Electronics > Beginners
understanding zener and high side switching
hsn93:
hello,
im design circuit for industrial application i want to have water detection sensor and planning to have electrodes (SS) with distance of 20mm or so...
the water is tap water so it would be "around 2K ohm"
so i dont want to put 24v to the electrodes if the water exist for long time as this will give electrolysis the water ... so i want to switch the sensor off and on ..
my circuit is like following i would like to see what you guys think and if someone have an advice i appreciate that
* oh (D2) is 3v3 zener but couldnt find it in LTSPICE
* (Rload) is water exist .
* (Rload) would be open circuit if water doesnt exist
* do you think my high side switching is good? 10K for R1 is good?(why)?
hsn93:
hello,
to make more sense of the subject title and to really understand what im designing here is my understanding (i know i go to details but i like to understand :bullshit:):
when Vpulse (V1) = 0V:
Q1 will be around 5M ohm or something "in worst case"? it has leakage current.
10uA leakage at VCE = 50v -> 5M ohm
so the 10k (R1) will not have a lot of voltage drop (less than 0.7V i guess) "if it had more than 0.7V, D1 will conduct in the loop and shunt it?"
hence, P-MOS VGS < thresh hold = off
when Vpulse = 3.3V:
Q1 will have saturation 1V. (assume)
R2 (5K) will have drop of ~18V "assuming Zener will have 11v drop" (V=IR -> I= 3.6mA)
D1 will have drop of ~11V (is that right?)
R1 (10K) will have (11V also) hence (I=1.1mA)
which means the zener will have 3.6mA - 1.1mA = 2.5mA.
I'm ignoring the dynamic resistance (to simplify)?
now, to my understanding R1 (10K) is defining how fast the P-MOS will switch off.
Gate is 32nC is worst case?
to turn off mosfet it will be as following:
which means to turn off the mosfet it will take around (in worst case):
5*R*C = 5*10K*32nC = 1.6mSec ?
are my assumptions correct?
nick_d:
Yes, your circuit looks like it would work and your calculations are broadly correct, although I didn't understand why you changed 32.2nC from the datasheet to 32nF in your diagram. It's better to use the gate capacitance for this calculation. A figure that I use for rough estimates is 1nF, it will be less for a signal-level MOSFET and more for a power MOSFET. In this particular case, referring to a Rohm datasheet on Digikey:
https://media.digikey.com/pdf/Data%20Sheets/Rohm%20PDFs/RSS060P05.pdf
See the line "Input Capacitance, Ciss, Typ. 2700 pF, VDS = -10V", so your gate capacitance is about 2.7nF, the RC time constant is then 10k * 2.7n = 27us for the turn-off and I guess 3.33k * 2.7n = 9.0us for the turn-on (because 10k in parallel with 5k is 3.33k). Since it will be affected by threshold voltage and drive voltage and all sorts of things, this is only an estimate, but it's certainly good to within an order of magnitude.
By the way, it would be simpler to use a PNP bipolar transistor for the high side drive and a FET to pull the base of the PNP down. This would eliminate most resistors and the zener. Also, I didn't understand the 1N750 section. What are we sensing exactly? Could we use an op-amp for the sensor, or a simple resistive divider to get the sensed voltage down to an appropriate scale so that the 2.5V threshold of the MCU detects it?
cheers, Nick
Zero999:
Why not simply use 3.3V for the water detection electrodes?
If you're that worried about corrosion, then use AC, rather than DC. Make one electrode 0V and send a square-wave to the other one, via a capacitor and high value resistor. Monitor the voltage on the electrode and if the square-wave falls to 0V, that means it's detected water.
spec:
Hi hsn93
You haven't asked this, but I seem to remember that stainless steel as a resistive water level sensor has problems and other materials are more appropriate.
There are a number of ways to detect water level, but the best compromise, in my opinion, is to use insulated terminals and capacitive sensing. If you are interested in this approach we can discuss- the circuitry is well proven, simple, and low cost.
Navigation
[0] Message Index
[#] Next page
Go to full version