Profets or SmartFets And Picaxe

[rmtucker]

Just wondering if anyone has done any experimenting with this or similar devices?.
http://uk.farnell.com/infineon/its436l2s/mosfet-profet-9-8a-to-220-5/dp/1426196

I want to use one of these to replace a fuse and relay in an automotive application.
As far as i can see all i require is a pulldown resistor between the picaxe and Profet.
The datasheet is here:
http://www.farnell.com/datasheets/85673.pdf

 

[hippy]

I want to use one of these to replace a fuse and relay in an automotive application.

Automobiles are safety critical systems and therefore so is anything connecting to them. The usual caveats apply. There are rules, regulations and laws which may be applicable and it is recommended such projects are only undertaken by those competent and qualified to do so.

 

[John West]

I haven't used them, but the specs certainly indicate they could do the switching job just fine as a relay replacement in a 12V system. (Unless you're switching an unusually heavy or inductive load.) Without further info on the nature of the load I couldn't say much more.

They certainly aren't inexpensive, however.

As far as a replacement for a fuse goes, it is NOT. Always use a fuse. When a fuse fails, it fails open, period. Semiconductors can fail short circuited as easily as open. They are NOT fuses and are not an equivalent substitute for them.

 

[Adamey]

Virtually all automotive manufacturers now rely heavily on smartfets, profets and similar. Relays sre still used for some high-current applications, but most loads in your car will use one of these devices. There's a reason why - they just work so well.

I think what you really need to be concerned with is providing a suitable power supply for the PICAXE to work in the harsh automotive environment and making sure any inputs/outputs are well protected if they need to interface to 12V circuits/switches.

As mentioned, fuses are still always used. You can have an "electronic" fuse using the diagnostic outputs of these devices to limit current in a circuit yourself, but you still need a regular fuse.

 

[Jaguarjoe]

They're not cheap but they solve a lot of problems. Be aware that their slow rise and fall times may throw a wrench into PWM activities. Can you leave the status output floating?

 

[rmtucker]

I intend to use these just as a Relay and fuse combined for switching,no pwm.
I have purchased one and have setup a bit veroboard and connecting blocks etc.
I am going to try and blow it up by attatching lot of amps and short circuits etc.
I am just going to connect an led to the status output for feedback
My electronics knowledge is very limited,so after reading the pdf i still can't figure some stuff out.
Maybe someone could take the time to explain:

1. What minimum ampage will this profet droput at(my intension was to connect some big bulbs and a meter and find out).This does not have a current sense output so it must dropout at a preset level.

 

[Andrew Cowan]

At 25 degrees C, the current limit is between 39 and 58A, typically 51A.

According to the datasheet, the limit is 'shut down by overtemperature, reset by cooling'. This means it could take a while (in ms) to heat up and cut out.

Very important - you MUST use a fuse, especially in automotive applications. The current limiting won't work if the device is damaged/faulty - a fuse MUST be in the circuit as well.

Also note the maximum current for the status pin is 5mA - connect an LED with a 330 ohm resistor, and you may well blow the chip up.

 

[rmtucker]

Andrew it looks like i have bought a profet that is far too big??
I was looking for one that would act something like a 10 amp fuse for indicators (4 x 21w bulbs) .
I was looking at the nominal load current of 9.8amps oops:

 

[Jaguarjoe]

Here's some incandescent lamp stuff for you:

http://www.allegromicro.com/en/Products/Design/an/an295012.pdf

 

[MartinM57]

I was looking for one that would act something like a 10 amp fuse for indicators (4 x 21w bulbs) .

The best thing that acts something like a fuse is...err as stated previously...a fuse.

These things heat up when there is a short circuit (probably due to the high amps), go pretty much open circuit (but not completely?), cool down (due to no amps), conduct again, heat up as there a short circuit (probably due to the high amps), go pretty much open circuit....

I wouldn't consider them as a fuse replacement - AFAIK even modern cars haven't abandoned the fuse box yet.

 

[premelec]

PTCs?

A PTC is a type of resettable fuse that has a thermal time constant but can work well in many instances - at least as well as thermal breakers I've met in some automotive applications. PTC is "positive temperature coefficient" thermistor... Their main shortcomings are not being good at high voltages and currents at the same time... anyhow if you don't want a one time blow type situation they can work well [fault / voltage must be removed for them to cool and reset].

