schottky diodes/battery back-up increasing voltage

[cyrusthevirus]

schottky diodes/battery back-up increasing voltage WHY?

I have two schottky diodes. One coming from the 3.3V power supply. The other coming from the 3.3V battery backup. When I measure the output voltage of the circuit, I expected the voltage to be 3V, but instead it is 4V. What happened? How do I fix it? The diodes I am using are 1N5817. The are wired similar to the circuit below, except I am not using an LM7833.

 

[#12]

What is the voltage before the diodes?

 

[4pyros]

Do you have a load on the output?

 

[cyrusthevirus]

What is the voltage before the diodes?

The voltage is 3.3V regulated.

 

[cyrusthevirus]

Do you have a load on the output?

Just the PIC24F. The only change I made to the system was to add the battery backup components. Before I added them the 3.3v went directly to the pic.

 

[#12]

You should keep those diodes carefully protected. They are the only ones I have ever heard of that generate voltage.

 

[Pommie]

Can you measure the battery voltage?

Mike.

 

[cyrusthevirus]

I thought it was wierd so I double checked my circuit and found the problem. To detect the need to switch over to battery and adjust my program, I am monitoring the original voltage which is running at 5V. The pins on the PIC24F are supposed to be able to handle 5.5v, but they are backfeeding into the pic. I am going to adjust and measure the 3.3v voltage instead. That should clear up my problem depending on how much the diodes leak.

 

[blueroomelectronics]

It's the diode clamps on most I/O pins.

 

[Pommie]

You could use a resistor divider to monitor the 5V supply.

Mike.