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Subject: Re: Need idea for a remote lamp status indicator.
References: <3E1C60A9.email@example.com> <firstname.lastname@example.org>
X-Newsreader: Forte Agent 1.93/32.576 English (American)
Date: Wed, 08 Jan 2003 13:16:52 -0500
NNTP-Posting-Date: Wed, 08 Jan 2003 13:16:58 EST
Organization: Bell Sympatico
Some cars have that feature.
You can be sure they use the most economically way of doing.
How do they do it ?
What about a opto-coupler with a shunt resistor in series with the
lamp on the ground side and a transistor and a LED at the other end?
On Wed, 08 Jan 2003 17:49:35 GMT, Spehro Pefhany
>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.
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