Blown fuse indicator design validation – Automotive

[Mr.B]

Hi all,

I would really appreciate some comments regarding my design below.

I have used Falstad to validate the design, however this simulator does not appear to allow me to enter all of the characteristics of the FET.

Purpose:
This is one channel of a 24 channel fuse board. It is to replace a classic car fuse board with a board that has modern blade fuses and a “Blown Fuse Indicator” LED for each fuse.
The indicator will only be on when the car ignition is on and the fuse is missing or blown.
The indicator will turn off if the fuse is intact.

The reasons for choosing the Toshiba SSM6P15FU are that it appears to be readily available from Mouser, it is a dual SMD package, reducing the number of parts I need to hand place, and hopefully I have read the datasheet correctly and it will do the job.
The purpose of the diode is to pass any voltage spikes caused when a fuse blows that is supplying an inductive load, such as the cabin air fan.
The resistors will also be 4 way SMD arrays to reduce part placement fatigue.

This is just a one off project for a car enthusiast friend of mine, but I want it to be reliable and last the test of time.

Link to the FET datasheet: https://nz.mouser.com/datasheet/2/408/SSM6P15FU_datasheet_en_20140301-1661037.pdf
Circuit example below.

Any comments greatly appreciated.

P.S. The greatest challenge of this project so far has been finding stock of high current wire to board connectors that are in stock without a 99 month lead time. I finally found some Phoenix at DigiKey, but the price would make anyone’s eyes water.

[NiHaoMike]

You're overthinking it. Just use LEDs with series resistors and 1N4007 diodes. Positive is switched and negative connects to load side of fuse.

[bdunham7]

2 things jump out at me.

First, I've never seen a diode across a fuse like that in an automotive environment and I'm not sure it is a good idea.  For spikes, perhaps a TVS to ground at some point after the fuse.

Second, if your FET were to somehow go molten and totally short out, would your 100R resistor catch on fire?  If so, can you simply use a higher value and still get the same result?

[bdunham7]

You're overthinking it. Just use LEDs with series resistors and 1N4007 diodes. Positive is switched and negative connects to load side of fuse.

That method assumes that there is at least some load on the circuit.  I suppose you could add a very small load just for that purpose, but that may create its own issues.

[Mr.B]

You're overthinking it. Just use LEDs with series resistors and 1N4007 diodes. Positive is switched and negative connects to load side of fuse.

I don't think I am.
LED with series resistor across the fuse has the following major drawback.
A non technical person trying to diagnose a blown fuse in the brake circuit, alone, cannot depress the brake pedal and see the LED at the same time.
Porsche fuse panel is under the front bonnet. 

[Mr.B]

2 things jump out at me.

First, I've never seen a diode across a fuse like that in an automotive environment and I'm not sure it is a good idea.  For spikes, perhaps a TVS to ground at some point after the fuse.

Second, if your FET were to somehow go molten and totally short out, would your 100R resistor catch on fire?  If so, can you simply use a higher value and still get the same result?

Thank you for your reply.
I do not know much about TVS diodes and will need to do some more research.
I may be able to get away with a 1K0 as the Gate Threshold Voltage is very low.
I will do some calculations.

[floobydust]

Littelfuse and others make automotive blade fuses with a built-in blown fuse LED indicator ATO Smart Glow Blade Fuse for example, given the cost they might be usable.

[bdunham7]

Something like this will likely suffice.

https://www.mouser.com/ProductDetail/Vishay-General-Semiconductor/SMBJ20CD-M3-H?qs=sGAEpiMZZMsItbzKdwyIe%252Bgj7Z%252BBsubrW%252BK5FZ7LTlB2aehgf2%252B3%2FA%3D%3D

But if you are only trying to protect the FET, then you should use gate protection right across the G-S terminals.  Automotive electrical is a rough environment.  There are automotive-specific self protected MOSFETS just for this environment and perhaps one of those would serve you better.

https://www.diodes.com/design/support/technical-articles/self-protecting-mosfets-deliver-improved-reliability-in-the-harsh-environment-of-automotive-applications/

[Mr.B]

Littelfuse and others make automotive blade fuses with a built-in blown fuse LED indicator ATO Smart Glow Blade Fuse for example, given the cost they might be usable.

