I work with a few tracking devices and the newest model keeps popping. I've narrowed it down to possible short term current spikes in the trucks they are installed on. The device receives power from the OBD port or a three wire harness if the OBD port is unavailable.

To try fix the problem without any interference from the driver, I want to place an auto resetting fuse in series with the device and a capacitor in parallel to keep the device powered when the fuse trips.

The only information the manufacturer of the device gave me was this: At 24 V Operating Mode: 35-180 mA. Operating mode + IOX: Up to 2 A. Sleep mode: 3.0 mA.

The IOX is an additional connection for things like NFC tags, bluetooth tags etc. These are not always used.

Can anyone help me identify if this circuit will work to stop the devices dying while keeping them powered? If so, what size capacitor and what rating fuse would I need?

Thanks in advance.

  • 2
    \$\begingroup\$ Likely cause: The driver doesn't like that you put a tracking device in their truck, so the driver zaps it with a barbecue lighter. \$\endgroup\$ Commented Sep 10, 2021 at 12:48
  • \$\begingroup\$ A surefire way to avoid dirty power would be to use a discrete battery pack instead of sourcing power from the trucks electrical system. Also I bet that thing uses WAY less than 2A normally...that's a max worst case rating... It HAS to be because 24V at 2A is 50W. Any small plastic box would surely melt subjected to 50W continuous. Something like a 24V battery pack used for RC cars would do the trick and likely last many many hours. \$\endgroup\$
    – Kyle B
    Commented Sep 10, 2021 at 15:13
  • \$\begingroup\$ ^^^^But don't use a Lithium batt if you do that. They tend to explode which would likely pi$$ off your drivers LOL. A NiMH pack would be the right choice. \$\endgroup\$
    – Kyle B
    Commented Sep 10, 2021 at 15:19
  • \$\begingroup\$ Hi Kyle, thanks for your answer. The devices are often installed without the drivers and many employees' knowledge and hidden away as the companies want to track their vehicles to prevent any organised theft from within the company. This is why the device needs to operate continuously for years without any interaction from drivers or the hiring company. Unfortunately I don't think a battery would work unless it was constantly charged by the vehicles power source and had a system built in to prevent any electrical spikes from badly maintained trucks. \$\endgroup\$ Commented Sep 11, 2021 at 8:58

1 Answer 1


What makes you suspect current spikes? The thing that confuses me regarding this is that it's not the truck's 24V power supply that determines the current flowing through something connected to it, it's the thing itself, the tracker.

If the tracker wants 180mA, it demands 180mA, and the power supply (batteries, alternator or whatever) is not able to just "shove" 2A through it.

What the power supply can do is exceed the maximum voltage that the tracker can tolerate, which would indeed kill the tracker. In that case, the fact that current draw may have spiked is a symptom of the tracker already being dead.

It seems to me that what you need is not a current limiter, but better voltage regulation for a poorly designed tracking device. I'm also assuming that the whatever goes on in the third wire in the harness is not complicit in this failure.

If I'm right, then your next step should be to find out the absolute maximum voltage that your tracker can withstand, and the absolute minimum voltage that it will operate reliably from. With this information, an appropriate regulator can be designed.

  • \$\begingroup\$ Hi Simon, thank you for your answer. The third wire just goes to the ignition sensor to determine when the ignition is on when OBD is unavailable. The device measures things like the incoming voltage from the power supply and from that data it seemed unlikely that the voltage was the issue. I will however test it to see the maximum and minimum voltages it can withstand. \$\endgroup\$ Commented Sep 11, 2021 at 8:38
  • \$\begingroup\$ The problem with the trucks where I live is that a lot of companies would rather have a cheap, unreliable fix to any problems they experience with the vehicles rather than a pricier sturdy one. This has caused current/voltage spikes in the past but our older devices were more robust and could withstand those spikes better. From inspection of the new devices, it seems they've installed a new sensitive fuse that usually is the component to go. On a couple of others an IC has blown. From this I suspect its a sudden spike as the fuse hasn't blown yet rather than a sustained high voltage/current. \$\endgroup\$ Commented Sep 11, 2021 at 8:46
  • \$\begingroup\$ The other problem is I'm only permitted up to about $20 worth of additional components to stop the devices from blowing so I'm trying to figure out the cheapest way to prevent spikes to the device without causing it to lose power at any point. Unfortunately I'm no expert at electronics so any and all help will be hugely appreciated. If you would like anymore info I'd be happy to add on. \$\endgroup\$ Commented Sep 11, 2021 at 8:49

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