NPN Switch with overcurrent protection

Hello.

im building a circuit with a NPN transistor to be able to switch relays with an arduino (see picture) the maximum load needed for the relay is around 1A and 12V. What i need in my circuit is some sort of overcurrent protection in case of short circuit to VCC (Battery).

what is the best design for an easy and reliable over current protection? i have looked at resettable SMD fuses is that a good way to protect this circuit?

A better way would be to use a constant current source, in my opinion. Try Googling constant current source. You should be able to find a description and an equation for any maximum current you want. You only need one more NPN transistor and a resistor in the range of 33 to 47 ohms for good LED output (15 to 20 ma)

If you need more help, come back here again.

Your NPN will beat any fuse. If you short out the load, no fuse or circuit breaker will trip before the transistor blows.

Very few circuits use fuses to protect individual sections. Most will only have a main fuse to prevent damage.

Weedpharma

Hi,

im building a circuit with a NPN transistor to be able to switch relays with an arduino (see picture) the maximum load needed for the relay is around 1A and 12V. What i need in my circuit is some sort of overcurrent protection in case of short circuit to VCC (Battery).

I think what you are asking is for a circuit to monitor the current through the contacts of the relay and through the load.
NOT the NPN and coil circuit.
If I am wrong, what are the specs for the relay, can you post a link?

Tom...... :slight_smile:

Hello!

I want to protect the NPN and the circuit board in case of a short circuit.

sebb898:
I want to protect the NPN and the circuit board against short circuit.

Place a fuse very close to the +12V.

Ok!
What about a resetteble fuse (PTC) on the circuit board?

Need some help with a good design.

As weedpharma mentioned above, your NPN will probably blow before the fuse.

Do you really anticipate a short across the relay coil? Under what conditions?

This is something that I typically wouldn't worry about (and I've been doing this for a long time). Just make sure you include the back-voltage snubbing diode across the relay coil (as shown in earlier posts) and ensure that the NPN can handle the requisite coil current (which is probably in the range of 20 - 40 mA).

As an example, here's my current project, with many relays (and my first Arduino project, by the way, which is why I'm here).

Note -- there's no fusing to protect the NPN drivers. There's only one fuse, and it's on the AC line going to the power supply transformer.

This is an automotive solution and therefore i want al the input / output pins to be protected.

Mabey its overkill to have short protection on this pin?

I think i found the answer to my question here: http://www.physics.unlv.edu/~bill/PHYS483/current_lim.pdf

Megasquirt ECU uses the same concept on ther V3 board when they activate the injector coils see sheet 4, note 1 :V3 Main Board

In post#0 you say you have a 1Amp relay coil. Must be a big one...
If you add a 1Amp current limiting circuit to the NPN transistor, 1Amp is going to flow in case of a short.
That will dissipate 12watt in the NPN transistor.
That means a power transistor with a big heatsink, or a small transistor and a big bang.
Or... the current limiting circuit, a power transistor, a small heatsink, and a polyfuse against the transistor.
A common power transistor has barely enough gain to switch a 1Amp relay with Arduino's 20mA drive current.
A darlington or mosfet might be needed.
A darlington plus the current limiting circuit has a volt drop of ~2volt.
2volt less for the relay.
A logic mosfet plus current limiter drops ~0.8volt.
Leo..

It's not clear to me if the 1A 12V rating mentioned in the first post refers to the coil rating (which would be 12 ohms!), or if this is the load that must be handled by the relays contacts.

I'm assuming the latter, but perhaps the original poster can clarify...

As weedpharma mentioned above, your NPN will probably blow before the fuse.

It is my experience electronic devices protect fuses.
Fuses protect wiring.

You need to implement a self resetting electronic trip circuit - one that cuts the current off when it exceeds 1 amp, and resets when it drops below a certain threshold, eg 0.5 amp. A current sense resistor feeding a schmidt trigger fed back to the base. Transistor will need to survive whatever transient overload that can occur.

yendis:
You need to implement a self resetting electronic trip circuit

I did mention polyfuse.

Leo..

If there is a possibility that there will be a short across the relay coil, then there is something wrong with your layout.

Weedpharma

Hi,
What is the spec of your relay, a picture if possible?
What is the resistance of your relay coil?

Tom.... :slight_smile:

FYI: You never want your driver transistor (or have any transistor) to be part of the Flyback Damping path (as shown in schematics referenced). It's just not a best practice.

In any case, the circuit shown in the second drawing are not
over current protection.
The TIP42 sets a maximum fly back voltage by dumping the
flyback current until the darlington has time to turn on.
These circuits are just there to help control the release
time of the solenoid. They do nothing to protect the driver transistor
from a short circuit if the battery voltage is connected to
the collector of the driving transistor.
The entire circuit could have been replaced with a diode and a zener
of the right voltage and current.
Dwight

here's my current project

Very Impressive !