# Best method for small ref voltage?

#### Porkchop

Joined Feb 5, 2007
17
Hi

I have designed a fan controller which is op amp driven

It uses an non inverting amp set up which is sensing voltage across a .01R So every 1A of load equates to 10mV across the current sence resistor.

The O/P of this is then fed in to a comparator with positive feed back.

Which switches a transistor to start the fan.

Question is i need a 1.7V reference voltage for my comparator.

Would a voltage divider be ok to use for this? the loading affect on the divider must be very small?

Is there an easy way to select the resistors. Or do i just select say R2 and experiment with R1 in the formula till i fine a good value?

Must be a better way lol

Is it best to use high values like 100K plus or keep it 10's of K's ?

Cheers

#### beenthere

Joined Apr 20, 2004
15,819
What comparator are you trying to use, and why does it need that value os reference voltage?

#### nomurphy

Joined Aug 8, 2005
567
Given that you need 1.7V, a good way is to use a voltage divider made up of a series resistor from the supply feeding some number of series diodes (depending on type and voltage drop) to ground. The resistor/diode junction is the input to a non-inverting op amp with gain set accordingly. (For higher reference voltages, one could use a single zener diode or a specific value reference diode.)

This design will keep your reference independent of supply voltage variations, which will not be the case with a simple resistor divider.

For example: if you used one "standard" diode with a 0.7V drop, and multplied by a gain of 2.43, you will get +1.7V from the op amp. Two diodes, let's say two schottkys in series at 0.5V each, would give you 1.0V for which you would need a gain of 1.7.

As a rule of thumb, set the diode resistor value for ~1mA through the diodes, but in general it's not very critical.

The diode resistor would be: X ohms = (Vcc - Vdt) / 1mA
where Vcc is the supply voltage, and Vdt is the total voltage drop of the diode(s). Use a 10-20 turn trim pot on the op amp resistors to adjust the gain and thus set the output for the needed reference voltage.

This type design is more flexible than using a 1.7V reference diode, if you can find one, but has more components and needs a +/- supply for the op amp.

BTW -- if you needed a negative reference voltage, just feed the non-inverting output into an inverting op amp using a dual version such as the TL082.