Voltage divider impadence

[Phorhas]

If I got it right, the impadence of a voltage divider, made of a pair of 10k resistors, is 5k?

oh, and while on the subject, what does mismatchen power supply impadence does to the sound, uf it does something at all, and whay some CRKT use a divider network made of up to 1M resistors?

[bioroids]

Hi!

The total resistance of two series resistors is the sum, so 10k + 10k gives you a 20k input impedance (as seen at the "top" of the two resistors)

I'm not sure what you mean with mismatched, you mean like using a 100k and a 220k resistor for the divider? That's done to set the voltage in the middle of the resistors to an arbitrary value (2 equal resistors get you half the input voltage, different resistors gives you different ratios, according to Ohm's law).

The values used gives you different current consumption (is that the word?), again you calculate it with Ohm's law:

V=I x R   =>  I = V / R

You want the voltage divider to carry more or less 10 times the current the circuit it feeds will sink.

Hope this helps!

Luck

Miguel

[spongebob]

If I got it right, the impadence of a voltage divider, made of a pair of 10k resistors, is 5k?

Yes, but normally there is also a buffer capacitor connected to the voltage divider, so the impedance for AC is much lower than 5K. Without a capacitor, the impedance is determined only by the value of the resistors.

Sometimes voltage dividers that are directly connected to the input of an opamp use large value resistors to maintain a high input impedance (for example 2 Meg || 2 Meg = 1 Meg).

I'm not sure if there is a general effect of a high impedance power supply on the sound, but a high impedance voltage source can only supply a very limited current, the supply voltage will go down if you draw too much current, depending on your circuit this could have all kind of effects.

[Jason Stout]

If I got it right, the impadence of a voltage divider, made of a pair of 10k resistors, is 5k?

oh, and while on the subject, what does mismatchen power supply impadence does to the sound, uf it does something at all, and whay some CRKT use a divider network made of up to 1M resistors?

1. Rod Elliot can help! http://sound.westhost.com/vda.htm
2. Read the whole site! http://sound.westhost.com
3. Voltage sources (be it battery or wall wart) should be seen as LOW impedance so they can provide the current needed without the voltage dropping. In many circuits increasing the power supply's impedance can cause problems, but sometimes it can provide a means to beautiful ugliness. Zvex suggests a 5K pot in series with a 9V as a good starting point for experimentation.

Hope this helps.

[bioroids]

Oh sorry, I misunderstood the question!

I was looking at the voltage divider from another angle!

You can follow Jason and Spongebob replies anyway, they're right

Luck

Miguel

[R.G.]

1. Go read http://geofex.com/circuits/Biasnet.htm if you're after how to pick resistors for voltage divider references.
2. The INPUT impedance for a voltage divider is R1+(R2||driven load) and the OUTPUT impedance for a voltage divider is R1||R2, where R1 is the input resistor, and R2 is the resistor to ground, and "||" means "paralleled with".
3. If you're using the divider to drive a DC load, then eithere design it with the load resistance in mind (i.e., R2||load) or make R1||R2 less than 1/10 the resistance of the load, remembering that R1||R2 loads the voltage source feeding them.
4.When capacitors are involved, you have to broaden the definition to include the capacitors. I don't recommend beginners mess with this in most cases, as the math is typically something they're not ready for. Instead, make the capacitors BIG so that they act like open circuits for DC and short circuits for AC, avoiding the nasty place in the middle where the math matters. You know the place in the middle when the frequency of interest is between 0.1*F and 10*F where F= 1/(2*pi*R*C).

[bioroids]

Hi!

R.G. if you read this, can you tell me if there's a way to purchase you book about PCB layouts? I tried to pm you but somehow I couldn't make it work...

Sorry to hack the thread  for this OT :roll: !

Luck

Miguel

[Satch12879]

*bump*

[Phorhas]

I'll be sure to read it all



thanx... so much to learn...

[petemoore]

I've been using LM386's as voltage dividers.
 They work great.
 V+ of 386 to battery+
 386 V- to battery negative
 ...output of 386 Is 1/2v.

[Phorhas]

I can't get the supply to be an even 1/2V+... it's always assymmetrical

[Jason Stout]

I can't get the supply to be an even 1/2V+... it's always assymmetrical

What's your setup?

[Phorhas]

A basic breadboard... but I think I know what the problem is.

PS, if I hook the 386's +V and -V terminals to the batt's + and - respectivly, and connect the output lug to ground will I get a BP PS (+9V to -9V) ?

[Jason Stout]

PS, if I hook the 386's +V and -V terminals to the batt's + and - respectivly, and connect the output lug to ground will I get a BP PS (+9V to -9V) ?

No, you will get nothing, the output is grounded! All you need to do is make sure you power the chip correctly(get the LM386 datasheet for the correct pinout) and 1/2 V+ will be on the LM386's output pin.

[Phorhas]

Silly me..

Thnx

I guess I wasn't really thing when I wrote that