 

[Adamey]

I'm currently building a power distribution system for my car which uses networked modules to operate everything from the lights to accessories. I'm using these to control my various loads (similar to a profet or smartfet). The reason I wanted devices with current monitoring is explained later.

Quad Switch PDF
Dual Switch PDF



In my experience with automotive, even having a fuse isn't 100% foolproof. I have seen countless vehicle fires or damaged wiring because of a partial short.

Say you have a circuit that usually draws about 5A, but can surge up to 20A on startup (lights, motors). The circuit is protected with a 20A fuse to handle the surges. Then you get a short or damaged wiring harness and get a draw of 15A continuous. The 20A fuse doesn't blow, but the conductor and connectors start to get hot because they weren't designed to handle that much current continuously. Then something melts (plastic connector housings are common).

This is where a profet works great. The designer knows the circuit draws 5A normally with a higher initial surge. Using the current feedback of many of these devices the module can monitor the current used and shut off the circuit if it sees something abnormal (like a continuous draw well above 5A).


That said, always use a fuse. It's your primary device for protection. The profets abilitiy to protect itself from current/temperature or short circuits is your secondary circuit protection. The combination of the two makes for an extremely robust (and safe) circuit protection system.

 

[MartinM57]

always use a fuse[/B]. It's your primary device for protection. The profets abilitiy to protect itself from current/temperature or short circuits is your secondary circuit protection. The combination of the two makes for an extremely robust (and safe) circuit protection system.

Nice. I like that...

 

[rmtucker]

The problem with some of the devices discussed here is that they require 2 pins on the picaxe,one for control and one for current sense.
My hope was to fit devices that simply shut down at 10amps,16amps,or 30 amps and to give some kind of feedback led.
I will of course after seeing the advice,fit fuses.

Would the fuse be fitted on the input of the profet or the output?.Or would it matter?.
If the status pin can only handle 5ma,what would i need to put an led in there for feedback?

Thank you everyone for the advice and information.

 

[kevrus]

I would suggest, first and foremost, to cope with differing loads, it is important to select the fuse with the correct characteristics, anti-surge/time delay fuses are available, along with motor rated fuses to deal with surges and mag inrush. Semiconductor fuses for fast acting responses to protect solid state devices.

 

[BeanieBots]

The fuse must be on the INPUT to your circuit.
The purpose of a fuse is to prevent the cable or supply from burning during a fault condition. It is NOT to protect your circuit. Your circuit is responsible for its own safety.

A PTC is NOT to be used as a replacement for a fuse. It can however, be used in conjunction with a fuse to avoid nuisence trips. However, it should NOT be used in circuits which involve movement because when it resets, it can result in "unexpected movements" which is a safety issue.

There is legislation which covers all of these issues.
Please check with your local regulations for the country you are in or which country your 'product' is to be used/sold in before commiting to any designs which utilise over current protection.

 

[Adamey]

I would suggest, first and foremost, to cope with differing loads, it is important to select the fuse with the correct characteristics, anti-surge/time delay fuses are available, along with motor rated fuses to deal with surges and mag inrush. Semiconductor fuses for fast acting responses to protect solid state devices.

The main problem with this is that automotive fuses only come in a very small number of configurations. I could see confusion with the owner of a car if they had to go to a store and select from several fuse types along with several amperage ratings.

The automobile is probably the only consumer product where the owner would be responsible for changing their own fuses on a regular basis. I don't ever recall changing fuses on my home consumer electronics, though I know most of them (especially my theatre equipment) have them installed.

Manufacturers always have multiple warnings in the owners manual about making sure to replace the fuse with one of the same amperage rating. Yet people who have a fuse repeatedly blowing will almost always put in a bigger fuse. Couple that with the fact that the fuse box is rarely labelled and you need to look at a legend in the manual to see what fuse should belong and it's easy to see people putting in a wrong fuse. Usually someone will put in the same size fuse they pull out, without checking to see if someone else put in the wrong size earlier.

This is the kind of stuff an automotive electrical system must put up with - owners changing the parameters of your circuits.

Now it makes sense why manufacturers would want a profet so they could more closely monitor their protected circuits.

 

[Adamey]

The problem with some of the devices discussed here is that they require 2 pins on the picaxe,one for control and one for current sense.
My hope was to fit devices that simply shut down at 10amps,16amps,or 30 amps and to give some kind of feedback led.

