the relay specs from the part number.
By measurement, but it could get a little tricky.
There are two ways to include a diode, for different purposes -- in
series with the coil, or in parallel. Series connection will simply
allow the relay to operate with correct polarity, and not with
reverse polarity; that's its purpose. Parallel (more common) will
allow the relay to operate in correct polarity, and will short out
and blow the diode (which will probably fail open, removing itself
from the circuit) if unrestricted power is applied in the reverse
polarity. The purpose of this setup is to prevent a large voltage
spike in the control circuit when the relay is deactivated.
If you're willing to *possibly* sacrifice the diode and conceivably
the relay, just hook it up both ways to 12v, with a one-amp fast-blow
fuse in circuit. Works both ways -- no diode. Works one way --
series diode. Works one way, blows the fuse the other -- parallel diode.
If you have a meter with a diode test function it *may* tell you --
first use the ohms function in both directions. If you get open
circuit both ways, you have a series diode. If you get a reading,
you have no diode or a parallel diode. Then shift to the diode check
function. Equal readings above about .8 volts, no diode. Equal
readings below half a volt, unknown diode or not. Noticeably unequal
readings, parallel diode; the higher reading shows operating
polarity, lower reading reverse polarity.
You can go farther with testing, but it's not as easy to explain.
>
>This leads me to wonder something else. Does it matter if terminals
>30 and 87/87a are reversed?
No, it doesn't. Relays like this are break-before-make, so switching
a relay wired this way will first disconnect the NC source and then
connect the NO source to the load. At no time will the sources be
connected together. The disconnect time will be maybe a few
milliseconds. Be sure your load doesn't mind this, and of course
keep your load within the relay ratings. And of course you'll need
an SPDT relay with both NC and NO contacts.
>of camping, I'm better off waiting until I get home and charge with
>a deep cycle battery charger than to use the alternator for charging.
>
>What do you think?
Works for me...
David
--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage," '85 GL "Poor Relation"
