DaleB

DaleB

Well Known Member
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.
 
Relays!

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
 
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.
 
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.
 
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.
 
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).
 
translation?

Flyingleap said:
..... 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. .....
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I just love it when you guys talk like this....it's like visiting a foreign country! :) almost as good as minion-speak!
 
What's the voltage drop across the mosfet's junction vs that across the mechanical relay's contacts?
 
DaleB said:
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).
Click to expand...

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.
 
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Seems to me that relays fail gracefully - well more gracefully than a mosfet. And when they do, I can fix them with a hammer, a screw driver and maybe a file. So I don't need to carry a spare in the plane away from home.

In reality, its an experimental. Do what you want. But dont discount stupid simple for stupid idea. If we did that, noone would use duct tape.

Same with VP-X. Not for my plane. If the thing goes up the fritz, all your electrical eggs are in one basket. Breakers, not an issue. I like manual; automated when necessary only.

And I'm an EE schooled in the digital age. Each to his own.
 
MCA said:
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/
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I figured it did. Did you find you needed any additional protective circuitry around the MOSFET?


Neal, the voltage drop depends on the current load. The one I mentioned has an Rds(on) of .026 Ohms, meaning a 5A load will see about a 0.13V drop across the MOSFET. I haven't explored the specs on automotive relays in depth, but a quick check of a spec sheet or two tells me the FET isn't significantly worse - not enough to make a practical difference, anyway.
 
Done that

I am using integrated high side switches through my aircraft. Think VertPwr but in a distributed fashion instead of monolithic. IMO single down point to SS switching is repairs. If you are using the aircraft for travel then it will be harder to make repairs in the wild. Soldering irons etc vs plugging in a new relay
 
maniago said:
Seems to me that relays fail gracefully - well more gracefully than a mosfet. And when they do, I can fix them with a hammer, a screw driver and maybe a file. So I don't need to carry a spare in the plane away from home.
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I hear you... spent many years cleaning up relay contacts with a burnishing blade. In fact, that's one reason I would like to avoid them where I can. I've seen relays and MOSFETs both fail open and shorted. In fact I've seen far more relays fail than FETs, but then I worked on equipment that used a lot of relays for a lot of years. MOSFETs generally don't arc -- not more than once, anyway -- and relays generally don't smoke when they fail. ::shrug:: six of one...

As for field repairs, carrying a spare just isn't an issue when you just screw the spare to the subpanel right next to where it might be needed. A few minutes and a screwdriver and you're back on your way, no hammer or contact file needed. If I used relays I'd do the same thing, attach a spare close to where it might be needed.
 
Dale,

There is protection circuitry for everything from thermal overload to ESD spikes.

I prefer FETs simply because they are much much much more reliable than mechanical relays and switches.

And with the VP-X you can wire backup circuits for redundancy for critical avionics. It's all described in the install manual.
 
jdeas said:
I am using integrated high side switches through my aircraft. Think VertPwr but in a distributed fashion instead of monolithic. IMO single down point to SS switching is repairs. If you are using the aircraft for travel then it will be harder to make repairs in the wild. Soldering irons etc vs plugging in a new relay
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I thought about that. It's a valid concern, since there's really no off the shelf equivalent to the 5-pin auto relay socket for a TO-220.

So... my solution: Solder ring terminals to the TO-220 legs, with the resistor already attached. Use a barrier block on the subpanel to screw down the wires and MOSFET leads. Have a few prepared in advance and just attach 'em to the subpanel next to the terminal block. Viola - a quick replacement strategy that requires no soldering in the field.
 
Agree 100%.


maniago said:
Seems to me that relays fail gracefully - well more gracefully than a mosfet. And when they do, I can fix them with a hammer, a screw driver and maybe a file. So I don't need to carry a spare in the plane away from home.

In reality, its an experimental. Do what you want. But dont discount stupid simple for stupid idea. If we did that, noone would use duct tape.

Same with VP-X. Not for my plane. If the thing goes up the fritz, all your electrical eggs are in one basket. Breakers, not an issue. I like manual; automated when necessary only.

And I'm an EE schooled in the digital age. Each to his own.
Click to expand...
 
Why use simple fets anymore?

My first gen boxes used an ST part (VN920). In the Pentawatt package it may do what you want with fewer parts and current control.

Tons of automotive rated high side switches to choose from.
 
When ordering parts for my first panel, I ordered the relays. By the time I finally settled on the wiring plan, I didn't need any. When I started my second panel I didn't even think of relays. By keeping every circuit or subsystem to 15A or less, it not only reduced the complexity of my wiring plan but also helped me focus on the purpose of each system. Ironically, the two "big" switches I have don't actually carry big current. I just wanted to use locking switches for Main and Standby. If I were to do it again, I wouldn't even bother with those two.

There are lots of things you can do with a power infrastructure like VerticalPower or you can design your own power plan and omit the relays or solid state solutions.

It really comes down to your goals.
 
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Whatever turns an avionics bus on, mechanical switch, electro-mechanical relay or solid state relay, is a single failure point. There should be an alternate current path. Bob Nuckolls' E-Bus has two current paths, one through a DIODE and one through a switch or relay. The diode is highly unlikely to fail if properly installed to a heat-sink. And even if the normal feed path fails, there is an alternate feed path. There is no need to carry spare parts.
 