How many devices do you need to control? There are "profets" out there which can control multiple loads over a serial interface.

 

[rmtucker]

I have 8 outputs to manage.
I am looking for 6 off at 10amps,1 off at 16amps,1 off at 30amps.
The purpose of this circuit is to control all the electronics on my trike.

It controls:
1. Left turn Signal
2. Start/Kill
3. Horn
4. Right turn signal
5. High/low beam
6. Sidelamps
7. Headlamps
8. Aux

The unit has a built in alarm.
Self cancelling Indicators.
Built in hazards.
Dallas one touch keys in place of Ignition.
I have the prototype programmed and running,i only have the output electronics to sort out.

To be honest i would have used good old relays etc,but i was looking at the size of the finished article.

 

[Adamey]

A PTC is NOT to be used as a replacement for a fuse. It can however, be used in conjunction with a fuse to avoid nuisence trips. However, it should NOT be used in circuits which involve movement because when it resets, it can result in "unexpected movements" which is a safety issue.

LOL. You should tell Jaguar that because I have seen them use a thermal breaker in many situations, especially things like power windows.

 

[Adamey]

I have 8 outputs to manage.
I am looking for 6 off at 10amps,1 off at 16amps,1 off at 30amps.
The purpose of this circuit is to control all the electronics on my trike.

It controls:
1. Left turn Signal
2. Start/Kill
3. Horn
4. Right turn signal
5. High/low beam
6. Sidelamps
7. Headlamps
8. Aux

The unit has a built in alarm.
Self cancelling Indicators.
Built in hazards.
Dallas one touch keys in place of Ignition.
I have the prototype programmed and running,i only have the output electronics to sort out.

To be honest i would have used good old relays etc,but i was looking at the size of the finished article.

I think you should still use a relay for the horn - even new cars still rely on a relay for this high-current item. What is the Start/Kill output for? If it's what I think, then a relay is probably better for this as well. After that, everything else is a great candidate for a Profet.

What PICAXE were you using? I know you're trying to save space by getting rid of relays, but if you have enough space to mount 7 TO-220 devices then surely you can squeeze in a 28X2. Then you'll have plenty of inputs/outputs to work with.

I'm not sure if the status output of a Profet could be used to drive an LED directly. Also, the Profet you listed won't change the status for overcurrent - only for no-load or temperature (at least according to the truth table I saw).

It would make more sense to me to have the status output go to your PICAXE and let it decide to do when there's a problem (turn on a general warning light or even flash a light in a specific sequence to indicate the problem - similar to early fuel injected cars flashing the check engine light in a pattern to show codes).


Sounds like an interesting project. I can see the appeal of having one box control all the electrics in your vehicle, and making simple changes to the software to change how everything works (like the speed of your signal flashing).

 

[rmtucker]

I am using the 28x2 picaxe.
I did not mention above the 9 inputs i am also using.

Button inputs:
1.Right turn Signal
2.Start/Kill
3.Horn
4.Left Turn Signal
5.Lights
6.High/Low beam

Other inputs:
7.Dallas Probe
8.Shock sensor Stage 1
9.Shock sensor Stage 2

As you can see i am rapidly running out of in/out pins.
I am also using both timers in the 28x2 so if technical is watching they both get used.

 

[Andrew Cowan]

I'm not sure if the status output of a Profet could be used to drive an LED directly. Also, the Profet you listed won't change the status for overcurrent - only for no-load or temperature (at least according to the truth table I saw).

As far as I understood the datasheet, it detects overcurrent by the temperature rise - ie there is no seperate current limiting feature.

Andrew

 

[Jaguarjoe]

LOL. You should tell Jaguar that because I have seen them use a thermal breaker in many situations, especially things like power windows.

I poked around the 'net last night, a whole lotta car manufacturers use breakers in lieu of fuses for their powered windows. Does anyone know why?

 

[Pauldesign]

I poked around the 'net last night, a whole lotta car manufacturers use breakers in lieu of fuses for their powered windows. Does anyone know why?

Irrespective of fuse and breakers applications, i think it's just because of the fundamental reason that fuses get destroyed when overloaded whereas breakers just stripped and can just be push back ON, hence no replacement needed.

