Diode drops to regular voltage

[wannaBinventor]

Opps: Title should say "regulate" instead of "regular."

Say I have a 5v power supply and I want to run everything off the 5v supply except for an IC that puts out readings in the form of analog voltages that needs max 3.3 volts. If I didn't have another regulator IC or a zener to clamp the voltage to 3.3v, could I just create the voltage by putting a few diodes in series to create the required drop in voltage?

What issues do you guys see with this, assuming the IC's power draw didn't exceed the max amperage of the diodes?

 

[SABorn]

A practice thats often used, just check your voltage with a meter as not all diodes give the expected 0.6 volt drop.

Pete

 

[ericgibbs]

hi wb,
Another tip is to add a lowish value resistor across the output of the diode chain and 0V [common]
A resistor of approx 220R/270R will sink about 20mA, this should help hold down the 5Vout.

Simple way would be a RED led and a 150R across the 5V

EDIT:
This should read as 3.3V not 5V, so adjust the 150R as required for 20mA

 

[MikeMl]

Not if the 3.3V chip needs a "regulated" voltage, which it uses for, say, a reference voltage for a A/D converter (a la PIC). The diode drops are not very precise, and change a lot with the current being drawn.

 

[ericgibbs]

Not if the 3.3V chip needs a "regulated" voltage, which it uses for, say, a reference voltage for a A/D converter (a la PIC). The diode drops are not very precise, and change a lot with the current being drawn.

It would be interesting see the voltage error plot with a fixed 180R load across I1 ie: Vout to 0V.

 

[MrAl]

Hello there, Another point to keep in mind is that the diode voltage changes with temperature a bit more than we usually want and when we put them in series this voltage change is multiplied by the number of diodes. A typical Si diode will change by about -2.2mv per degree C, so for two diodes the voltage will change by about -4.4mv/degC and for three diodes -6.6mv/degC. As others have pointed out already too the load changes the voltage drop somewhat, which can be quite a problem. You can include a load resistor to keep the load current more constant, but that wastes power if this is battery operated. In short, using diodes to regulate voltage is one of the worst ways to create a regulated voltage so almost anything else works better (except of course a series resistor) unless the voltage can be allowed to change a lot and not bother the application. There are very cheap linear regulators out there with fixed outputs of 3.3v which would probably do nicely.