Thank you for the suggestion, but they suffer from the same problem I mentioned above when using just an LED and series resistor across the fuse.

[Mr.B]

Something like this will likely suffice.

Thank you @bdunham7.
I will look further into both of those options.

[Monkeh]

You're overthinking it. Just use LEDs with series resistors and 1N4007 diodes. Positive is switched and negative connects to load side of fuse.

I don't think I am.
LED with series resistor across the fuse has the following major drawback.
A non technical person trying to diagnose a blown fuse in the brake circuit, alone, cannot depress the brake pedal and see the LED at the same time.
Porsche fuse panel is under the front bonnet.

A non-technical person shouldn't be trying to diagnose a blown fuse. What, exactly, is a little light telling them the fuse is blown going to achieve? Them inserting another fuse which will promptly blow?

[floobydust]

Littelfuse and others make automotive blade fuses with a built-in blown fuse LED indicator ATO Smart Glow Blade Fuse for example, given the cost they might be usable.

Thank you for the suggestion, but they suffer from the same problem I mentioned above when using just an LED and series resistor across the fuse.

But you can never sense a blown fuse if the load is off, as in your brake-light example. There is no current drain or "pull down" which all methods here rely upon.

[Mr.B]

Littelfuse and others make automotive blade fuses with a built-in blown fuse LED indicator ATO Smart Glow Blade Fuse for example, given the cost they might be usable.

Thank you for the suggestion, but they suffer from the same problem I mentioned above when using just an LED and series resistor across the fuse.

But you can never sense a blown fuse if the load is off, as in your brake-light example. There is no current drain or "pull down" which all methods here rely upon.

In my example, the brake light switch is between the fuse and the load.
So, yes, my circuit will indicate a blown fuse irrespective of the brake switch position.
As long as there is 12v on the load side of the fuse, the LED will remain unlit. Remove the fuse and the LED will light.
The 'pull down' is the 100K gate resistor.

Of course this only applies to circuits that have 12v on the feed side of the fuse, either always or when the ignition is on.
In the case of headlights for example, the fuse only becomes supplied when the headlight switch is on.
But unlike the brake pedal, this can be left switched on while inspecting the panel.

[JPortici]

You're overthinking it. Just use LEDs with series resistors and 1N4007 diodes. Positive is switched and negative connects to load side of fuse.

I don't think I am.
LED with series resistor across the fuse has the following major drawback.
A non technical person trying to diagnose a blown fuse in the brake circuit, alone, cannot depress the brake pedal and see the LED at the same time.
Porsche fuse panel is under the front bonnet.

A non-technical person shouldn't be trying to diagnose a blown fuse. What, exactly, is a little light telling them the fuse is blown going to achieve? Them inserting another fuse which will promptly blow?

I suppose we can counth mechanics as non technical. Se do aftermarket electronics and most of our customers are mechanics and oh boy... more than 90% of them don't even have a diagnosis, let alone a multimeter. They have never moved on from pre-OBD era and that level of electronics, their younger assistants get taught the same way and appear to be even more dull.
I can recall of at least ten occurences in the past couple of years at our workshop, of all places, in which they were diagnosing an issue, the first thing done is check the fuse and it appears okay but it's actually blown, because it didn't blow on the filament, on the window, it blew where you can't see it, with newer fuses being more prone to this type of failure.

**I also wouldn't use LEDs directly in series with the load. A lamp, maybe, but an LED not. High brightness surerly not. I wonder how many cold starts it would survive

[Mr.B]

**I also wouldn't use LEDs directly in series with the load...

I don't think anyone was proposing LEDS in series with the load...

