Electronics > Projects, Designs, and Technical Stuff

Can you use a voltage divider to translate UP?

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ledtester:

--- Quote from: ejeffrey on August 12, 2020, 06:48:17 am ---
--- Quote from: ledtester on August 12, 2020, 04:03:21 am ---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.

--- End quote ---

A variac is not a voltage divider.  It is a transformer.  You can make an inductive voltage divider with two inductors (possibly variable) but it won't boost voltages and as a divider it won't keep the output voltage constant under applied load.  A variac (like all transformers) works because the multiple coils or coil sections are wound on the same core allowing them to exchange energy.

--- End quote ---

Mathematically it operates the same as a resistive voltage divider which is my rationale for calling it a "voltage divider".

It is used to boost an AC voltage, e.g.:

https://www.circuitspecialists.com/blog/variables-of-a-variac/

It doesn't keep the output constant under applied load but neither does a resistive voltage divider.

Zero999:

--- Quote from: ledtester on August 12, 2020, 12:07:51 pm ---
--- Quote from: ejeffrey on August 12, 2020, 06:48:17 am ---
--- Quote from: ledtester on August 12, 2020, 04:03:21 am ---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.

--- End quote ---

A variac is not a voltage divider.  It is a transformer.  You can make an inductive voltage divider with two inductors (possibly variable) but it won't boost voltages and as a divider it won't keep the output voltage constant under applied load.  A variac (like all transformers) works because the multiple coils or coil sections are wound on the same core allowing them to exchange energy.

--- End quote ---

Mathematically it operates the same as a resistive voltage divider which is my rationale for calling it a "voltage divider".

--- End quote ---
Except it doesn't.

alank2:

--- Quote from: T3sl4co1l on August 12, 2020, 09:23:17 am ---Not sure why people are saying no... you can divide towards +V just fine. ???
https://www.seventransistorlabs.com/Calc/ResDiv.html#three
This calculates both as a general case.  If you enter an unreasonable value, you will get a negative resistance somewhere.  Which is still very reasonable: if you had a negative resistor, that's exactly what value you would need.  Negative resistance means you need active gain, rather than passive attenuation (which is all a positive-resistance divider can do).  Negative resistances can be implemented with op-amps, though that probably won't be very fast, which is annoying for a logic translation application, and a traditional level shifter would be better.
--- End quote ---

Interesting; I wondered if it is possible even if it isn't ideal.


--- Quote from: T3sl4co1l on August 12, 2020, 09:23:17 am ---Those "automatic" translators (TXBxxxx) are janky and far less behaved than the 74AHCT125 already.
--- End quote ---

I found a test TXB0104 I had ordered but never tried because like you said, I've read about them being janky, but after the SN74AHCT125 fail I thought why not.  It actually worked well with the same test I tried on the SN74AHCT125.  I would say an AVR output should be plenty strong to push SS, MOSI, and SCK towards the SD card, but until the SD card outputs MISO I suppose the direction there will be undefined.  Hopefully the output of the SD is strong enough for that (that TXB0104 datasheet says it has to be able to output 2mA).  There is an OE pin on it that they show pulled down to ground and then driven with a uC pin, to HiZ the connections during power up and power down, but I'm not sure if that is completely necessary or if it can just be tied high to I think it was VCCa.  I saw a board one of the board makers put together with this IC and they used a solder jumper to do exactly that.


--- Quote from: T3sl4co1l on August 12, 2020, 09:23:17 am ---And yeah, static testing the AHCT was probably oscillating, so the values read were averages of some waveform, meaningless as thresholds.  Check with an oscilloscope.  Try again after installing it on copper clad (dead bug style, etc.) with bypass and stable (resistive) sources and loads.
--- End quote ---

It was, I at first was looking at its output on a LED, but noticed different brightness levels so I connected a scope.  It was oscillating around 27M.  I wasn't surprised that it was oscillating at certain voltages, but I expected them to be within the 0.8V-2V range, not 0.4V-4.4V range.

I was using a DP832 output channel to change the voltage and one thing I noticed even with the TXB0104 is that there was quite a transition phase (around 20ms) or so of noise with the TXB0104 between transitions.  If I simply connected the input to GND or VCC, it was a nice clean switch from low to high or vice versa, but not when I changed it from the power supply.  Perhaps this has something to do with the AHCT range being wider, but I don't see how it wouldn't stabilize eventually.

T3sl4co1l:
Yeah, clip leads on a power supply, or something like that, and a part either just completely loose in the air, or on a breakout board or breadboard with no ground plane, bypass or termination -- those leads aren't connections, they are transmission lines, rather high impedance at that (~200 ohm Zo?).

Such connections make great resonant stubs, with which to amplify the voltage from, say, power or ground pin to input.  Plenty of opportunity for feedback.

Whether it eventually stabilizes, or oscillates over some voltage range and with some amplitude, is impossible to say.  You'd need to measure the setup for the mutual inductances and all that; but it's useless because a mere shake of the leads and the setup is completely different.

Putting resistors at the ends of those wires might help -- trying to terminate those transmission lines, so they at least don't resonate so strongly.  But really, you need a ground plane with supply bypass, so the chip is in its intended environment, signals fed in and out along terminated coax cables, say.

Tim

alank2:
Thanks Tim - I'll play with it some more and see if shorter leads change the way it behaves.  I was using some 3" leads with push pins on the end of them in a breadboard so maybe that is the problem.  I did give it a bypass cap though.

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