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From: "Roger Hamlett"
References: <3E1C60A9.email@example.com> <firstname.lastname@example.org>
Subject: Re: Need idea for a remote lamp status indicator.
X-Newsreader: Microsoft Outlook Express 5.50.4807.1700
Date: Wed, 8 Jan 2003 21:34:59 -0000
NNTP-Posting-Date: Wed, 08 Jan 2003 21:34:02 GMT
Organization: ntl Cablemodem News Service
"Spehro Pefhany" wrote in message
> On Wed, 08 Jan 2003 12:32:25 -0500, the renowned Glenn Ashmore
> >I am working up a design for a MOSFET based switch box to control the
> >lights on my boat's mast from the cockpit. These lights cannot have
> >more than a 3% voltage drop from the battery so wire runs have to be as
> >short as possible and the run up the mast and back to the switch box is
> >130' long. The switch box will eliminate about 50' of wire from the
> >mast to the cockpit but there is a drawback.
> > From inside the cabin I can't see the switch position or the lights. I
> >need a circuit design that will light an LED on the switch box when the
> >light is on. Because of the possibility of a burned out bulb or broken
> >wire the circuit has to know that current is actually flowing and not
> >just that the MOSFET is on. The circuit must also have minimum effect
> >on the power reaching the light.
> >Specifics: The voltage is 12-13.8VDC and the load ranges from 2A to 7A
> >depending on the light. The MOSFETs are 30 amp IRFZ34 driven at close
> >to max Vgs.
> Why are you using such a wimpy MOSFET? You can get one with about
> half the Rds(on) and double the Idm for a few cents more.
> You could measure either the voltage across a shunt (which might be a
> few feet of wire) or you could put a window comparator on the drain
> voltage of the MOSFET. Too low and we assume the bulb is gone, too
> high and the MOSFET is not working. You should probably also try to
> detect the MOSFET failed ON. If we assume that 10 millivolts can be
> reliably detected, you'd need a shunt of 5m ohms. That is about 5'
> of AWG10 wire. Or an SMT resistor or two, if you prefer. You could
> use an op-amp such as Microchip's MCP6022* for the comparator; it
> has a very low Vos and uses little power.
> Now, does it have to detect ONE of several parallel loads that have
> failed, or is there just one lamp per MOSFET?
> * Not confirmed, but I could have sworn I saw something like phase
> reversal with this part when BOTH op-amps were out of the CM range,
> although the data sheet claims that phase reversal doesn't occur
> with this part.
Or stick a Hall effect current transducer into the feed wire, and check for
the current drawn falling below a specific level according to how many
lights are on. One transducer in the power feed to the switch box, combine
it's output with voltage feeds from the switches, so the unit knows how many
lights are lit, and you can have a single LED, which signals that a bulb has
failed. I must agree that given the cost, a lower Rds MOSFET would be a
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