[JahJahBinks]

This is for my school project. I have designed a digital circuit for my team and due to safety concern I would like to make sure it does not fail on the exhibition day. I think the three main reason for failure are 1) plugged in Vdd for GND and GND for Vdd. 2) reverse current 3) over-voltage, 4) over-current.

The first case can be prevented by checking the connection before turning the power supply on. The second case can be prevented by using Zener Diodes. However for the latter two cases I have to trust the power supply. If 20 something IC chips on my circuit all short out at once, or even one stops working then I will be really screwed.

So to prevent over-voltage I decided to over a voltage regulator (such as LM7805). My questions are: 1) Hooking input and output pins to capacitors can reduce the noise. What's the formula for calculating the cutoff frequency? It behaves like a low pass filter, right? 2) Extra voltage is transformed into heat, but if I use a 5v voltage regulator and it receives 5v from power supply over a long period of time, do I need to attach a heat sink to it?

If you have better idea in prevent over-voltage, please tell me. As for over-current, I am not sure if that's a major problem or not, I need your two cents. Thank you!

 

[OtterMarc]

JahJahBinks,
I'm no expert on formulas for filters. Maybe someone else will get a moment to post them. It still takes me hours to figure em out.

sound dot westhost dot com (Elliot Sound) has some great resources on power supply circuits and also using a fixed voltage regulator to control a power transistor (to get more current if needed). I think I've also seen some circuits there with current limiting features.
You'll get a voltage drop over your 7805, so input volts will need to be at least 1.2 to 1.5 volts higher than 5v.
Most of us will eventually make mistakes with polarity. One way I avoid it with a "valuable" circuit is to wire the power input into a diode and buzzer such that it will buzz on reverse polarity only. I use a second switch downstream of this level. No buzz means ok to connect second switch. The buzzer can be replaced by a diode (for higher reverse voltages), resistor - LED combo. You can wire one normal with a green LED, and one backwards with a red LED. Using such a circuit means you remember to plug in with switches off, or it may buzz as your ic's make smoke.

Marc