I am a geneticist (retired) by profession and neither an engineer nor a programmer, so each step I take requires quite a bit of learning, and I'm likely to ask some pretty naive questions.
At the moment, it's time for me to learn something about controlling high-current loads. In particular, I need to simple ON/OFF, very low frequency, control of, for example, a 2A resistive load in a nominally 24V lead-acid battery powered system. The actually voltage can, of course, be substantially higher than 24V; near 30V during charging, and with high transient voltages (even with proper snubbing) from major, inductive loads elsewhere in the system. Clearly, one wants a lot of head room here and I'm thinking along the lines of a source-drain spec of 60V.
What I'm really nervous about, however, is the possibility of exceeding source-gate maximum voltage. It is rare to see a MOSFET that tolerates greater than +/- 25V absolute maximum source-gate voltage. With a low-side n-channel MOSFET this is not going to be a problem when it is switched on -- voltage drop across the load will keep source voltage down. What I'm nervous about is what happens if the gate is 0, the MOSFET is off, and a 35V transient hits the source. Isn't this likely to fry the MOSFET, or am I misreading things?
If this is an issue, how does one go about protecting the circuit?
LROBBINS:
What I'm really nervous about, however, is the possibility of exceeding source-gate maximum voltage. It is rare to see a MOSFET that tolerates greater than +/- 25V absolute maximum source-gate voltage. With a low-side n-channel MOSFET this is not going to be a problem when it is switched on -- voltage drop across the load will keep source voltage down. What I'm nervous about is what happens if the gate is 0, the MOSFET is off, and a 35V transient hits the source. Isn't this likely to fry the MOSFET, or am I misreading things?
This would be a problem with a p-channel MOSFET but not an n-channel.
The source pin of an n-channel MOSFET should be connected to ground (the same ground as the Arduino) so the transient would hit drain, not source.
The usual way how to protect gate and drain (against source) is with Zener diodes.
The n_fet's gate: simply feed the signal to the gate via a 1k resistor (for example) and from the gate to source connect a 15V (or less) zener diode, cathode on the gate, anode to the source.
The n_fet's drain: from the drain to the source connect a 25V (or less) zener diode, cathode on drain, anode to source.
Thank you fungus. My thinking was upside-down as I didn't really understand "source":
The source is so named because it is the source of the charge carriers (electrons for n-channel, holes for p-channel) that flow through the channel; similarly, the drain is where the charge carriers leave the channel. (from Wikipedia)
So, in an n-channel MOSFET with electrons as charge carrier source is negative with respect to drain, i.e. ground as you've said. That makes life much, much easier.
Ciao,
Lenny
LROBBINS:
So, in an n-channel MOSFET with electrons as charge carrier source is negative with respect to drain
Yes, they decided to label MOSFETs the opposite way to all the other electronic components ever created. Technically it's the correct way, but I'm not sure it was the smart thing to do.
I thought "source" doesn't imply a polarity. "source" v. "sink" is relative to ground, ground doesn't have to
be the negative rail? Its just like "emitter" in NPN / PNP transistors.
Yes, they decided to label MOSFETs the opposite way to all the other electronic components ever created. Technically it's the correct way, but I'm not sure it was the smart thing to do.
There is an agreement the arrow always points from P to N.
NPN has the arrow towards the emitter (P->N).
PNP has the arrow towards the base (P->N).
nchannel MOSFET has its channel N induced in P substrate, so the arrow points to gate.
pchannel MOSFET has its channel P induced in N substrate, so the arrow points to source.