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Need to detect under/over voltage condition...

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mdredmond

New Member
Hi folks, this is my first post and I hope it's okay to just jump in with a question. I am a VERY amateur hobbyist and have no background in electronics. I can make way way around the digital stuff but beyond Ohm's law, analog is a mystery to me. That said...

I have a PIC project running on 5v. It is to be connected to an aircraft DC bus that provides either 12 or 24 volts, depending on the plane.

When the alternator is running properly, the voltage should be somewhat higher in both instances - approximately 13.5 and 26 volts, give or take.

I need to detect undervoltage and overvoltage conditions, and preferably make the upper and lower thresholds configurable.

Can someone help me with a straightforward way to do this? I do have an a/d converter on the pic.

Do you think a simple 2-resistor voltage divider and stick the output into an a/d pin on my PIC will do it?


Thanks!

mdr
 
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ericgibbs

Well-Known Member
Most Helpful Member
Hi folks, this is my first post and I hope it's okay to just jump in with a question. I am a VERY amateur hobbyist and have no background in electronics. I can make way way around the digital stuff but beyond Ohm's law, analog is a mystery to me. That said...

I have a PIC project running on 5v. It is to be connected to an aircraft DC bus that provides either 12 or 24 volts, depending on the plane.

When the alternator is running properly, the voltage should be somewhat higher in both instances - approximately 13.5 and 26 volts, give or take.

I need to detect undervoltage and overvoltage conditions, and preferably make the upper and lower thresholds configurable.

Can someone help me with a straightforward way to do this? I do have an a/d converter on the pic.

Do you think a simple 2-resistor voltage divider and stick the output into an a/d pin on my PIC will do it?


Thanks!

mdr

hi,:)
For the 13,5V I would allow a 15V possible max.
Use a resistor divider, 4.7K in top and a 2.2K on the bottom, at 15V the junction will be +4.7V.

For the 26V allow 30Vmax, so its a 2.2K at the bottom and a 12K at the top.
Giving approx 4.64V for a 30V input.
 

MikeMl

Well-Known Member
Most Helpful Member
A couple of suggestions:

The ADC on the PIC is usually configured ratiometric to its own Vdd supply. Its absolute accuracy is only as good as the accuracy of its Vdd. If you divide down the aircraft bus voltage to near 4V, then if the 5V Vdd drifts up or down, this will have a big effect on the reading...

If you are building your device for a certified aircraft, you would have to subject the device to DO163 certification and testing, which would put transient voltages of ~+-100V on the 14V bus. Make sure you "protect" the PIC's analog input pins against such transients using series resistance, a shunt zener?, TVS, and capacitor. Even if this is for an experimental aircraft, you will have to design in some glitch tolerance...

Been there, done than...
 

mdredmond

New Member
Make sure you "protect" the PIC's analog input pins against such transients using series resistance, a shunt zener?, TVS, and capacitor. Even if this is for an experimental aircraft, you will have to design in some glitch tolerance...

Thanks, Mike.

This is for an experimental ac (mine). I don't want to burden you, but can you elaborate on what you said above?

I have a second question...

My PIC is using SPI to talk to a group of 14 devices that draw 50-ish mA each (i.e., 600-800 mA). I know that running 7805s in parallel is fraught with peril, but if I power those devices with one 7805 and the PIC and associated hardware are powered with another 7805, will that cause me issues? The only connections between the two circuits will be SCK, SS, SDI and GND. Can I do that?

I'll offer my apologies again for being no so good at this...
 
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