DC relay for 48V Power Vent [RE-wrenches]

[John Raynes]

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Thanks to everyone for the helpful replies -

I did take Chris's suggestion about using a 4-Pole relay with the contacts 
wired in series.  I didn't turn up a 4PST right away, but I found a 4PDT 
Magnecraft (P/N 782XDX1C-12D) available from Mouser for $4.16 in single 
quantities, with a nice little DIN-Mount screw terminal relay socket and 
holding clip to match for about $4.00 additional.  Its contacts are rated 
for 3A at 30VDC.  There's a 10 Amp rated version of the relay for $3 more 
but I figured that was overkill.  They also have 24VDC and 120VAC coil 
options available.

I wanted to stay with a well-established Mfr. such as Magnecraft or P&B or 
Omron, available through distribution so ordering replacement spares 
wouldn't be an issue.  And at that price, I don't mind passing through the 
cost for providing an on-site spare from the get-go.

Windy, I'm not sure I totally concur that ~1/2 VAC contact rating is 
sufficient for figuring a safe VDC rating.  It seems more like 1/4 to 1/10 
ratio is what I find more commonly when there is both an AC and DC 
rating.  And those DC ratings when they are given, tend to be purely 
resistive loads.  A DC fan is likely to be somewhat inductive, and if I 
remember correctly from my school daze an inductive load can raise the 
circuit voltage relatively high for an instant when it is interrupted, as 
the energy stored in the inductor attempts to maintain the flow of current.

After working with PV for a number of years I've become really sensitized 
to the arcing issues involved with interrupting a DC circuit, even a small 
one such as this.  Mechanical relays and contacts fail, I'd rather play it 
safe.  At least in this case there wouldn't be great harm if the relay 
fused shut, it would just mean that the vent fan stayed on all the time.

John Raynes
RE Solar
Torrey, UT



At 02:58 PM 2/19/2008 -0800, you wrote:

>John -
>
>You are right that most of the small relays you will find do not have a
>very high DC rating - but I have not seen any problems with using 30VDC
>rated relays in applications like vent fans.  They are very low current
>and are also operated from a brushless fan drive - so the currents are
>well controlled and there is virtually no start up current involved.
>
>I would not advise using a typical "automotive" relay for this however -
>they do not have the contact spacing - these are usually not rated for
>120VAC either.  Go with a socketed type relay with a clear housing so
>you can see if the arcing is excessive or discoloration of the cover
>occurs.
>
>One trick to also keep in mind is that you can also wire multiple poles
>of a relay in series to increase the voltage tolerance - although this
>is not shown in the product literature it is pretty common in the
>control world and also the electric vehicle conversion marketplace.  So
>wire up a 30VDC 4PST relay into a 120VDC SPST relay for example.  I have
>seen these work well for years on EV conversions with big loads at high
>voltages (120VDC) such as heaters etc. which have high inrush currents.
>
>
>Christopher
>
>Christopher Freitas
>Director of Research of Development
>OutBack Power Systems, Inc.
>cfreitas at outbackpower.com
>Tel 360 435 6030
>cell 360 202 4239
>19009 62nd Ave NE
>Arlington WA 98223 USA
>www.outbackpower.com
>
>
>-----Original Message-----
>From: John Raynes [mailto:john at raynes.com]
>Sent: Tuesday, February 19, 2008 1:15 PM
>To: RE-wrenches at topica.com
>Subject: DC relay for 48V Power Vent [RE-wrenches]
>
>
>I'm sure this has been covered some time in the past, but with the
>Topica
>search function being kaput...
>
>Does anyone have a part number for a relay for switching a 48V Power
>Vent
>from an MX60 aux output, one that has worked well for them?  I know I
>could
>have ordered the 12V model, and powered it directly, but I didn't.  I
>assume the relay I need is going to have a 100V-125VDC rating, since no
>one
>to my knowledge makes a relay with 70-80 volt ratings.  That usually
>means
>mercury-wetted or solid state for DC relays.  Without getting too exotic
>or
>pricey I thought someone may know of a model that works well and mounts
>to
>a relay socket or DIN rail.
>
>Thanks,
>John Raynes
>RE Solar
>Torrey, UT


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