Am intending to build arduino nano controlled LIPO 2cell battery fast discharger. Is the FQP30N06L suitable for switching the load, 8.4v down to 7.4v at 30amps. I know it has logic level switching but is 7.4v high enough to switch the 30 amps? All examples of this I have seen use 12v to drive load.
Thanks
Hi Andy,
One of those MOSFETs can definitely switch 30A at 7.4V, but you’re going to struggle to keep it cool enough to be able to do that, so it’ll heat up and de-rate quickly.
You’d be much better off using one of these:
Datasheet:
These also have a logic level Vgs on threshold, but can handle twice the current.
The key characteristics I look for when selecting a MOSFET are the:
Drain-source breakdown voltage (The max load voltage it can switch)
Continuous Drain Current (at 100°C) - this will usually be the value it can handle all day and night with just a heatsink.
Maximum Gate-Source Voltage - This is the maximum voltage you can use to switch the MOSFET
Gate-Source Threshold Voltage - This is the minimum voltage you can use to switch the MOSFET
For basic applications, these values are usually all you need to look at on the datasheets.
Regards,
Oliver
Support | Core Electronics
Thanks for that Oliver. Do they have a minimum voltage they can switch? With the logic level Vgs. I forgot I am also switching separately a 1 cell lipo from 4.2v down to 3.4v with a 5v or 3.3v trigger level.
Thanks,
Andy Wallace
Hey Andy,
No, there’s no practical lower limit to the voltage they can switch (by the time you’re getting down to sub-milivolt range you’re doing something quite special, and you’re not going to be using one of these).
The MOSFET I suggested has a 2.35V Vgs Threshold rating, so 3.3v microcontrollers will still switch it just fine.
Cheers,
Oliver
Support | Core Electronics
Does the 3.3V input need a resistor to drop the voltage to avoid damaging the MOSFET?
Hi Michael,
No it doesn’t. You can use any voltage up to the Maximum Gate-Source Voltage. For the 30V / 60A MOSFET above this is 20V:
In an extreme case, note that because the current through the gate is effectively zero, a resistor won’t work to drop the voltage. You’d need to use something else to switch a lower voltage on the gate (or use a different transistor).
Regards,
Oliver
Support | Core Electronics
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I thought that would be the case. Having just spent a few days staring at those datasheets for my project, the numbers were all starting to blur into one. It can become a bit confusing when every guide / howto / blog uses a different component for the same task.
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Hi Michael,
Yeah no worries. They definitely do tend to do that.
If you can, I’m keen to see a preview of this bike of yours when it gets up and running 
Regards,
Oliver
Support | Core Electronics
I will hold off posting anything in a public forum until the episode goes to air. With most components remaining the same as last year, I don’t anticipate many problems. And as with the cross trainer, the Pi will be well hidden.
I got a message from someone at Raspberry Pi asking about a feature in MagPi, but never got a follow-up to my reply. That said, I will try document it better this time. The last build went quickly from proof-of-concept to production-ready. By then, there wasn’t much to get photos of.
With Melbourne’s lockdown, most things are up in the air at the moment. By the time I am back near the bike, the design guys may have already hidden the bulk of the hardware from view. But I may be able to post photos of my Lego-based prototype.
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Using PWM to drive this do I need a gate driver similar to MCP 1407 (low side non inverting)?
Hi @Andy127258 - if your PWM signal is only 3.3V then you can still expect about 3 A will pass through the transistor at room temperature.
A 5V signal will turn the transistor fully-on.
