Holding Tank Indicator LED bulb voltage?

[HandyDan]

I want to express my complete ignorance in electronics.  Is there anyone out there that can help me with the holding tank led lights.  I managed to break one in trying to pull the panel so I could work on some other things.  I need to replace it.  Soldering it isn't a problem but finding the right led voltage is.  Researching these I find they come in a variety of voltages but all look exactly alike.  All I know is the resistors to the bulbs have 3 red stripes and a gold stripe.  Does anyone have enough savvy on these things to tell what the voltage would be?

           

[ClydesdaleKevin]

Hey Dan,

The color code on the resistor comes out to 2.2k Ohms 5%.

I am extremely rusty on this, so I'm sure Dave can come to the rescue here on calculating what the final voltage to the LEDs is going to be.  I'm going to assume its 12 volts coming in before the resistor.

I memorized this stuff just long enough to pass exams, and then immediately forgot it...lol.

Kev

[DaveVA78Chieftain]

The gold band is related to resistance accuracy, not voltage.  2.2K +- 5%
Physical size of the resistor normally indicates wattage.  That is either an 1/8 or 1/4 watt resistor.
Most LED resistors are 1K or less so there is more to this circuit than just an LED.  The resistor is most likely also be used for a gating transistor.  I suspect you should be able to use a 5 volt LED.

http://www.kpsec.freeuk.com/components/led.htm

http://unclean.org/howto/led_circuit.html

Dave

[HandyDan]

Thanks Dave, I went to Radio Shack and asked for a red 5mm LED bulb and was inundated with questions that I had no idea how to answer.  It was "what amps, what voltage, what current, etc."  All I want to do is replace the bulb I broke.  I just thought all LED bulbs were the same.  I told you I know nothing about electronics.  I ended up with a 12 volt bulb but it doesn't work.  I'll go back and try a 5 volt job.  At least they are cheap.  Thanks.

[brians69d24]

I don't know much about electronics either, but there is a right and wrong way to hook them up. The flat spot indicates polarity.
brian

[DaveVA78Chieftain]

You might have to measure voltage on one of the good circuits to see what it is.  Your typical 5V Red LED only requires 2 volts for it to trigger.   The resistor is used to limit the current flowing through the circuit.  As such most of the voltgae is dropped across the resistor.

Supply voltage (VS) = 12 volts
LED Voltage (VL) = 2 volts
LED max current (I) = .02 amps  (max current that can flow through LED driver circuit)

(VS -VL) / I = Limit resistor value

(VS - VL) = 10 volts (amount of voltage that has to be dropped across the current limiting resistor)
10 / .02 = 500 ohms

So, as you can see, that 2.2K ohm resistor is there for more than just the LED itself.  Without knowing what the complete circuit is, I am having to just guess at this.  I have had to replace a transistor before in my tank monitor circuit.  That may be the root of your problem.

As Brian indicated, a LED also has + and - leads.  Has to be connected properly to work.  Information about how to determine proper connection is on the first web page I linked to above.

For ease of installation, many of the 12 Volt LEDs bulbs on the market have the current limiting resistor already built in and the bulb used is actually 5 Volt.  That 5 volt rating is meant to mean 5 Volts max with normal operating voltage between 2 and 4 volts.

Dave

[HandyDan]

Mission Accomplished!  I went to Radio Shack again and got a couple of lower voltage LEDs.  Took a chance and soldered one of them in.  It works!  I sure learned a lot about LEDs.  Since I didn't know anything to begin with, it didn't take much to learn me much.  (I know that is horrible English).  Anyway this is what I was dealing with and the LEDs I used.  As a whim, I hooked up the 12volt LED straight to the battery and it worked without a resistor.  It would be good for something on the instrument panel or maybe in a cupholder.

[ClydesdaleKevin]

Ah the custom circuit boards used by the Holiday Ramblers back then...I even have them UNDER the dash, with fuses and LED indicators...and they look a lot like your monitor panel board.  In 89 they were using a different monitor panel, with a separate panel for the battery disconnect solenoids and volt meters.

Glad you got it all back up and running!  Sweet!

Kev

[DaveVA78Chieftain]

If the 12 Volt buld did not blow when you hooked it to the battery then it has current limiting resistor incorporated.

Glas to hear it worked out ok

Dave