Well, it's always good to diagnose or know the reason for the stripping before pushing the breaker back ON otherwise you'll keep pushing back on forever.

 

[Adamey]

^ I don't ever recall seeing a breaker in a car that could be reset manually - usually they reset automatically after cooling off. They are also often in a location you couldn't access anyway.

As for windows I think it's to prevent damage. In case the window is stuck or frozen shut.

 

[Pauldesign]

^ I don't ever recall seeing a breaker in a car that could be reset manually

You're right, may be in Canada.

 

[hippy]

As for windows I think it's to prevent damage. In case the window is stuck or frozen shut.

That would be my understanding, to handle temporary fault conditions and there's probably a higher rated fuse for permanent fault conditions. When the trip resets itself the window winder will not be active and/or the window is no longer stuck.

If the fault still exists and the circuit is active you'd end up with trip, cool, reset, trip again cycling. Some voltage regulators do that with shorts and overloads as thermal protection cuts in, regulator cools, tries again.

 

[rmtucker]

I did some simple testing with the profet today,Tried to let some smoke out?
Connected it up to a 55w car lamp with no heatsink,No problem at 4amps.
Connected 2 x 55w bulbs with no heatsink and it started flashing after about 5mins.
Put a small piece of pcb board about 1" x 1/2" in as a heatsink.
Still 2 bulbs,without any problems.

Tried 3 55w bulbs at 12 amps but cut out after 5mins and started flashing.
So it's all about the heatsink and heat dissapation.

Further tests to follow.
The only problem is that the heatsink is going to be so big that it will take up the same room as a relay?

 

[Adamey]

^ Automotive manufacturers have little in the way of heatsinking. What I see more often is adding additional devices instead of running multiple loads per device.

When they do use a heatsink, it's usually just the case itself or a "rail" with all devices mounted side-by-side and sharing a single long heatsink. I've also seen the PCB itself used as a heatsink with large planes on both sides of the board.

 

[John West]

As long as you stay within the voltage and current limits on this sort of app it is indeed all about the heatsink.

While vehicle manufacturers spend all their time trying to figure out how to knock off 2 cents worth of material, someone doing their own design can do the job right, and add in that extra 2 cents worth of material, perhaps even three cents worth. It keeps the active devices very happy.

As for the window circuit breakers, they are indeed essential to keep the mechanism from either causing damage or self destructing when the load is too high. Fuses would need to be replaced on a regular basis, especially in cold weather climates where everything often freezes up.

There's also the possibility of arms (or heads) getting stuck in closing windows. It wouldn't do to allow too much current to flow too long in such circumstances. I'd expect there also is (or will very soon be) electronic current limiting on all such circuits, in addition to circuit breakers.

I imagine a self-resetting circuit breaker is the cheapest reliable solution to such problems.

 

[premelec]

Just noticed a product announcement from Diodes.com ZXMS6006DT8 - dual FET with protection... aimed at the automotive trade I think...

 

[Adamey]

^ That's a dual low side switch for things like engine and transmission modules. In vehicles, most loads are high side (like your lights) with a local chassis ground. Computer modules like engine ECU's use almost exclusively low-side devices where things like injectors, ignition coils and relays are provided with 12V and the ECU provides the ground.


John: They've had current sensing in power windows for some time now, depending on the manufacturer (I know VW/Audi did this extensively at least 10-12 years ago). They use it to implement anti-trap (person getting stuck in the window). Not only do they stop the motor, but they also apply 1 second of reverse to open the window back up.

As well, many automatic one-touch windows actually count the commutation pulses of the motor. To initially calibrate the window it's moved from fully closed to fully open so the total number of pulses can be counted. Current sensing is used to determine end stop positions during calibration. After calibration, any current increase that happens before the pulse count indicates fully open/closed indicates a fault (when closing, anti-trap is also activated). Some motors also use hall sensors for this as well.


As to whether or not to use a heat sink, I always prefer to use one. But I see manufacturers choosing to use a higher-rated device or multiple devices with only the PCB as the heatsink. They must have their reasons, as I'm sure they put a lot of design effort into their product. Some of these decisions (like using multiple Profets) are actually more expensive than a simple relay, so I don't think cost (saving money) is the only issue.

