Lm3914 as volt drop across a automotive fuse.

[Gazza836]

Hi, apologies if this is posted in the wrong thread. I'm looking to use an lm3914 chip to monitor the voltage
Drop across an automotive fuse and drive a LED away to show any changes in current across the fuse.
Any help with the project would be much appreciated.

 

[BradtheRad]

* Depending on which fuse you're talking about, expect voltage changes across it to be tiny. You probably need to build an amplifier which applies gain to the input in order to bring it up to a range that affects 3914 operation.

* It matters how you supply power to your project. It depends on which supply rail the fuse is closer to. And 3914's outputs sink current, not source it.

I've built several voltmeters using 3914 IC's. Both for automobile and house voltage.

 

[Gazza836]

Thanks for your message.
Yes I figured I'd need ti include an opamp in the project. The reason I'm looking into this route is because I have an issue with my infotainment system on my car which is intermittent. The system uses fibre optics to link about 8 modules and one or more of them are failing. No useful doc's to point to a particular module though. So I'm looking to use a led array for each module to see which one is failing. It would be a doddle if it failed completely but it recovers within a seco d or two.

 

[BradtheRad]

If your vehicle is like the majority...
Automotive fuses usually are located close to the positive supply rail. Appliances all over the car draw power from a wiring harness, then connect directly to 0V ground.

It may not be simple to detect small voltages near the positive rail. You may need a certain type of op amp.

Expect it to be tricky when working around the fuse. Heat from soldering could dislodge nearby components.

 

[danadakk]

Keep in mind an automotive fuse not considered a precision shunt.
You could do a cal routine with a substitute accurate load when you
install, or add to switching a known load and use the incremental current
as a gain correction factor.

Or just simply consider the readings relative and use that
as indication of fault.

 

[crutschow]

You could use a high-side current-sense IC such as one of these to generate a signal for the LM3914.

 

[Gazza836]

If your vehicle is like the majority...
Automotive fuses usually are located close to the positive supply rail. Appliances all over the car draw power from a wiring harness, then connect directly to 0V ground.

It may not be simple to detect small voltages near the positive rail. You may need a certain type of op amp.

Expect it to be tricky when working around the fuse. Heat from soldering could dislodge nearby components.

Your comment on 0v ground has given me an idea to measure on the ground circuit of each module instead of the positive feeds. It would eliminate the issue of the variable voltages in an automotive system due to the modern smart charging systems the power rail could vary by around 3v.

 

[Gazza836]

Keep in mind an automotive fuse not considered a precision shunt.
You could do a cal routine with a substitute accurate load when you
install, or add to switching a known load and use the incremental current
as a gain correction factor.

Or just simply consider the readings relative and use that
as indication of fault.

I was thinking about connecting both sides of the fuse and use the feed ide of the fuse as the reference part of the circuit to try and eliminate the issue of the variance in voltages found in automotive systems. What do you think about measuring on the ground side of each module instead ? Would this not reduce the issue of the variation in voltage from the smart charging system.

 

[betwixt]

Ground sides are typically bonded or bolted to the vehicle chassis in a nearby convenient place so connecting there might be difficult.
You can do it 'high side' if you suitably drop the voltage on both sides of the fuse with a resistive divider then amplify the difference between them to counter the overall voltage division and the scaling you need to drive the LM3914.

Don't forget you need to cater for peak load dump voltages across an open fuse as worst case.

Brian.

 

[danadakk]

To betixt's point : google "load dump automobile pdf", lots of ap notes on the topic

 

[BradtheRad]

It may be easier (and more practical) to build an automotive voltmeter. That was my first experiment with a 3914 IC when Radio Shack first carried them decades ago. Ten outputs to ten led's calibrated in steps of 1V (overall range 7V to 16V). I plug it into a cigarette lighter socket using a plug from my junk box or Radio Shack.

Tells me running V, idle V, cranking V, charging V. Response is instant and undamped as volt level droops momentarily starting the engine. Analog or digital meters cannot match that speed.