I have often wondered why the auto industry still uses relays. One would think that if FET's were cheaper and more reliable, the auto industry would have adopted the technology for all cars.
 
I use Honeywell switches in my own plane and prefer them for the panels I mod, they have proven to have superior reliability in the airline world I came from. They have literally millions of flight hours over decades of use and I don't ever remember seeing one fail. For circuits like the avioincs bus I use a single DPDT with both sides tied together for a little extra load capacity/redundancy.

Honeywell swithches and Klixon breakers make for a bullet proof electriical system, old school perhaps, but its simple and it works.
 
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Walt said:
I use Honeywell micro switches in my own plane and prefer them for the panels I mod, they have proven to have superior reliability in the airline world I came from. .
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All I get on a search are squat switches. Got a model number or some such on the Honeywell toggle switches?
 
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Honeywell!!!!

Thanks Glen, now those are the ones I have been looking for also. I have a new retract project and gear switches must be of the proper quality for me!! I have often thought a Hall effect switch might be the way to go for being foolproof but Honeywell is not far from being perfect. Also found a bunch of relays in this bird with a questionable mounting structure, quality, and wiring to boot. Its not fun to be flying at night over water, in thunderstorm conditions, with a load of ice, having declared an emergency AND have to worry about the type of electrics around you. John
 
Flyingleap said:
I have often thought a Hall effect switch
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After more microswitch failures than I care to count, I'm now using proximity sensors in my gear indication system with great success. I highly recommend pursuing a solid state solution if you have the spare energy to do so! Commercial jets use prox sensors just about everywhere.
 
Last century, I was employed at a nuclear research facility to design high power switch-mode power supplies before they became a commodity.

The key factor in using transistors (bipolar or FET) is what is called SOA or Safe Operating Area protection. This is very much dependent on the load (resistive, capacitive, inductive, combinations thereof). Operating outside of the SOA could lead to disaster, as my garbage can full of transistors would attest to.

Modern automotive solid-state power switches are very robust as long as the load behaves itself. Under certain fault conditions, however, it is possible to let the smoke out. Electromechanical devices (relays) are more tolerant and fool proof, but can degrade with the number of operations.

In my experience, the most hostile loads we have in an aircraft are motors (flaps, gear), gas-discharge strobe power supplies and incandescent lights. With these disappearing due to advances in LED technology, we can see the latter two being a non-issue. With motors, prudent design will avoid any problems.

Nothing, however, beats the convenience of unplugging a relay and plugging in a spare. A few years ago, I designed a circuit board that duplicates an automotive relay footprint with fast-on tabs. It used FET switches and a SPDT configuration. Problem was, it was expensive and polarity dependent... it was not a drop in for every application. Hard to compete with $2.00 relays.

Vertical Power is the best implementation that I have seen, if you want to be locked into a particular achitecture. I would never, however, put high currents through a toggle switch no mater who makes it. My experience is that this is very stressful on the switches and even more stressful on the pilot when the switch fails in flight.

Relays or FETs, both are used in cars for different reasons. Same with aircraft... after all a master contractor or starter contractor is just a relay.
 
Here's an update. Rather than change the world by designing a SPDT relay, also known by telecom folks as 1 form C, i think just the SPST type would handle most of the loads in an aircraft.

Here's an off the shelf solid state automotive mini-din relay...
http://hellahd.com/index.php/default/electrics/relays/mini-iso-relays/12v-spst/h41773001-931773987/

This baby is about $30 and a typical aircraft would use about 10 of them. They can't handle reversing apps like flaps or trim, but they are good for 10 million operations.

I have a bank of automotive relays in sockets in my Rocket. Easy upgrade to the Hellas when the time comes.

It's also an interesting read to go through the entire relay catalog from Hella... Flashers, time delay relays, battery relays and so on.

I will point out that these devices are software-free and interchangeable with the standard mechanical versions.
 
vlittle said:
Here's an update. Rather than change the world by designing a SPDT relay, also known by telecom folks as 1 form C, i think just the SPST type would handle most of the loads in an aircraft.

Here's an off the shelf solid state automotive mini-din relay...
http://hellahd.com/index.php/default/electrics/relays/mini-iso-relays/12v-spst/h41773001-931773987/

This baby is about $30 and a typical aircraft would use about 10 of them. They can't handle reversing apps like flaps or trim, but they are good for 10 million operations.

I have a bank of automotive relays in sockets in my Rocket. Easy upgrade to the Hellas when the time comes.

It's also an interesting read to go through the entire relay catalog from Hella... Flashers, time delay relays, battery relays and so on.

I will point out that these devices are software-free and interchangeable with the standard mechanical versions.
Click to expand...


Ahhh....now that, is very interesting. Thanks for the post.
 
I didn't post it here, I guess, but here's what I am planning to test out once I get a little further along:

MOSFET-with-resistor-267x300.jpg


Easy mounting to a standard barrier terminal block, easy to have a couple of spares attached to the panel right next to the terminal block. Change it out in a couple of minutes with a screwdriver, costs less than an automotive relay, etc. I'm also looking at different terminal block types that would eliminate the need for the lugs.

The pic is actually of a sacrificial dead P-MOSFET used just to build a TO-220 form factor test item. Once I have some time and a couple of loads I'll see how it actually works.
 
Do a Google search for "MOSFET zener diode protection". On another forum, I was advised to add a zener diode between the gate and source to protect the MOSFET from transient voltage spikes.
 
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