Compass light "dimmer"

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Tiderace

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We have a new compass on, and need to wire the compass light in. The old compass had a dimmer switch (rheostat?) which is long dead.

I'm struggling to find anywhere that sells a dimmer/rheostat for such an application. (As you can tell I'm not a sparkie, so looking at £40-60 rheostats on RS Online baffles me!)

Any advise much appreciated.

Thanks
 
Red LED. I don't see the need for a dimmer, the light's only on when you want it dimmed, ie in the dark.

I rigged mine through two relays though diodes would have done the same job, so that the compass can be on with the tricolour under sail, or on with the running lights under power. Without that, all the lights would come on regardless of which one selected.
 
Catalina36 said:
Red LED. I don't see the need for a dimmer, the light's only on when you want it dimmed, ie in the dark.

I rigged mine through two relays though diodes would have done the same job, so that the compass can be on with the tricolour under sail, or on with the running lights under power. Without that, all the lights would come on regardless of which one selected.
Click to expand...
Thumbs up on the red LED: There is no excuse IMHO for incandescents in any application these days, especially not on a boat.

Relays? Really? Every authoritative figure in electrical design will say that diodes are required across the coils of relays to prevent the inductive spike that severely shortens the life of the connected switch. If you have diodes for that then you might as well use them to implement your other solution; it is the better one afterall. The components are cheaper, easier to connect and will increase the lifespan of your switches for your tricolour and your running lights. I would also include a series resistor to limit the current through the LED.
 
The LED will probably want something like 20mA?
It will already have some resistance in series to control that current, maybe 10v/20mA = about 500 ohms.
So adding a potentiometer of 0 to 5kohms or so in series might be a useful range of brightness?
The wattage is only about 60mW max so you don't need a high power rheostat.

Potentiometer POT 470R to 1M 16mm Mono LIN or LOG Variable Resistor Control *UK* | eBay
4k7 linear

You could measure the current to make sure.
 
TernVI said:
The LED will probably want something like 20mA?
It will already have some resistance in series to control that current, maybe 10v/20mA = about 500 ohms.
So adding a potentiometer of 0 to 5kohms or so in series might be a useful range of brightness?
The wattage is only about 60mW max so you don't need a high power rheostat.

Potentiometer POT 470R to 1M 16mm Mono LIN or LOG Variable Resistor Control *UK* | eBay
4k7 linear

You could measure the current to make sure.
Click to expand...

Con't you mead in parallel. If you put a 5K ohm in series there would be very little light from the LED.

I very often use 1K ohm fixed resistor in series with a red 5 mm LED as a switch indicator to keep the brightness level low so they do not glair.
 
Rogershaw said:
Con't you mead in parallel. If you put a 5K ohm in series there would be very little light from the LED.

I very often use 1K ohm fixed resistor in series with a red 5 mm LED as a switch indicator to keep the brightness level low so they do not glair.
Click to expand...
It won't be in parallel. If it gets turned right down to increase brightness the resistance of the combined resistor and pot will be 0 ohms, which as sure as eggs is eggs will fry the LED. By default, I go with 330 ohm for a 5v supply, so I'd start with 660 ohm as my starting minimum resistance for 10v (12v less the voltage drops across blocking diode and LED).

For course, you could do away with the two blocking diodes and just use two separate LEDs (if there is space in the compass dome). There is an advantage in that It would give an extra visual clue to the tricolour and the running lights being on at the same time.
 
Yes you are correct but the 5k in series will give very little pot adjustment.

As you say 330 and 0 to 660 will give a better adjustment and brightness range. then a 500 and a 0 to 5K will
 
HissyFit said:
Thumbs up on the red LED: There is no excuse IMHO for incandescents in any application these days, especially not on a boat.

Relays? Really? Every authoritative figure in electrical design will say that diodes are required across the coils of relays to prevent the inductive spike that severely shortens the life of the connected switch. If you have diodes for that then you might as well use them to implement your other solution; it is the better one afterall. The components are cheaper, easier to connect and will increase the lifespan of your switches for your tricolour and your running lights. I would also include a series resistor to limit the current through the LED.
Click to expand...
All that may well be true. When I fitted the relays 10 years ago I didn't know about diodes or what they were for.
 
TernVI said:
The LED will probably want something like 20mA?
It will already have some resistance in series to control that current, maybe 10v/20mA = about 500 ohms.
So adding a potentiometer of 0 to 5kohms or so in series might be a useful range of brightness?
The wattage is only about 60mW max so you don't need a high power rheostat.

Potentiometer POT 470R to 1M 16mm Mono LIN or LOG Variable Resistor Control *UK* | eBay
4k7 linear

You could measure the current to make sure.
Click to expand...
20mA?? That's one high powered LED - for a compass light? More like5mA.
Just had a look at an LED sold for 12v, 1K resister in series on the positive, assuming the OP existing LED is similar (pretty safe assumption) then your linked pot will add another 470R to that 1k even when turned all the way down. Not a good choice of pot.
 
Rogershaw said:
Yes you are correct but the 5k in series will give very little pot adjustment.

As you say 330 and 0 to 660 will give a better adjustment and brightness range. then a 500 and a 0 to 5K will
Click to expand...
You can put a significantly smaller resistor in parallel to the pot so that the minimum resistance of the whole line of resistors and pot will be the value of the serial resistor on its own. The maximum resistance will be the value of the serial resistor plus approx the resistance of the small resistor paralleled to the pot.

You will need the 660 ohms minimum to stop the current through the LED going over its maximum forward current, thus blowing the poor thing. 4k7 pot paralleled with 470 ohm will give 0 - 420 approx ohms, which is good enough for meaningful control. Nearest standard value to 660 ohms is probably 680 ohms.
 
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Catalina36 said:
All that may well be true. When I fitted the relays 10 years ago I didn't know about diodes or what they were for.
Click to expand...
Wow, ten years! You must have coil paralleling diodes already. Either that or you've been preying to the God of switch protection. :ROFLMAO: ;)
 
GHA said:
20mA?? That's one high powered LED - for a compass light? More like5mA.
Just had a look at an LED sold for 12v, 1K resister in series on the positive, assuming the OP existing LED is similar (pretty safe assumption) then your linked pot will add another 470R to that 1k even when turned all the way down. Not a good choice of pot.
Click to expand...
Given that the OP is replacing a rheostat, I wouldn't be surprised if the compass is lit by an arclight. :ROFLMAO: Honestly: rheostats, incandescent bulbs, relays! Before boating gets into the 21st century we need to get out of the 1950s!
 
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GHA said:
20mA?? That's one high powered LED - for a compass light? More like5mA.
Just had a look at an LED sold for 12v, 1K resister in series on the positive, assuming the OP existing LED is similar (pretty safe assumption) then your linked pot will add another 470R to that 1k even when turned all the way down. Not a good choice of pot.
Click to expand...
20mA was a guess at the most it could reasonably be. Adding 0 to 4k7 in series will give adjustment down to something around 2mA.
I'm surprised you didn't suggest a PWM output from a microcontroller?
(-:
 
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