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Hi all...

I'm sure this is a very basic question, and I should know the answer, but my knowledge is patchy.

I'm modifying a tyre warmer set for a radio-controlled car, and want to have four LEDs to indicate that the four warmers are working. They're all run from one 12V supply, and draw a little over 3A in total. They will sometimes also be run on an 8.4V (averages 7.8V) battery to maintain heat whilst waiting at the track-side.

I toyed with an elaborate Arduino controller, but realised all I really need is to power them from my 12V bench supply, then plug a battery in when necessary. I might introduce an auto charging system, but I digress...

I can isolate the live and return for each of the four tyre warmers (they're a metal cup with a heating wire), but how would I wire in an LED to each one that only illuminates when they're working? I'd like to know immediately if one of them has failed. If it's easier to have a light come on only when the circuit in the heater breaks, then that's fine, but I'd prefer a light that goes out when the circuit breaks.

I'd have thought that wiring an LED and resistor in series wouldn't work with the heater element, but I could be wrong. Or should I measure the resistance of each element, and go from there?

So, my quick question grew, as they always do! Thanks VERY much for any help any one can give me...
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If I am reading you correctly, try this circuit (for 12V) :


* Heater.jpg (102.68 KB, 562x429 - viewed 13 times.)
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The way you have it in your schematic isn't the same as how you have it wired up!

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Are you trying to detect if the heater element fails?  That implies sensing current through the heater,
not as simple as detecting the presence/absence of 12V.  A shunt resistor that gives 0.7V or so
could be used to turn on a transistor that powers an LED, but the shunt resistor will get warm/hot
and reduce the efficiency of the tyre warmer a little.  A smaller shunt with a comparator to sense
a smaller voltage would be better.
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Measurement changes behavior
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Are you trying to detect if the heater element fails?  That implies sensing current through the heater,
not as simple as detecting the presence/absence of 12V.  A shunt resistor that gives 0.7V or so
could be used to turn on a transistor that powers an LED, but the shunt resistor will get warm/hot
and reduce the efficiency of the tyre warmer a little.  A smaller shunt with a comparator to sense
a smaller voltage would be better.

Why not go whole hog and implement tire temperature sensors and use four PID feedback loops so one could know for sure that the tires have reached the desired optimum temperature for the specific race conditions. One does want to win the race after all and any edge can make the difference.  smiley-wink
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Are you trying to detect if the heater element fails?  That implies sensing current through the heater, not as simple as detecting the presence/absence of 12V.  
A shunt resistor that gives 0.7V or so could be used to turn on a transistor that powers an LED, but the shunt resistor will get warm/hot and reduce the efficiency of the tyre warmer a little.  A smaller shunt with a comparator to sense a smaller voltage would be better.

Although I agree that your idea is probably a good solution, I must comment that each of his heaters draws 0.75A (3A overall/4) so a resisistor to drop 0.7V would be around 1 Ohm and need to be at least a 1/2W type, if not bigger. Dropping a smaller voltage would require an even smaller resistance. Resistors of less than 1 Ohm are not standard products and are, therefore, quite expensive.

//Edit//
How about a couple of diodes to drop 1.4V instead of the resistor?
« Last Edit: May 30, 2013, 10:46:47 pm by Henry_Best » Logged

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Thanks everyone for the replies... Yes, I am trying to detect a failure in the circuit, not simply the presence of power.

I had considered independently controlled warmers, or simply one temperature setting but controlled for all four heaters, but to be honest, my driving is so bad it wasn't worth the expense. I'm confident I could make the controller for that, but in the end I just bought a cheap 12V temperature controller from eBay to measure one tyre, as it wasn't worth the hassle of four relays or MOSFETs. The difference in temperatures won't really matter by the time I've actually got to the race stand (maybe for the guys with pit crew it matters, but not at my level!).

I'm relieved this isn't a totally simple circuit! Does anyone have any further ideas please? Of course, I could use a haptic feedback system (does it feel hot?!) to test the warmers, but I wanted to put a little flare into project.
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