Measuring Vxx with zero load Divider

For people who want to save energy – a voltage divider which does not load the Vxx (V1).
Vxx could be any reasonable voltage, not only 5V as depicted above.
The Voltage divider does not load the Vxx, only during a measurement.
The resistors in the Divider could be of any value you need to fit into ADC range.
Nice to have when you talk uA power consumption and 1-10k source impedance for fast and reliable ADC
Provided as-is, no warranties of any kind are provided, use at your own risk
The sketch is easy to create, left as an exercise for the readers

Vxx Trick.JPG (15.99 KiB) Viewed 1256 times

That is a neat trick. I wonder how high a voltage on the V1 input you could get away with. Switchable attenuators for the pig-o-scope are one use case that comes to mind.

Guys

I just tried to model this in LTSpice but I get strange results

I thought I’d turn on the control signal for 100 microseconds, (actually I used a square wave of period 200 microseconds), but the waveform I get on the middle of the divider is a series of increasing voltage pulses

Maximum voltage I get on the middle of the divider (for 5V input ) is 600mV

I’ve attached my LTSpice file (perhaps I made a mistake)

It works such you toggle the Pinx (output) from 1 to 0.
That opens the Q1 for
time t = aprox R3 * C1.
During that time you have to provide ADC. As it takes say 1us for STM32, you may try with
R3 = 10k C1 = 4n7-10n
to get t = say 100usecs. You may try even smaller t. The goal is to get the smallest t while the measured ADCin is still stable/precise.

After the measurement you do nothing, the C1 discharges, Q1 will be closed and Vxx and the Divider (say 9k/1k) will be switched off from MCU. No current drained.
Good if you want to measure Vxx = 3V on a CRxxxx coin battery from time to time.. Or something like that.

PS: mind the t changes with Vxx a little bit. You need a logic Q1 pmosfet with lowest level threshold, ie. 1.5V.

Below simulation: it took 1usecs to get the ADC Vin stable. And it lasts for 30usecs stable. You do ADC there. R4 and C2 are parasitic for simulation only.
I do not have such low threshold logic level Pmosfet model for simulation, so I used power mosfet instead (not good, it has high threshold voltage and big gate capacitance).

Zero power DIVIDER.JPG (66.72 KiB) Viewed 1235 times

For those not wanting to complicate things too much, the online Falstad Circuit Simulator is easy to use.

CircuitSimulator.jpg (46.02 KiB) Viewed 1217 times

@mrburnette: Your C1=10uF, shall be 10nF.. Try my second example with Vxx=30V and 9k/1k divider (R3=10k, C1=10nF) if you get similar results as I got.
Mind there is a current via capacitor C1 as well (during charging/discharging), not only via the divider.

I tried the circuit again in LTSpice, but it still didn’t give the same results.

I get a big volt drop across the FET , and I don’t know why.

But I seem to have issues with FETs in LTSpice, which I need to fix sometime.

@Roger: Works fine your simulation
With some fixes, of course
OMG that graphics, icons, like a children’s toy
Red – Gate, Blue – mid of the Divider, Green – Pinx pulse

Roger LTspice Zero Power DIV.JPG (80.51 KiB) Viewed 1194 times

Thanks Pito

I will try it when I get home

Pito can you zip and post your simulator (asc) file

Edit.

No need.

What the problem seemed to be is that the FET needed to have its type set, ie so it modelled a real device, rather than a theoretical “perfect” FET.
I’m not sure why this is, as normal bipolar transistors in LTSpice are OK using the theoretical model most of the time (they seem to be a good close match to real world bipolars)

P.S.

@Pito, which simulator do you normally use ?
I thought LTSpice had a good reputation for accuracy and flexibility, with its main drawback being its very clunky user interface

The bipolars have got much smaller spread of params so all work fine basically.
For Spice circuit simulations (not for the pcb design) I can recommend the “Circuit Maker 2000” (the old one, before Altium took over). The only 2 issues with CM 2k – it is EOL since 2k, and the latest part types are missing (but hundreds still in). But you can add them, afaik.
PS: there is a new edition – CM – 2015 version free, from Altium, you may try. DaveJ did a teardown

Pito wrote:The bipolars have got much smaller spread of params so all work fine basically.
For Spice circuit simulations (not for the pcb design) I can recommend the “Circuit Maker 2000” (the old one, before Altium took over). The only 2 issues with CM 2k – it is EOL since 2k, and the latest part types are missing (but hundreds still in). But you can add them, afaik.
PS: there is a new edition – CM – 2015 version free, from Altium, you may try. DaveJ did a teardown

Btw, Spice simulator could be run on the stm32 with EXRAM too – ie BlueZEX. Late 80ties I ran it on AtariST (with 1MB ram). As far as I remember, the binary itself was about 350kB. Command line, no graphics, standard Spice 2G5 outputs. We may try

FYI – I’ve added pmos BSS223PW transistor’s model into my CM2k and the stuff simulates when driven from Pinx’s pulse ampl 1.5-3.3V fine.