The circuit to be fed is a simple attiny + RF module (5v; 20 - 100mA required). I was thinking that the next circuit would be suffice to feed and protect the attiny. The components in the schematic are the ones I have available. The question is if would be enough portection and if the order of components from left to right is optimal (?).
1 Automotive power. From 11.5v to 14.8v.
2 Varistor. 18vDC (High peak voltage protection)
3 Diode. (Reverse protection)
4 Zenner diode 18v rated
5 Electrolytic capacitor
7 Voltage Regulator (Vout =5v ; Max Vin = 25v)
6&8 Ceramic capacitors (values specified by V. Regulator manufacturer )
9 Electrolytic capacitor
The regulator (with the required/recommended capacitors) is probably enough.
There is enough current capability in an automotive system to fry your varistor & Zener if the voltage goes high enough to "turn them on". Typically if you are using that kind of "crowbar" protection you need to add a fuse so the fuse blows in an over-voltage situation.
Okay... If you use the 7805 regulator then you will be safe with just the capacitors (5,6,8,9) and the diode (3). You could also add a fast blowing pcb fuse rated a few milliamps more than your maximum current drawn. The variable resistor and the zener should be skipped.
The 7805 regulator works fine with input voltages from 9 to 18 Volt DC if i remember correctly so you definitely are well within its working range..
Keep in mind though that the maximum current output for an 7805 regulator without heat sink is 500ma. So drawing 100ma, you should be safe without heatsink.
I use the PSU circuit i describe above for all my microcontroller projects and never had problems.
There is a long and complex SAE standard on automotive electronics. You won't be able to buy a copy of it unless you are a member of the SAE. There are no free copies online.
But you can find lots of people talking about the standard online. You will learn phrases like "load dump" which is a lot more violent than it sounds.
There are a large number of electrical events which can attack the power supply you are trying to build. They can be categorized by the voltage, amperage and duration of the fault current. It all depends on the energy. A static electricity zap can be thousands of volts but it carries very little power. Hooking up the jumper leads backwards can give your poor little box enough power to make it glow red hot.
The Zener is basically useless. Either it does nothing or it explodes just slowly enough to let enough damaging current flow past it to cook everything else.
The circuit to be fed is a simple attiny + RF module (5v; 20 - 100mA required). I was thinking that the next circuit would be suffice to feed and protect the attiny. The components in the schematic are the ones I have available. The question is if would be enough portection and if the order of components from left to right is optimal (?).
1 Automotive power. From 11.5v to 14.8v.
2 Varistor. 18vDC (High peak voltage protection)
3 Diode. (Reverse protection)
4 Zenner diode 18v rated
5 Electrolytic capacitor
7 Voltage Regulator (Vout =5v ; Max Vin = 25v)
6&8 Ceramic capacitors (values specified by V. Regulator manufacturer )
9 Electrolytic capacitor
Why not use an automotive USB phone charger? (A quality one I mean, not a rubbish one) - all the
problems are solved for you.
MarkT:
Why not use an automotive USB phone charger? (A quality one I mean, not a rubbish one) - all the
problems are solved for you.
I've been using an arduino(speedometer gps+oled display) with cheap usb £1 adapter for a long time without an issue in my motorbike.
But in this case I need something really small (1206 smd components).
What intrigues me is how simple Led bulbs (Rear+braking) can survive that long with only resistors, and an attiny can't survive with additional safety components.
MorganS:
The Zener is basically useless. Either it does nothing or it explodes just slowly enough to let enough damaging current flow past it to cook everything else.
Thanks, I already have the components so I'll mount them, but if implies further damage I'll miss the zener one.
Resistors are surprisingly effective as protection components. A 1K on an LED simply can't carry a lot of current without a big voltage drop, which brings the voltage down to a survivable level on the protected side. A 15KV static electricity spark will dissipate in nanoseconds and the resistor will survive 15KV for that short time. (So long as it doesn't arc around the outside of the resistor. Then you use 2 in series.)
