My first prototype (Looking for opinions)

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rmtucker

Member
This is the next iteration of my circuit.
The power supply is pretty much a direct copy of the megasquirt power supply shown elsewhere on the forum.

I just thought this may be a good starting point as it is well proven.

The circuit is basically a fusebox/wiring/controller for my Trike.
All of the ampages for the 9 outputs will be preset in the software from 1 to 30amps.
I have already Built a 1 off test circuit for the outputs and it seems to work well interfaced to the picaxe.

This is my first full blown attempt at the circuit,so feel free to critisize.
 

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premelec

premelec

Senior Member
Increase power input impedance

You have several protective measures for the input DC put nothing that I see that limits the
current from the 13.5 VDC which magically becomes 12 VDC on the diagram - there should be a fuse or resistor in there in series to limit the current into your protective circuits... otherwise smoke may result when the input voltage surges... I haven't checked further down the line as my brain burned out at the input... :)
 
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rmtucker

Member
Ooops
I have altered the 13.5v,it should have been 12v.
I have updated the pdf.
So i should have a fuse at the end of the 5v on the power supply?
Forgive my ignorance but this is my first attempt.
 
premelec

premelec

Senior Member
Well... I'd limit it at the 12volt end and put in a resistor in series to the regulator - Perhaps a circuit breaker that will take the full 12V current on the input and then a series resistor to the regulator circuits - say you are running 200ma to the regulator and can 'afford' to drop 4 volts before the regulation and transient suppression circuits then put 20 ohms in series which drops 4 volts in the resistor at maximum current [.2x20=4] and also dissipates some power [.8 watts] that the regulator otherwise would have to turn into heat... The resistor is cheap and takes some of the strain... The circuit breaker prevents fires when something shorts out...:)

Looking further at your circuit and noting you've called out as much as 30 amps on a branch circuit - you need fuses or PTCs on each branch output so one short doesn't kill every output - it's a pain but that's why we have fuse blocks and it make trouble shooting when there is a short so much easier... I made a circuit breaker wires soldered to an open fuse which I'd put in the defective circuit for trouble shooting rather than blowing one fuse after another - it worked very well when I was looking for a tail light short...
 
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MartinM57

Moderator
If your switches/alarms are between the PICAXE inputs and ground (as is the normal situation in vehicles) you will need pullup resistors between the inputs and Vcc, otherwise your inputs will be floating (and read as who-knows-what logic value) and Bad Things (tm) will happen.

I would also put 100n capacitors from PICAXE inputs to Ground to help (and probably completely) avoid switch bounce.
 
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rmtucker

Member
Martin

The resistors shown on the inputs are pullups,it is just a sil resistor so it is not clearly shown.
They do look on the schematic to be in line.
 
R

rmtucker

Member
Martin

I should have enough room on the board to fit the caps,Can you recommend a type of capacitor for this purpose.

Premelec
I have thought over the fuse situation and tried to collect as much information as possible.
My problem here is what exactly is the fuse protecting.
I thought this was the job of the smartfet?
If i were fitting a fuse i suppose smartfets would not be needed (Just Fets)
Please accept my apologies in advance for my numb thoughts but i really know very little on these matters and rely on information collected from the web.
 
premelec

premelec

Senior Member
Don't know...

I've not looked at the FET controllers - the issue is how they fail when they do fail - many solid state devices fail shorted in some way [gate punch through, diode short etc] but also some just fuse open [overheated zeners & such]... So read the data sheets and perhaps only protect the overall high current path... it's always a toss up as to causing more problems whilst trying to prevent them... I look toward easy trouble tracing when setting up or future so at least a terminal board to isolate areas without having to cut and splice wires... You'll have the answers when you build it :)
 
hippy

hippy

Ex-Staff (retired)
rmtucker said:
I have thought over the fuse situation and tried to collect as much information as possible.
My problem here is what exactly is the fuse protecting.
I thought this was the job of the smartfet?
Click to expand...
I have a feeling of deja vu here; has this not already been discussed ? Perhaps it was a different project.

The smartfet protects itself / the system / load from load failure, a fuse protects the system from smartfet and other circuit failure.

For example, the smarftets won't help one iota if the power supply regulator decides to become a dead short across the supply.
 
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rmtucker

Member
Hippy

Do you mean add a fuse to the 5v regulated output?.
I have just gone back and checked and i have missed the polyfuse in the original schematic.
 
hippy

hippy

Ex-Staff (retired)
No, because a fuse of the 5V regulated output won't protect the vehicle supply from regulator failure. If the fuse is on the regulator input that will be okay.

On fusing individual smartfets it's more difficult to say; are smartfets infallible, would a fuse help if one does fail ? I can't make the judgement on that but if an extra fuse doesn't interfere with operation, could save the day if a smartfet does fail, it's an extra level of safety at low cost.

Besides, you're not just fitting the fuse to protect the smartfet; what happens if a power cable to a smartfet gets worn or damaged and shorts against the chassis ? You'll need something to protect from that though a fuse would be placed closer to the battery in that particular case so wouldn't be part of the circuit here.
 
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rmtucker

Member
Not sure what current it will draw until i build it.?
Do you think if i put a std fuse holder on the board and then put in the correct fuse when testing.?
Just i will have to try and squeeze it onto the board before i cut it on the cnc.
I can't cut the board anyway till the lcd comes from tech-supplies.
 
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