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I connected a dfrobot DC-DC Buck Converter 7-24V to 5V 4A to a 12 VDC battery via a switch, as per the supplier's diagram. When I turned the switch on the input of the converter fried.

It's the second one that's fried but the first one worked fine for a few uses. Eventually it fried when the switch was closed. The product is rated up to 24 V but on the back it says 30 V.

Removing the converter and using a multimeter connected to the supply, there are OL (overload) and 35 V readings before it settles to 12.96 V in about 1 sec.

Is this normal for a 12 VDC system or is it an artefact of the multimeter measurement? A 35 V spike would obviously fry the converter but why would a 12 V battery output almost three times its rating?

The wires are 11 A rated and there is about 1 metre between battery and converter via battery protect, fuse box and switch.

Everything else connected to the 12 V system is fine, including a buck/boost converter that drives a 10 W LED, another circuit that powers a USB socket and another one that powers a small 12 V LED.

Should I be using a Zener diode and resistor on the positive input?

Any advice greatly appreciated.

There isn't a schematic available for the product. All it says is:

The module integrates durable ceramic chip capacitors of large capacity on the ports and large inductor that can reduce heat generation

Wiring diagram:

enter image description here

I replaced the fried converter with an LM2596S variable DC-DC converter set to 5 V output and it works perfectly in the above circuit.

The fried input on the converter: enter image description here

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    \$\begingroup\$ Schematic of the buck converter please (questions on commercial products that don't have a schematic are likely to be closed). Also, a picture of your input wiring from battery to converter. \$\endgroup\$
    – Andy aka
    Commented Sep 4, 2022 at 10:37
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    \$\begingroup\$ You can't diagnose spikes with a multimeter ... especially if it's an autoranging one. \$\endgroup\$
    – user16324
    Commented Sep 4, 2022 at 11:21
  • \$\begingroup\$ thanks for the pointers. I've added wiring diagram and the blurb from the product page. Unfortunately there isn't a schematic available. \$\endgroup\$
    – codebrane
    Commented Sep 4, 2022 at 12:40
  • \$\begingroup\$ In engineering and in general, please remember to put a space between a quantity and its unit. So it's "5 V" rather than "5V". \$\endgroup\$
    – TonyM
    Commented Sep 4, 2022 at 13:08
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    \$\begingroup\$ I think that space is not mandatory and situation dependent. Some people agree that for example with a capacitor, it's more clear to type 4u7 then 4.7 uF. (Like on a schematic). \$\endgroup\$
    – RemyHx
    Commented Sep 4, 2022 at 14:41

1 Answer 1

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Put an electrolytic capacitor, or TVS diode, across the input.

The inrush overcharges the ceramic capacitors (which quickly lose capacitance as voltage rises, accelerating the overcharge*), until the regulator pops. This is driven by excess supply inductance, which for these components can be merely on the order of 10 cm of wiring length.

*This is a unique property of type 2 ceramic capacitors; if this phenomenon didn't occur, the overvoltage would be limited to a bit less than double the input voltage, which would still be safe in this case. Though a solution may still be desired in others.

The electrolytic dampens the resonance, drawing much more inrush current as a result (this will wear your switch), but keeping the voltage stable. Or the TVS clamps the excess, so should be chosen for 12 V nominal and clamping somewhat less than the 24 V maximum; a P6KE15A might do.

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  • \$\begingroup\$ thanks for that. The first time I tested one of these converters was with a smaller 12 V battery and about 10cm wires. The first one fried instantly. The second was fine for a few starts then it fried also, with the same wires. \$\endgroup\$
    – codebrane
    Commented Sep 4, 2022 at 18:05
  • \$\begingroup\$ I read up on TVS selection and thanks very much for pointing me in that direction. Now I understand what it's all about. \$\endgroup\$
    – codebrane
    Commented Sep 4, 2022 at 19:06
  • \$\begingroup\$ @codebrane You're welcome! \$\endgroup\$ Commented Sep 4, 2022 at 19:08

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