Simple question: Why relays?

[DaleB]

In looking at my electrical system design in progress, I have a few relays - one for the boost pump, for example, and one for the avionics bus.

Is there a good reason to use a relay instead of, say, a P-channel MOSFET? I'm looking at devices that have pretty decent looking specs, like the FQP47P06 at 60V, 47A, .026 Ohm Rds(on). It seems to me that one of those could replace a SPST relay.

I like the size, weight and simplicity of wiring advantages of the MOSFET... but am wondering if there's a reason to stick with relays instead.

[Flyingleap]

Dale,
I have given this the same thought, IR (International Rectifier)devices seem to be the best. I did buy a few China types that donot perform as well. They also need a driver to power all of them. Seems odd, but the impedence on the gate drive rail to rail must be with a low imp and no hiccups at the zero crossover even thought the current drive is near zero. At least, my scope showed that to be the case. I don't have that much experience with these but I think they would be a better choice than relays. When I am able to put them in parallel ( with the right circuit ) , start the engine ( in place of start relay), I've arrived. Not there yet. John

[RVDan]

If you use switches rated at 15A@ 12VDC you won't need a relay for much. Probably, only master, starter and depending on your alternator arrangement the output of the alternator. Typically not the fuel pump or even the landing lights.

It makes for a simple arrangement.

[DaleB]

Yes, except in those situations where you can't use a 15A rated switch (like switches on the stick grip), or where you need or want to switch a load by grounding a low-current wire rather than running heavier gauge wire to and from the panel.

Most of the system uses simple switches. A couple of places need something else.

[RVDan]

There have been some companies make MOSFET relays for aircraft. For DIY, if designed properly they should be reliable. There are lots of inexpensve auto relays that are rated up to 40A that are small and light weight and have been environmentally tested for only about $8. Available from Allied, Newark and so on.

[DaleB]

Yes, I'm aware of the wide selection of relays available - both electromechanical and solid state.

My question remains... is there a good reason to use a relay instead of a dead-simple MOSFET switch? I'm not talking about a MOSFET solid-state relay. I'm talking about just a P-channel MOSFET with a switch grounding the gate to turn it on, used as a high-side switch. Of course you'd also need a resistor -- say 10K Ohms or so -- between the gate and source, so add another penny to the cost.

So let's take a simple test case. You want to control a 5A DC motor using a pushbutton stick switch. And for the sake of argument, let's say you can spare the ounce or two of usable load to carry a spare for either solution, since you can't walk into Auto Zone and buy a P-channel power MOSFET.

MOSFET pros: Small, light weight, simple wiring, no inductive spike from turning it off, little/no extra current draw.

MOSFET cons: You're on your own for wiring. Is there anything else I haven't thought of?

Relay pros: Readily available plug connectors. Makes noise when actuated, possibly making troubleshooting easier.

Relay cons: Takes more space. Weighs more. Causes inductive spikes when turned off. Draws more current (minor nit).

[flyboy1963]

Originally Posted by Flyingleap

..... need a driver to power all of them. Seems odd, but the impedence on the gate drive rail to rail must be with a low imp and no hiccups at the zero crossover even though the current drive is near zero. .....

I just love it when you guys talk like this....it's like visiting a foreign country! almost as good as minion-speak!

[Neal@F14]

What's the voltage drop across the mosfet's junction vs that across the mechanical relay's contacts?

[warrenkm]

Originally Posted by DaleB

Yes, I'm aware of the wide selection of relays available - both electromechanical and solid state.

My question remains... is there a good reason to use a relay instead of a dead-simple MOSFET switch? I'm not talking about a MOSFET solid-state relay. I'm talking about just a P-channel MOSFET with a switch grounding the gate to turn it on, used as a high-side switch. Of course you'd also need a resistor -- say 10K Ohms or so -- between the gate and source, so add another penny to the cost.

So let's take a simple test case. You want to control a 5A DC motor using a pushbutton stick switch. And for the sake of argument, let's say you can spare the ounce or two of usable load to carry a spare for either solution, since you can't walk into Auto Zone and buy a P-channel power MOSFET.

MOSFET pros: Small, light weight, simple wiring, no inductive spike from turning it off, little/no extra current draw.

MOSFET cons: You're on your own for wiring. Is there anything else I haven't thought of?

Relay pros: Readily available plug connectors. Makes noise when actuated, possibly making troubleshooting easier.

Relay cons: Takes more space. Weighs more. Causes inductive spikes when turned off. Draws more current (minor nit).

I don't have any experience with power MOSFETs in aircraft switching but if I were considering it I might think through whether or not I'd want to use a PMOS transistor that requires an active high to shut off a load and whether or not it performs at VGS=0V the way you want for a given load. You can also get a good spike, VDS, on switching an inductive load so a healthy derating is always good. Even with small Rdson be careful to consider heat dissipation. Again, I've not tried to implement these in any sort of aircraft system but I have helped address certain types of reliability issues for power MOSFETs in switching converters, mostly with IR devices.

[MCA]

Dale, the Vertical Power VP-X does exactly this. It uses FETs to eliminate the need for relays and high-current switches. http://verticalpower.com/