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| Help with Resistor divider |
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| hanno00:
For my project I have a 5 Volt output, a ground and two inputs. I want to give 3.3V to one of the inputs at a time. I need to scale the %v down to 3.3 Volt using a resistor divider. The LEDs I use for testing havent broken yet so I think its working, but when I'm trying to measure the voltage with a multimeter, I keep getting 0 V. So my question is: Should this resistor divider technically work? TLDR; Does this scheme work? PS: I know I used outputs for the Inputs but that was just to make it more clear for myself. |
| OM222O:
you should get 2.5V not 3.3V (same value resistors divide by 2) if you're getting 0: 1) check multi meter leads, they might be broken 2) check for shorts to ground |
| hanno00:
--- Quote from: OM222O on March 04, 2019, 01:16:03 pm ---you should get 2.5V not 3.3V (same value resistors divide by 2) if you're getting 0: 1) check multi meter leads, they might be broken 2) check for shorts to ground --- End quote --- Yes sorry, I meant a maximum of 3.3 Volt, but thanks for the help. Really appreciate it! |
| ebastler:
Your voltage divider does give you 2.5V, which the inputs (assuming they are 3.3V CMOS inputs) will interpret as a logic "high". But in the alternate state of the switch in your schematic, the inputs will be open. Depending on your chip, this may result in an undefined state, or may be interpreted as a logic "high" as well. So in this respect your scheme does not work. You want to switch the inputs between 0V (GND) ad 2.5V. Alternatively, connect a pulldown resistor form each input to GND permanently. That resistor's vaule needs to be significantly higher than the resistors in your voltage divider -- a few kOhm. |
| hanno00:
--- Quote from: ebastler on March 04, 2019, 07:49:34 pm ---Your voltage divider does give you 2.5V, which the inputs (assuming they are 3.3V CMOS inputs) will interpret as a logic "high". But in the alternate state of the switch in your schematic, the inputs will be open. Depending on your chip, this may result in an undefined state, or may be interpreted as a logic "high" as well. So in this respect your scheme does not work. You want to switch the inputs between 0V (GND) ad 2.5V. Alternatively, connect a pulldown resistor form each input to GND permanently. That resistor's vaule needs to be significantly higher than the resistors in your voltage divider -- a few kOhm. --- End quote --- So when the inputs aren't connected at all, there isn't necessarily 0V, at least what the chip can detect? |
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