Electronics > Projects, Designs, and Technical Stuff
Can you use a voltage divider to translate UP?
alank2:
I've got a project where I am translating 5V to 3.3V for SPI. MOSI/SS/CLK could use a voltage divider to tap a 3.3V signal from a 5V signal. Can this also work in the opposite direction?
Quick aside: I thought about using a SN74AHCT125 - but got some odd results from it. I'm feeding it 5V and have a 104 cap on it. If I start at 5V and go down in 0.1V steps:
1.6V it becomes unstable
0.4 goes low!
Then back up in 0.1V steps:
1.6V it becomes unstable
4.4V goes high!
I tried to different gates on it with the same results. Should it for sure be HIGH at 2V or above? LOW at 0.8V or lower?
Back to a voltage divider question. If a SD card is 3.3V and I want to make sure it can set the AVR input pin high, the AVR says 5 *0.6 = 3V which is a little close. Can I bias it by using a voltage divider so that instead of being 0-3.3V is is more like 0.9-4.2V by adding 0.9V to it? A divider with 5V - 10K - MISO - 2K - GND. Would that be a problem for MISO, it being raised up.
Peabody:
I'm not sure what's happening with your 74AHCT125, but it may be that you're not meeting the transition time requirements - i.e. changing the inputs too slowly.
I don't think the divider will work to bring the voltage up. The SD card output is pretty low impedance, so to have any effect on the voltage, you would need to insert a series resistor in the MISO line ahead of your divider. But that just complicates things for both high and low.
All of the microSD modules you find on Ebay and elsewhere use a translator to drop 5V to 3.3V for MOSI, SCK and CS, but they send MISO back to the processor at 3.3V, which seems to work fine even for 5V processors. I'd suggest you try that first to see if it works for you.
alank2:
I'm going to try using a TXB0104D and see how it does.
OM222O:
There's a reason it's called a divider, not a transformer or amplifier: you CAN'T get higher voltages out of it. Only lower. If you want bi directional level shifting then you're better off with a dedicated IC. If you need only one way level shifting then you can use an inverter (not gate) connected the desired output level and invert the signal twice.
In a more general case: you must use an op amp to increase the voltage.
ledtester:
Simply addressing the subject of this thread...
A variac is an example of a "voltage divider" which can boost the voltage. However it does so with AC power. The variable resistor in a conventional DC voltage divider is replaced with a variable inductor in the variac.
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