# LM339/LM324 Voltage Controlled Oscillator Circuits

Thread Starter

#### jarek319

Joined Jan 24, 2010
18
I'm looking over the datasheets for the LM339 and LM324 chips and was wondering about their Voltage Controlled Oscillator circuits.

http://www.national.com/ds/LM/LM324.pdf pg. 13
http://www.national.com/ds/LM/LM339.pdf pg. 13

First off, I heard that the LM339 is faster than the LM324, from a previous thread I posted, but that was for a different circuit. Would the speed difference be the same in this case? I am looking to generate 60Hz->10KHz square and triangle waves.

On both datasheets, there is a marker for V+/2. Am I correct in assuming this is the voltage input going through a voltage divider with R1 = R2?

The LM339 mentions that V+ = 30V. Does this mean I can only drive it with 30 Volts? I am looking to make this powered by a 9V battery; which parts would I have to change in order for it to work with a lower voltage?

On the LM324 datasheet, it mentions that the wide voltage range is 0Vdc<=Vc<=2. I understand that bit, but what does the second part mean (V+ -1.5DC)?

#### SgtWookie

Joined Jul 17, 2007
22,221
First off, I heard that the LM339 is faster than the LM324, from a previous thread I posted, but that was for a different circuit. Would the speed difference be the same in this case? I am looking to generate 60Hz->10KHz square and triangle waves.
The LM324 will "run out of gas" at around 7kHz; even at that low speed, your square wave will look closer to a triangle wave.

The LM339 will not have that problem.

On both datasheets, there is a marker for V+/2. Am I correct in assuming this is the voltage input going through a voltage divider with R1 = R2?
Yes.

The LM339 mentions that V+ = 30V. Does this mean I can only drive it with 30 Volts? I am looking to make this powered by a 9V battery; which parts would I have to change in order for it to work with a lower voltage?
Reduce all of the resistances by roughly 2/3; for example a 100K resistor becomes 33k, a 50k resistor becomes 16.7k.
Don't forget your input voltage range will reduce correspondingly as well.

On the LM324 datasheet, it mentions that the wide voltage range is 0Vdc<=Vc<=2. I understand that bit, but what does the second part mean (V+ -1.5DC)?
The LM324 can't "see" within about +V-1.5v.
The actual formula reads:
0Vdc <= Vc <= 2(V+ -1.5DC)
so, if V+ is 9v, that would evaluate as 2(9v-1.5v) = 2(7.5v) = 15v; then:
0Vdc <= Vc <= 15v

Thread Starter

#### jarek319

Joined Jan 24, 2010
18
Wow, thank you very much for the prompt responses! I'll be putting this together right now using the LM339! The 10 ohm resistor going into the capacitor gets changed as well? I'll have trouble finding a 3 ohm resistor that's not a few watts....As for the other awkward value resistors (16.7k, 6.6k, 1.7k) would it be better to round up or down to the next closest common resistor?

#### SgtWookie

Joined Jul 17, 2007
22,221
You can leave the 10-Ohm resistor as is.

Here's a calculator for generating possible combinations of resistors in series/parallel:
http://www.qsl.net/in3otd/parallr.html

You can do some rounding if you'd like. After all, you'd be hard-pressed to find a 50k Ohm resistor to begin with; they should've used 51k or 47k in the schematic.

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