Voltage regulating pulses!

Thread Starter

alistair

Joined Mar 18, 2010
10
SgtWookie,
the resistance of the wire i intend to use varies from 0.4 to 0.7 Ohms, and i have no idea what the output impedance of the 12v pulse signal is.

I would have to get the specs for this type of flow meter, but another similar one was around 450 pulses per litre and expected flow is between 9 and 15 litres per minute giving pulse range of 4050 - 6750 pulses per minute.

Looking at your oscilloscope, am i right in saying both your idea and that of rjenkins give similar results. But would i also be right in saying that if the 12vdc wavered, prehaps a spike from starting the tractor, your design would give more protection?
 

SgtWookie

Joined Jul 17, 2007
22,230
Having the specifications of the flow meter would make selection of an appropriate circuit much easier.

We also do not know what the capacitance of the input circuit of your Arduino will be, nor the impedance (inductance) of the wire between your flow meter and the divider. If the signals are at a relatively low speed, this may not be an issue. At moderate frequencies, it will become an issue. Plain wire has an inductance of about 15nH per 10mm, so 6.5 feet is about 3uH. However, I still do not have a good idea about the pulse width of the input signal.

I arbitrarily used a 1kHz input signal. I see now that I need to step that up to around 10kHz, but I don't know what the width of the pulse is.

At the moment, both give similar results, but the input capacitance of your Arduino circuit is not being considered. I do not have enough information yet to create a reasonably accurate simulation.
 

SgtWookie

Joined Jul 17, 2007
22,230
Here is the simulation repeated, with Rwire set to 1 Ohm, and 3uH inductance, and input changed to 10kHz, 10uS pulse.

Input capacitance for the simulated Arduino load set to 100pF (likely much higher than it actually is, but better to go for exaggeration).

Note that the 7805 circuit is starting to show what might be interpreted by the Arduino as a double pulse.
 

Attachments

Thread Starter

alistair

Joined Mar 18, 2010
10
The more data we put in is just confirming that your design seems to handle the pulse best and that the regulator design is a no no, if i'm reading this right.

I can see if i can get some specs tomorrow on the duration and frequency of the pulse.

If those figures then worked in your simulation, in your opinion would that make the design a goer?
 

SgtWookie

Joined Jul 17, 2007
22,230
Well, if you really want to make sure that your Arduino has more than decent protection, it would be a good idea to add a resistor between the Arduino input and the junction of the diodes (Resistor/Zener/diode version). That way, if there is some kind of fault somewhere, the Arduino ESD protection diodes might save it from destruction. The output from the 1k/Zener/1N4004 looks pretty good, but if the Zener and the 1N4004 fail, the Arduino is toast. Of course if that happened, you probably had a direct lightning strike, and have more problems than a fried Arduino.

That's what is so difficult about projects like these via the Internet; getting all of the relevant information so that a reasonably accurate simulation can be constructed.

As I suggested before, the 7805 circuit will likely cause you problems.

The all-resistor version is arguably the most safe and inexpensive.

The resistor/Zener/diode version arguably demonstrates the best performance.

But, this is all in simulation. Your actual mileage may vary - even considerably. Caveat Emptor. Simulation is not a replacement for actual testing; it is only as good as the accumulation of the errors in the simulation model.
 
Top