Best method for small ref voltage?

Discussion in 'General Electronics Chat' started by Porkchop, Feb 22, 2007.

  1. Porkchop

    Thread Starter Member

    Feb 5, 2007

    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 :rolleyes: lol

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

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    What comparator are you trying to use, and why does it need that value os reference voltage?
  3. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    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.