[floobydust]

But you can never sense a blown fuse if the load is off, as in your brake-light example. There is no current drain or "pull down" which all methods here rely upon.

In my example, the brake light switch is between the fuse and the load.
So, yes, my circuit will indicate a blown fuse irrespective of the brake switch position.
As long as there is 12v on the load side of the fuse, the LED will remain unlit. Remove the fuse and the LED will light.
The 'pull down' is the 100K gate resistor.

Of course this only applies to circuits that have 12v on the feed side of the fuse, either always or when the ignition is on.
In the case of headlights for example, the fuse only becomes supplied when the headlight switch is on.
But unlike the brake pedal, this can be left switched on while inspecting the panel.

I have seen brake-lights switched on the high-side, or on the low side. So you need a micro-power sense of voltage drop across a fuse, load or no load seen.
It reminds me of firewords/explosives igniters that have loop detection circuits to show continuity or open-circuit.

[Mr.B]

I have seen brake-lights switched on the high-side, or on the low side...

I accept that may be the case.
For the sake of clarity, my friends 1976 Porsche 911 has all momentary and most other stuff switched after the fuse.
There are a couple of exceptions, like the headlights I mentioned above.

[JPortici]

**I also wouldn't use LEDs directly in series with the load...

I don't think anyone was proposing LEDS in series with the load...

You're overthinking it. Just use LEDs with series resistors and 1N4007 diodes. Positive is switched and negative connects to load side of fuse.

[Terry Bites]

I think a MOSFET is unnecessary here, a cheaper bipolar will do just as well. It still needs some protection against negative spikes and over voltage on the 12V bus. Here's an idea. As long as the base is pulled high, the transistor will be off. Circuit protection is a must both on the power and sense inputs.

[wraper]

I've never seen a diode across a fuse like that in an automotive environment and I'm not sure it is a good idea. 

Certainly not a good idea. If for any reason the diode fails short, or battery is connected in reverse direction, this effectively bypasses the fuse.

[bdunham7]

I think a MOSFET is unnecessary here, a cheaper bipolar will do just as well. It still needs some protection against negative spikes and over voltage on the 12V bus. Here's an idea. As long as the base is pulled high, the transistor will be off. Circuit protection is a must both on the power and sense inputs. (Attachment Link)

His specified MOSFET is $0.39 for a dual-package in single quantity.

An indicator circuit like this should not require crowbar protection sized for the entire power circuit--you're not redesigning the whole car.  It should be isolated from the power circuit with some resistance and that smaller circuit can be protected.  A failure or short in the indicator system should be of no concern to the car itself except perhaps for some small drain current.  And in the case of your circuit, it won't work without a load--which may not be there in many cases where you still would want a fuse blown indication--and if you add a pull-down load, you'll have a quiescent drain of some mA, which becomes unacceptable if added over 15-20 circuits.

I think the OP is more or less on the right track (excepting the diode across the fuse) and the only issue would be a false indication on circuits where the fuse is unpowered at times.

[Mr.B]

I've never seen a diode across a fuse like that in an automotive environment and I'm not sure it is a good idea. 

Certainly not a good idea. If for any reason the diode fails short, or battery is connected in reverse direction, this effectively bypasses the fuse.

Valid point, thank you @wraper.
Would a bidirectional TVS diode be the answer?

[Mr.B]

I think the OP is more or less on the right track (excepting the diode across the fuse) and the only issue would be a false indication on circuits where the fuse is unpowered at times.

Thank you @bdunham7.

Regarding false indication:
The supply to the indication side of the circuit from the ignition only applies to fuses that are either always powered from the battery or powered from the ignition.
Fuses that are switched prior to the fuse, such as the headlights, will have their indicator supply fed from the high side of the fuse.
That way indication can only happen when power is switched on to the supply side of the fuse.