voltage divider equation

[Andy Le Blanc]

here's a handy equation

good for bias, floating heaters, and screen grids

[tubeswell]

Elevating the heaters that way from the B+, doubles as bleeder resistors (and there should also be a decoupling cap to ground at the 'knee" of the divider - in parallel with R2 - otherwise there will be trouble)

[Andy Le Blanc]

the little equation is the point...

bypassing to decouple and improve stability is always a good thought.

[overtone]

these little tidbits, along with the squiggles, should really be sticky.
A drop at a time is great, eventually they will stick to that blank screen where my brain should be.
Best, tony

[Richie]

here are also some helpfull sites for about everything. With easy calculators.

http://beyondttl.com/calculator-vdiv.php

http://ampbooks.com/home/amplifier-calc ... iled-pair/

http://www.raltron.com/cust/tools/voltage_divider.asp

http://www.bowdenshobbycircuits.info/r2.htm

http://www.allaboutcircuits.com/vol_6/chpt_3/4.html

[vibratoking]

Sorry if I sound smug, but isn't the voltage divider eqation already firmly entrenched in all of our heads? It is one of the most basic equations in electronics.

[Andy Le Blanc]

reality is usually the opposite of what you assume

Especially for DIY, kit makers and cloners...there's alway's something missing

I went to school for biology, and worked some in the fisheries
did some time in early childhood development, and a little in retail
but now I hold a gig as a sound tech doing celtic trad, and as parent I
don't have the time to remember what day of the week my kid has Kung Fu
let alone any thing else sometimes....

so its good to post things like this so I have chance to look it up when I forget...

[overtone]

Once I got an electricity lesson.
I was eight or nine years old.
My grandfather clipped some wires to me in his kitchen and hit me with a load of electricity.

After laughing like a madman, he taught me what voltage, current and resistance was.
He explained Ohm's law and I never forgot that lesson.

We proceeded to wind coils, build solenoids, telephones and things like that.
Grandfathers are great for that kind of thing.

But everyday life takes over…
grandfathers pass on…
and we never got to the voltage divider stage...

Somehow seeing that little hand drawn sketch in the OP was like being in the kitchen again and for the first time the voltage divider equation clicked.

[vibratoking]

That's a great story. Grandpa's are good for lot's of stuff for sure. Mine never taught me Ohm's law. The voltage divider equation is just an applicatiohn of Ohm's law (V=IR). If you only know Ohms law, you can figure it out as follows.

First, there is only one current. Call it I.
Vi-IR1 = Vo and Vo-IR2 = 0. So,
I = (Vi-Vo)/R1 and I = V0/R2. Combining gives,
(Vi-Vo)/R1 = V0/R2 which can be solved through some easy algebra:

Vo = Vi*(R2/(R1_R2))

You can memorize the formula, but it is so much more interesting and useful to be able to solve it yourself so that you understand where it comes from, what it means, and how to figure it out if memory fails. Give a man a fish or teach him how to catch his own...

[Super_Reverb]

here's a handy equation good for bias, floating heaters, and screen grids

This is accurate if there is no load on Vout, as a load represents an effective impedance to ground or maybe some other potential.

One rule of thumb is to set the series current in the divider to 10X the estimated load current. For example, I am biasing a device input to midsupply and the device draws 10uA input current: setup the divider impedance to allow 10*10uA = 100uA (more if you need more precision: can also tweek the ratio)

Voltage dividers such as this work with reactive elements as well, substituting (2*pi*f*C) or 1/(2*pi*f*L) for capacitive or inductive reactances, respectively.

This is basic network analysis without the LaPlace transforms.

Cheers,

rob