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Help with schematic

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dmc0162

New Member
I am trying to build this schematic for a project. The only thing that has me confused is the "+V LED" at the bottom of the array of LEDs. How much voltage do I need to supply here? The rest of the circuit goes off 12v, but this voltage isn't labeled. Thanks for your help! (this is a circuit for an LED tachometer)

 
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davenn

Active Member
The examples in the datasheet for the chip indicate VLED = Supply

the why's and wherefores are not overly clear

Dave
 

Diver300

Well-Known Member
Most Helpful Member
The LM3914 output must be more than about 0.6 V with the LEDs on so that it can sink the correct current. To that you need to add the LED voltage, which will be about 2 V for red LEDs.

So your minimum VLED supply voltage is 2.6 V.

The maximum is controlled by the heat generated by the LM3914. I think that the tachometer runs in bar mode. That is to say that the lower LEDs are lit when ever the engine speed is above their level. The alternative is dot mode when only one LED is ever lit. That means that all the outputs could be on at the same time.

The current is controlled to around 12 mA per LED. If all 10 are on, that is 120 mA. If the supply is 12 V, and the LED voltage drop is 2 V, that leaves 10 V across the LM3914, so the heat generated is 1200 mW. That is less than the maximum of 1365 mW.

However, I wouldn't run a chip that close to its limits. If you reduce the supply voltage to the LEDs the LM3914 will run much cooler. The simplest solution is to put a few diodes, maybe 8, in series, and put the LED supply through that. The diodes will each drop about 0.7 V, so that will reduce the supply by between 5 and 6 V, which will about halve the heat generated in the LM3914
 
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