Mosfet Automotive lighting controller

[julie_m]

Away the naysayers! Relays are for switching AC, or for when you need electrical separation between "switching" and "switched" circuits! You'll be fine switching lamp loads with a MOSFET, as long as you take all the usual precautions. That means a hefty Zener diode, cathode to source, anode to drain; a resistor/Zener voltage limiter on the gate; and a fuse. Also remember that you will be switching the earthy side.

You shouldn't need any heatsinking as the MOSFET will either be on (and so have almost no voltage across it), or off (and so have almost no current through it). However, with the load being a filament lamp, it's conceivable that the startup cycle could cause some heating. Under normal circumstances, the hot filament is limiting the current through the MOSFET. When the filament is cold, though, the roles are reversed: the MOSFET (remember, it is a triode after all) is acting as the current limiting device, and will be dissipating power until the filament warms up. Whether or not this is significant will have to be determined by experiment.

One other thing to watch out for: power MOSFETs are highly addictive, and you will want to use them for switching everything!

[BGmidsUK]

Originally Posted by ajs_derby

... you will want to use them for switching everything!

I wonder if they'll be suitable for switching loads such as invertors? I have a 600w unit which I'm planning on fitting but suitably rated relays (150A peak current rating) are rather expensive. I suppose a hefty, very low value resistor could be used in series with the supply but this might upset the device and in any case the cables will have some resistance. I don't know how the inrush current compares to resistive loads such as lights but I expect it to be rather higher!

>EDIT<
Just checked the specs for a couple of relays at CPC - Part Numbers SW02653 & SW02655 seem to fit the bill, with 70A rating and 240A peak, 70A giving a maximum load of 840 watts continuous at 12v. This will be more than adequate for 500W of lighting, as well as my intended application. Cost is less than £3.50 including VAT. The peak rating should be ample for the inrush current and a cap can be connected across the terminals to absorb switching spikes and protect the contacts to a degree.


Brian

[AlanBeckett]

Brian,
I would think you'd be OK. Lights are, in many ways, a menace due to their low cold resistance. Inverters, on the other hand, often deliberately limit the inrush current - so called soft start.
Alan

[BGmidsUK]

Thanks Alan. My reason for wanting to use a relay, apart from being able to isolate the load, is that I assume the unit draws current even when switched off via the small on/off switch on the front of the unit, plus I will need to have both the switch and socket(s) mounted remotely to the unit. Having thick cables and a manual isolation switch inside the car isn't ideal, even without the associated voltage drop/power loss this would cause! I imagine the same is true of Sean's lighting arrangement.

What I would do in Sean's case (and mine, when I get round to it ) is to mount the relay assembly in a sealed box close to the battery, preferably level with it, using screw terminals so that the water can't get in via cables and wrapping them with amalgamating tape. I'd put plenty of padding round the relay (in the box) to absorb some of the vibrations.


Brian

[julie_m]

Originally Posted by BGmidsUK

I wonder if they'll be suitable for switching loads such as invertors? I have a 600w unit which I'm planning on fitting but suitably rated relays (150A peak current rating) are rather expensive.

You shouldn't need to switch the inverter with a relay or anything!

There are already two hefty MOSFETs right there inside your inverter; one pulling each end of the centre-tapped first transformer primary to 0V. All the power that comes out of the inverter must go through one or the other of those devices. Make sure they are both turned OFF, and you'll cut the power consumption to a matter of milliamps (just the two oscillators, plus any leakages).

If the manufacturer hasn't already thoughtfully provided a remote switching input (it may be strapped to one of the battery terminals), it should be a simple enough matter to add one of your own.

[Sean Williams]

Think I will stick with the potted Mosfet idea for a while - they appeal to my sense of humour!

Will do some more research on driving these beasties, and lash a couple of test circuits together.

Should be fun

Sean

[murphyv310]

Originally Posted by Sean Williams

Graham is right - this is the main reason.

My reasoning behind wanting to use a solid state solution is simple really - modern automotive relays are to be honest, Rubbish, Control panel stuff does not really respond well to vibration and damp.

I do not want lots of high current cabling behind the dash - indeed the feed from the battery needs to be as short as possible - when the project is finished there will be nearly 500w of lighting available for use.

Sean

Hi Sean
Although not knocking the solid state device here, relays have been in automotive use for many decades and still are. A good OEM relay will last for years. I have fitted relays inside the headlamp units of my Audi 80 to control the lights and give less losses to the bulbs, they remain totally dry and have been 100% reliable.
The scenario though with MOSFETS, what happens if it goes short and you are giving out 500w of light? Will it be easy to dump the feed, maybe a master switch, better than dazzling others or running the battery down while pulling wires off! I have never had a relay go faulty with the contacts stuck closed.
Cheers
Trevor

[richrussell]

Originally Posted by murphyv310

I have never had a relay go faulty with the contacts stuck closed.

I have had two - both due to water ingress. A radiator fan control relay on a Renault 5, which is conveniently clipped just next to a headlight where water gets sprayed up from the road particularly when driving in slush or deep puddles. The other was the horn relay - which is also in a location that can get wet due to spray and also water seeping down the side of the bonnet. Unfortunately the standard horn on a Renault 5 GT Turbo is an extremely loud air horn, so it was quite an embarrassing situation. I did a brief toot to chastise someone who'd pulled out of a junction in front of me at 60mph, then had to pull over are rip the battery negative connector off.

However - these are relays that were fitted to the car in 1988, and survived around 20 years before developing faults, despite being in a very hot environment (I do drag racing with the car, so the engine is quite modified) and getting regularly wet. And 1980s Renault relays have no sealing rubber in the base. Decent modern relays with rubber sealed bases should have no problem, even in an offroad vehicle.

[flyingtech55]

ISTR you can get solid state relays. We had them on a machine where I used to work. There were some spares in the cabinet but they are probably long gone now. They looked like solid black plastic cubes about 1.5" square with the connections coming out of the bottom. They plugged into standard relay bases in the machines control panel.

TimR

[Denis G4DWC]

I've only just caught up with this thread.

I have been using Infineon High Side Driver PROFETs for several years both in professional Automotive and industrial applications and also for my own use.

Two of my Home built cars have quite a number of BTS555 and other Profets.

http://www.farnell.com/datasheets/317113.pdf

These are designed specifically for Automotive Lamp and Motor drivers. They can switch 160A though god knows how the small leads can withstand that sort of continuous current. On resistance is 2.5mR and is lower and more stable than a cheap relay contact. At 10A, dissipation is around 250mW and will just take the chill off it so it won't need a heatsink.

They are so simple to use. Volts in to Tab and Pin 3, output to load from Pins 1 and 5. Take Pin 2 Low (Sinks about 2mA) to turn on the load. So simple. Additionally Pin 4 is a Current source with source current dependant on load. By putting a resistor to ground you can develop a voltage proportional, within reason, to load current. Monitor this if you want with a Micro and you can elliminate the fuse if your daring enough.
Frankly they are almost bullet proof. A while back I designed these into units used in all cars in a recent motor racing series to drive high current inductive loads.

Sadly In small Quatities Farnell want £10 each but if you send me a PM Shaun I'll send you a sample to try if you are still in need.

HTH

Denis

[murphyv310]

Hi.
These BTS555 Profets look really good and I am going to give them a try on a couple of projects on my Audi 80.
Perhaps I might change my mind about relays
Trevor

[Sean Williams]

Hi Denis,

Many thanks for your kind offer - one I will avail myself of, as soon as I get a few minutes!

Cheers
Sean