Max Amperage on Ground?

Hi all, I am using a MOSFET as variable resistor in an external circuit where the current is up to 5amp. The gate is connected to a PWM pin of Arduino. The drain is connected to the +V of the external circuit and the Source to the ground of the Arduino.

Question, will the 5amp kill the Arduino if it is connected on the ground pin of the board?

Thanks! Riccardo

Possibly. It depends which "ground of the Arduino" you are connecting to. The socket header pins are rated for 3A each I believe, though you have two side by side so you could connect to both to split the current. The traces are no problem since a ground plane is used. The DC power jack is probably of a type that is only rated to 2.5A (like this for example: http://products.cui.com/CUI_PJ-202A_Datasheet.pdf?fileID=4976), so that might be the greatest area of concern.

How are you connecting the ground of the Arduino to the ground of the +V external circuit?

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This is how the circuit looks like. I have two wires coming from the external circuit (+V and GND), the external circuit GND goes directly to the arduino GND.

I am designing a PCB with Fritzing to connect those wires to the arduino. I am also concern about the width of the traces on the PCB, looks like they would have to be massive to allow for all that amp.. Should I use wires instead?

I actually have 4 of these MOSFET for 4 external circuits with 5amp on each. Is it feasible to use only one arduino board or will I definitely damage it?

Thanks Riccardo

giraz: Hi all, I am using a MOSFET as variable resistor in an external circuit where the current is up to 5amp. The gate is connected to a PWM pin of Arduino. The drain is connected to the +V of the external circuit and the Source to the ground of the Arduino.

Question, will the 5amp kill the Arduino if it is connected on the ground pin of the board?

Thanks! Riccardo

The way you describe the circuit the 5A doesn't go near the Arduino - assuming the ground of the external circuit only connects the Arduino in one place (via the FET's source). If that's true no problem (unless the FET fails of course). Place a 220 ohm resistor between the gate and arduino pin to prevent over-current on the pin though, the gate is very capacitive.

I have two wires coming from the external circuit (+V and GND), the external circuit GND goes directly to the arduino GND.

You have to be a bit more specific. Where is your power source? What is this "external circuit"? the MOSFET? HOW does the external circuit GND go directly to Arduino GND.

A sketch of your wiring (not the schematic...the wiring) would be helpful including all of the components, including the power supply.

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Apologies, let me try to explain better. I am using arduino and the little electronic circuit I'm designing to replace slot car controllers. What I called "external circuit" is the track.The car track has up to 5amp on each lane when at max speed. The MOSFET as variable resistor replaces the potentiometer which is the only component inside the traditional slot car controller.

Here the connections I made (2 MOSFETS for 2 controllers). Where you see "circuit" it means that there are two cables coming from the track +V and -V (I called it GND). These are going to the Drain and Source of the MOSFET.

Just to recap, I know that the circuit should works fine when the current is not very high. But now I am going to use a track which runs with 5amps and I am concerned of mainly three things:

  • Is there a problem with higher currents if I keep the GND of the track connected to the GND of the Arduino?

  • In these images you see two MOSFETS to replace 2 controllers (the current on each is up to 5 amp). What if I have four of these on the same arduino board?

  • using online calculators seems that I should use very VERY wide traces on the PCB to cope with 5amp. Should I instead use wires in certain parts of the circuit? (I am not sure of how the circuit should change in this case)

Thanks for your time!

R

To reiterate the high currents are not going through the gate/source circuit at all, only te drain/source circuit. Connecting the grounds together at the source doesn’t create a circuit (which is a loop) but merely a link (so the voltages are the same). Unless the MOSFET fails current can’t flow from gate<->drain at all.

Wide traces are good - load them up with extra solder if worried (assuming no solder-resist)

For 5A ensure the logic-level MOSFETs have a Rds(on) less than 0.02 ohms or you’ll need heatsinks.

Ah...OK. No, there is not going to be a problem with higher currents because these high currents will not flow through the Arduino GND, but instead back to the track power supply. Electrons will come through your "yellow" wire, through the MOSFET, then back to the "circuit" through the black wire. The GND connection to the Arduino will not carry any significant current (just the one to turn the MOSFET's on and off).

What your diagram does not show, however, is how you are going to power the Arduino. That power supply and its GND connection can also make a difference.

If you put this 5A circuitry on a PCB you will have to be a bit careful. I suggest 40mil traces to keep temperature rise 50C above ambient or less (the board will get pretty hot in those areas but nothing should be damaged at 25C+50C --> 75C). Or you can use 2oz copper (more expensive PCB manufacture) to use thinner traces or run cooler. Or run traces in parallel on top and bottom sides, or as you say, just use wires instead of traces. A common thing for people to do in the past was run solder on top of the traces to give them lots of thickness so they can carry lots of current. You can't do that if the board has a soldermask on the traces, but you can design it so the soldermask is exposed (or get a cheap board with no soldermask at all -- in which case you should tin it to avoid oxidation).

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Guys you are great. Thanks a lot.

This is the MOSFET I am using, it says RDS(on) = 40m Omh http://www.irf.com/product-info/datasheets/data/irf540n.pdf Not sure how to search for one with lower RDS(on) online : /

The Arduino is powered via USB at the moment!

As for the width of the traces, I used this online calculator to get the values http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/

If I put 5amp and thickness 35um, it suggests a width of 186mil for internal traces and 71.5 for external ones. Not sure if the PCB you get with Fritzing are internal or external traces ( http://fab.fritzing.org/fritzing-fab ) but are very wide! I am doing something wrong here?

That MOSFET is not logic level - you want Rds(on) <= 0.02 ohm at Vgs=4.5V

An IRF540 is not appropriate since it is not a "logic-level" MOSFET, i.e., it won't turn on at only 5V at the gate. I suggest something like an NXP PSMN022-30PL:

http://www.nxp.com/documents/data_sheet/PSMN022-30PL.pdf

Your width of 71.5mil for external traces is fine, but one important parameter of your trace width calculator is "temperature rise". There are 3 variables in this calculator, not just 2. The 2 obvious ones are trace width and current: the third is temperature rise. Basically, how hot are you wiling to let the traces get? My previous post suggested you can use 41mil traces IF you are willing to let the traces get 50C hotter than ambient.

Unless you are planning on an expensive (relatively) 4-layer board then all your traces will be external.

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Brilliant! Thanks a lot. I am learning a lot thanks to you and this awesome forum