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Basic Circuit Review

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007tiger

New Member
I am attempting to design a robust circuit board that will operator a pneumatic solenoid.

Here are the basic functions:
1. Monitor battery voltage
2. Read an input from a switch
3. Turn on a pneumatic solenoid to blow air

It has been a long, long time since I have worked with electronics at the component level so this is my first pass at trying to make this device work.

This is a preliminary schematic. What I am looking for is some feedback on the direction I am going for each of the functions.

1. For the battery monitor I want to protect the analog input in case the wrong battery is connected to the board. The diodes (D2, D3) maybe redundant since I tried to put a crow bar circuit in place to protect the whole board from high voltage.

2. For the field switch input I used a relay to keep the PIC isolated from the field device. Diode D6 is in place to help control voltage spikes when the relay is de-energized.

3. I am using a MOSFET, logic level, to turn on the pneumatic solenoid. There is a Diode, D5, in the circuit that is another attempt to protect against voltage spikes when the solenoid is turned off. The MOSFET I have listed, IRL510 is something I already have on hand. R7 is in place to protect the PIC output from the MOSFET.

4. The power supply is from a 12VDC lead acid battery. I attempted to design a protection circuit that will keep from overloading the board if, say a 24vdc battery were connected. I think there are IC's that do this but I haven't looked for them yet.

My main concerns are building a board that can withstand some abuse from voltage spikes or wrong batteries, or maybe even a battery hooked up backwards.

I am not looking for help in sizing anything, I will try my best to work through that once I figure out what the final circuit will look like, just trying to get all the general concepts/components correct first.

Any thoughts or suggestions on alternative concepts or components would be appreciated.
 

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Chippie

Member
Reverse polarity....omit the fuse in the -VE leg.....F1 provides sufficient protection if suitable rated.
Just connect a diode straight across the supply after the fuse...Cathode to +ve...If you apply the battery the wrong way round the diode will blow the fuse....simple and effective..

Everything else looks ok to me.......
 
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jimmythefool

New Member
Just a few thoughts,
If you are not going to use a reset switch, disable Mclr fuse in software, so you won't need the pullup. This will also stop your 5v line being raised to 13.5v when you program via ICSP. If you really want to pull the MCLR high, then use a schottky diode to isolate the VPP from the 5v.

It appears that the caps to Vdd are is series with the 5v supply, rather than parallel across Vdd/Vss.

A pull-down on the gate of the Fet would stop it being briefly turned on when the PIC is powered up (All I/O are default input state).

For a robust circuit in a potentially hostile enviroment, a Transzorb across the 12V would do no harm to clamp unwanted transients.

Use a double-sided board, and use the underside solely for a 0v Ground Plane

Keep the PIC away from the noisy parts of the board, and use short tracks.

The clamping diodes on the analogue pin, may not be necessary, if your series resistor is high enough.

This is just a quick look, I'm sure some of the other guys can add to this.


Regards

Jim
 
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