Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Will this burn out the LM3914?

Status
Not open for further replies.

joecool85

Member
There is a fan controller based on the LM3914 LED driver here: https://www.ssguitar.com/index.php?topic=2591.0

**broken link removed**

I'd like to build it, but I'm concerned that the LM3914 would burn up driving a 12v 150ma fan. Am I right in thinking that if you used a 12v fan as in the schematic that the chip would need to handle 1.8w (it's only rated for 1.36w), or is that rating how much heat it can dissipate? In which case, it shouldn't be dissipating more than maybe a half watt or something for it's own purposes as the fan will "eat" whatever voltage the LEDs don't use.
 
Last edited:
The circuit WILL NOT WORK because pin 8 of the LM3914 is not connected to anything (it should connect to ground).
The total resistance from pin 7 to ground is 1k ohms in parallel with the transistors and 680 ohm resistor that causes a max output current of the LM3914 to be about 22mA for each output. Then the total output current is 220mA if there are no LEDs and the fan is a short circuit. But the LEDs and saturation voltage loss of the LM3914 reduce the max voltage to the fan to about 9V then its max current might be about 150mA.

If the fan gets 9V then the outputs of the LM3914 are saturaterd with a low voltage drop so they operate cool.
 
The circuit WILL NOT WORK because pin 8 of the LM3914 is not connected to anything (it should connect to ground).
The total resistance from pin 7 to ground is 1k ohms in parallel with the transistors and 680 ohm resistor that causes a max output current of the LM3914 to be about 22mA for each output. Then the total output current is 220mA if there are no LEDs and the fan is a short circuit. But the LEDs and saturation voltage loss of the LM3914 reduce the max voltage to the fan to about 9V then its max current might be about 150mA.

If the fan gets 9V then the outputs of the LM3914 are saturaterd with a low voltage drop so they operate cool.

Sounds like the only issue is the ground not being shown or something. I know it works because this fellow has built it and used it for several years. Glad to hear that it can power the fan without overheating the chip due to low voltage drop.
 
A 12V DC fan is not going to run worth crap if only 8 or 9V is powering it. It may not run at all if only a few LEDs are on because it will starve the fan for current. DC motors don't work that way, you can't power them from a current restricted source. I don't think this circuit is going to do what you want.
 
Last edited:
A 12V DC fan is not going to run worth crap if only 8 or 9V is powering it. It may not run at all if only a few LEDs are on because it will starve the fan for current. DC motors don't work that way, you can't power them from a current restricted source. I don't think this circuit is going to do what you want.

The PC fan I was playing with ran fine at 9v. It started spinning at just over 4v and started moving substantial air around 6v. By 9v it was pushing pretty good and still quiet. Honestly, anything above 10v was loud enough I didn't care for it, but it did move more air.

As for the circuit and it's current limitations, from what I understand from the fellow that built it, it takes about 4 LEDs during on with the circuit set at 25ma per step before the fan starts up, but once moving it will keep spinning all the way down to 1-2 LEDs being on.

Honestly it doesn't matter too much as I've decided that at least for this project I'm going to go with the LM317 circuit I came up with.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top