Simple electronic fuse

Posted on Monday, September 3rd, 2012 in part review by DP

Sjaak shared this simple electronic fuse design in the forum:

This is perhaps one of the simplest electronic fuse circuit one can make. The circuit uses only one transistor, one SCR, one push button switch and two resistors.

Via the forum.

This entry was posted on Monday, September 3rd, 2012 at 1:00 pm and is filed under part review. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.

5 Responses to “Simple electronic fuse”

  1. Drone says:

    Where is the link to the post on the DP Forum? I can’t find a search function on the DP Forum and a site search for Sjaak’s post text turns up nothing.

    • Sjaak says:

      It wasn’t in the public part of the forum. It was posted in the ATX v2 thread (improved ATX breakout board) as an idea to implement. We haven’t discussed it yet, but it seems simple enough.

      Interested in your findings in testing it.

  2. philwatcher says:

    Eh, I am not sure if this will work reliably. It seems as if this relies on some source impedance that limits the total current so that the transistor can “suck” it away from the SCR.
    Also, I don’t see how the voltage across R1 could drop below ~700mV while the transistor conducts.
    It seems to me as if this requires a slow transistor that will keep conducting long enough for the SCR to reliably turn off even though I_base is dropping simultaneously.

    Additional notes: the SCR turn-off current has to be above (base current plus residual R1 current).
    Also, the transistor must be able to withstand the peak supply current during switch-off.

    I have not tested this circuit yet, which I will do later today. Maybe it does work, which would be really neat (a use for some thyristors collecting dust here), but I don’t see how.

    • Drone says:


      Yep… At first-glance I’ve got some issues with this circuit. e.g., dV/dt issues (snubber helps?), and a few other concerns. That’s why I was asking about the Sjaak’s Forum post, to see if there was a thread discussing this circuit.

      BTW, there is a model lib that includes the SCR part shown in the schematic. Look up the part on the ST Web site, the library is a free download. Again, at first-glance, the ST library looks like it will work OK with LT-SpiceIV (read the model for sim limitations, try the default SCR symbol in LT-Spice). The 2N3055 NPN part is native in the current release of LT-SpiceIV, and happens to use an ST-Micro model.

      So this should be pretty easy to simulate without having to smoke solder, and perhaps smoke other stuff on the bench (guess that’s why they call this Dangerous Prototypes.. he he).

      This site I bookmarked long ago (still working at post time) shows a simple circuit that can be used to verify the ST SCR model operation in Spice:

      Best Regards, David

  3. mike says:

    Assuming this is intended to be for DC and is connected in series with the positive lead, with the load on the right side, I do not see how this could work as a “normally on” device that cuts off above some level of current (which is how I would define a “fuse”). Once the SCR is triggered by S1, Q1 will stay on as long as enough current flows to keep the SCR on. There will be VsatQ1 volts across Q1. As current increases there is no mechanism to turn off Q1 and it will eventually fail shorted when exceeding Q1 max Ic or power dissipation.

    This is more of “a way to use a momentary pushbutton switch as a latching electronic switch” in my view.

    I agree with the other comments, model it in Spice or build it before posting it.

    Good luck.

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