Adding protection to circuit board

Hi, I just designed my first schematic and PCB in eagle, I want to add reverse polarity protection and protection from shorts or overloads.

I've been reading about diodes and it sounds like the 1n4007 will be ok, is this a good choice and will it work for both?

The board will have 13.8-24v DC going into it (ignore the 12v tag), there are 4 voltage regulators, 3 L7805's and 1 L7812. The board will be powering small dosing pumps, relays, 7" screen and other low current devices. 4 regulators is overkill but I want them running as cool as possible. The absolute max current that could go into the PCB is about 2-2.5amps but never for one device. The highest load on a regulator won't be more than 700ma.

So where would be the best place to put these diodes?

If you choose to use a series diode to prevent reverse polarity, you will need a diode rated at say 5A. The 1N400x series is a 1A diode.

A diode is not usually used for over current protection although they can blow on excess current above their rating.

Weedpharma

ok what would be the best way to protect the circuit from only reverse polarity? There will be a main power coming in which could be hooked up backwards and each output after each regulator could be hooked up backwards as well.

If your power supply has sufficient voltage overhead to be able to stand the voltage drop across a diode, this is the quickest simplest way. Put a diode that can carry 5A (safety margin) in series with the supply.

Weedpharma

ok where would I put it? If I put it where the power comes in I can see it protecting that but would it also protect the regulators if the output of them were reversed?

I'm thinking not and I would need a diode for each regulator on the out path?

If so I should be able to use the 1amp diode as nothing higher than that will come out of the regulator. If I right up to here I would then add the 5amp diode to the main power coming in? So a total of 5 diodes for complete protection?

The supply power is where the protection is required normally. What do you mean by “output of them were reversed”?

A diode on the output of the regulator would reduce the voltage to the load and would not protect the regulator from reverse voltage at the input.

If the load is plugged in in reverse, no diode on the board is going to prevent damage.

Weedpharma

ok, what I meant by the output reverse is because there will be connectors going to the output of the 5v's that the regulator puts out so when someone connects something to that 5v it could be backwards.

I did forget about the .7 power drop so yeah that wouldn't work as it needs to be 5v. Is there any way to protect those?

Then you need polarised connectors that cannot be put in wrong.

Weedpharma

Yes but its kinda a DIY project so others will be doing it. Is there some kind of fuse like a diode that would work?

At your maximum input voltage of 24 V and a load of 700 mA on one regulator it will be dissipating over 13 W. I hope you have provided for some pretty big heatsinks.

Russell.

Typically there will only be 13.8v going in but thought it would be ok with up to 24v. I'll change it to 18v.

The one that has the highest load will power a 5" or 7" touch screen, mega2560 and an RTC chip. One will be for 8ch relays, one will be for another mega and other very low power devices and the 12v will be for small dosing pumps. Only one pump will be powered at a time and they draw only 200mah or something and will run for no longer than 10-20 seconds.

Think they will be ok?

robsworld78:
Yes but its kinda a DIY project so others will be doing it. Is there some kind of fuse like a diode that would work?

You cannot put anything in series with the regulator output to stop reverse connection of the load.

As far as protection goes, the regulators have their own protection and will shut down on excess current or heat.

There is however no way to protect the add on device except to put the protection in the add on device.

Weedpharma

It sounds as if your 700 mA was an over-estimate. You need to try to calculate the maximum average power dissipated in each regulator at the maximum input voltage and heatsink accordingly. The regulators won't be damaged by overheating they will just shut down.

Power dissipated in the regulator = (Vin - Vout)/Load current

Russell.

Yeah I went high on the numbers. You got me thinking though. I do want everything running cool as it will be in a sealed box so I will look into adding a voltage drop before the 5v regulators. I'll try and have them powered with 7v which should help considerably. I only need the 13.8 because of the 12v regulator.

weedpharma:
You cannot put anything in series with the regulator output to stop reverse connection of the load.

As far as protection goes, the regulators have their own protection and will shut down on excess current or heat.

There is however no way to protect the add on device except to put the protection in the add on device.

Weedpharma

Ok, I wasn't worried about the device so much, more about protecting the regulators. I'm sure it will be ok without.

Is it ok to add some diodes to the line to lower the voltage?

Which line are you referring to? The input power? If so, it will simply drop the voltage by dissipating heat.

If you will be using a voltage higher than 9v, you should use a switch mode power supply to reduce the higher voltage to around 9v that can then be fed into the board. The higher efficiency of the switch mode will generate less heat.

Weedpharma

So a switching power supply for main power will apply lower voltage to the 5v regulators but higher voltage still to the 12v regulators?

Would a 1N5822 Schottky diode 3amp 40v be good for reverse polarity on the board input?

There will never be 3amps going into this board, the power supply will only be rated for 3amps. I'm thinking typical use will be maybe 1amp and if everything was on at once 2amps.

You illustrate your input connector wired backwards - the centre pin should be the positive.

Power supplies wired the other way around are a real hazard.

It is most ironic to see you speaking about reverse polarity protection if you already have it wired in a non-standard (except for Panasonic) way!

Another thought, recently on another thread, there was a mention of 5v switch mode supplies with pin out same as the 7805. If they fit, there is no problem with what ever voltage you feed it.

I cannot remember the part, maybe someone else can.

Weedpharma