One thing I've noticed in the datasheets for many of these devices (which are often surface mount) is specs for the PCB layout and the amount and size of copper planes needed to provide a heat sink. The original poster is using TO-220 devices which don't apply, but I can see the entire automotive electronic industry moving to surface mount as devices with through hole leads are becoming rather scarce.

This actually initially put me off on my own project as I wasn't comfortable with surface mount (soldering I don't mind, but devices with heat slugs or pads underneath need to be done in an oven) and was trying to avoid using it. I've now resigned myself to the fact that I will need to become proficient with surface mount if I want to use the latest automotive grade devices in my project.

 

[papaof2]

As to whether or not to use a heat sink, I always prefer to use one. But I see manufacturers choosing to use a higher-rated device or multiple devices with only the PCB as the heatsink. They must have their reasons, as I'm sure they put a lot of design effort into their product. Some of these decisions (like using multiple Profets) are actually more expensive than a simple relay, so I don't think cost (saving money) is the only issue.

Weight is another issue: less weight = more mpg. With the fleet standards for fuel economy getting higher, using two devices or a higher rated device instead of a single device with a heatsink saves maybe an ounce or two of weight. Doesn't sound like much, but the manufacturers are looking for any saving.

Consider Toyota's "built-in bedliner" ads - that's just putting a positive spin on the fact that they have replaced the traditional metal pickup bed with a plastic one. Are the changes making a difference? Yes - my 2008 Tacoma is bigger than the 1989 Nissan it replaced. The engine is slightly larger (2.7L vs 2.4L) and the transmission is a 4 speed automatic vs the 5 speed manual in the older truck. City fuel usage is comparable (~20mpg for both) but the highway fuel usage is much improved (25-28mpg new vs ~20mpg old).

John

 

[rmtucker]

Would i require any protection between the picaxe and logic input on the profet?
At the moment i only have a pulldown resistor.

And what circuit would i req to drive an led from the status pin which according to the datasheet will only handle 5 ma.?

 

[Adamey]

^ How many LED's? A simple logic gate like a hex buffer of hex inverter should do the trick.

A 1K resistor between the Profet and the Picaxe should be good.

 

[Pauldesign]

And what circuit would i req to drive an led from the status pin which according to the datasheet will only handle 5 ma.?

I'm a bit confused what will only handle 5mA. Is it the profet or LED?

Critics will say, my below explanation is pedantic and trivial and yes it is but it's worth knowing esp for rookies in electronics.

If is an LED and except you're using a specialized type, standard LED (be it the 5mm or 3.5mm in size) operates between 5mA and 25mA, with 15mA being the nominal current value. There is no standard LED resistor as it all depends on the value of the voltage source connected to the LED and the colour of the LED.

E.g If the logic or DC voltage is 5V, then according to voltage divider rule, you'll need 600 ohms (E-series 560 or 680 ohms) resistor to limit the current to 5mA as per the datasheet (assumed red LED with Vf of 2V).

Always bear in mind and although not very critical, different colour of LEDs requires different quiescent voltages (with IR and blue/white LEDs requiring 1.5Vf and 4Vf resp.) and current (which will be indicated in the data sheet) to place then into forward conduction.

Therefore, by using the LED datasheet, the value of the limiting resistor (though mostly thumb suck) should be calculated based on the logic level or voltage sourced to the LED.

Likewise, if your logic or voltage source to the LED, is outputing 3.3V/5mA, then, you need no limiting resistor if the LED used is green.

Don't be surprised if you replace a red LED with a green LED and it doesn't work. (well, it might not just be dimmed though).

 

[Andrew Cowan]

Most high-intensity LEDs will be quite bright at 5mA.

For a red LED, Vf~1.8v.

5-1.8/0.003 = 1K resistor should be fine.

 

[rmtucker]

Yes it was just 1 led.
I will try the above 1k Resistor and Led.
I touched on this earlier but i was wondering if it would be feasable to put the fuse ie 10amp fuse on the output of the smartfet?.
The reason for this being that i wanted to feed all the smartfets through a Positive Busbar/Heatsink,so it would be easier to fuse the output.

 

[hippy]

I touched on this earlier but i was wondering if it would be feasable to put the fuse ie 10amp fuse on the output of the smartfet?.

But ask yourself what the purpose of the fuse is; to protect against load failure ( isn't that the smartfet's job ) or to protect against failure of the smartfet ?