Re: Tripping a relay.

Quoting Dan Novy <[email protected]>:

>    I'm constructing something similar those "solar lanterns" you see 
> in people's gardens, except on a larger scale.  It charges up during 
> the day, and automatically turns on at night.  I'll be using a 
> photvoltaic panel to charge a 12v 12Ah battery. The battery will be 
> driving, through a driver of course, about 90 feet of ELWire. 
> Basically what I want to do is just use the light sensor and the RCX 
> to detect when the sun has gone down and send its 9v signal to a 
> relay, to let the juice flow from the battery to the EL driver.  My

There are simpler and less power comsumptive ways to do this (a CdS photocell
and a simple transistor circuit or comparator - you will probably see
something in IC "cookbooks" or with Google), but the RCX will certainly work.

> question is about the naming of relays. If the relay is labeled at 
> 9V, does that mean it takes 9V to trip it? How is the load that the 
> relay is capable of sustaining denoted. Basically I need 9v in and it
Check out a datasheet such as at omron.com or datasheets at DigiKey.

Relays have two rated "sections" - the coil, which moves the contacts, and the
load-carrying contacts. They are separate and independent.

Heres a relay at DigiKey for $1.45: Z1641-ND, Omron G5SB-14-DC9. Here's the
data:
http://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V=39&M=G5SB-14-DC9.
You can see that this "9V" coil requires 44 mA to operate (about 360 mW) and
it must operate at 75% of the coil voltage (about 7 V) and once active, must
release at no less than 5% of the coil voltage (0.5 V). It's max voltage is
110% of the nominal 9V at 70 degC, more at lower temps. These specs vary from
relay to relay.

The contacts on this relay are rated for 3-5 A at up to 125 VAC, resistive
load. An inductive load (like an EL driver?) would be less, maybe half as
much. This data sheet doesn't include an inductive rating.

But a major problem with relays is they require a significant continual
current in the coil to hold them in the "on" state - almost 400 mW for this
example. A solid state circuit will use much less current to stay in the on
state - 1/100 as much (4 mW) would be typical.

So you could find a simple circuit which would use a photocell, a FET or
transistor switch, and would turn on your EL driver a lot more efficiently
than an RCX and a relay.

> has to only turn on 12V. Anybody got a favorite relay they like? If I 
> get a relay that's rated well above the voltage and amp load that I 
> need, will it still work? Thanks.

If you mean the contacts, yes, within limits. Look at the data sheet
referenced above and the min load is 10 mA at 5V. Telcom relays typically
have a min load of 10uA and 10 mV. Heavier duty relays usually have coils
with higher drive requirements, and that's an issue running off batteries.

You can use a latching relay which only consumes power when it switches. This
might be what you want if the solid state circuit doesn't tickle your fancy.
But the latching relay stays in its given state until switched again. So if
your battery ran down it might be stuck in the 'on' state.

Here's a site which claims 11,000 circuits:
http://www.discovercircuits.com/list.htm

Regards

Bruce Boyes