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The board connected to power supply with 50-100m cable. There is C1, 1000uF 50V capacitor on input of board, next to power connector. Schematic of power circuit is below. I think it is for bypass or decoupling. It is really big in physically, so i think the purpose of it and any different solution instead of 1000uF cap. Text

Also, 20-30 board can be connected in same power line sometimes. All boards are same. Has any impact on purpose of that cap?

Thanks.

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  • \$\begingroup\$ It's weird to put D1 after C1. D1 will not protect C1 against polarity inversion at the input... \$\endgroup\$ – bobflux May 30 '20 at 18:37
  • \$\begingroup\$ this is a typical buck converter scheme, except from the D1 diode, between C1 and C2. Caps serve no reason other from stabilizing the bias voltage. Sometimes C1 is a "slow" electrolytic cap and C2 a "fast" ceramic or tantalum cap. \$\endgroup\$ – thece May 30 '20 at 19:01
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    \$\begingroup\$ @peufeu You are right probably. But I saw reverse polarity didnt blow up that cap. I dont know why but I will try to figure when I have time for it. Thanks for remark. \$\endgroup\$ – ExpinElectronics May 30 '20 at 19:31
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One of the purposes of C1 is to help damp-out input voltage overshoot. The 50 to 100m cable has a lot of series inductance. If you were to put, for example, a 10uF ceramic cap in place of C1 you will probably have a very large voltage overshoot when you first connect the regulator. Very possible to blow up the regulator first time you plug it in.

I do not know if 1000uF is needed. But if you make any changes, make sure you test for overshoot. Electrolytic caps generally don't overshoot as much as ceramic because their effective series resistance provides damping. It is also possible to add a resistor in series with a ceramic cap to get the same effect (but you may have trouble finding a 1000uF ceramic cap!). Also, if you add a fuse or PTC to the circuit, that may provide some damping. The amount of resistance needed for damping may less than an Ohm. Simulation can be done if you have very accurate models. Otherwise you can test it and monitor the overshoot with an oscilloscope.

Application Note 88 by Goran Perica of Linear Technology goes into a lot of detail about input voltage overshoot.

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    \$\begingroup\$ Link is great. Also, PTC idea. I was thinking about putting TVS and PTC before big cap to improve this circuit. link \$\endgroup\$ – ExpinElectronics May 30 '20 at 22:01
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30 power supplies like this taking their input from the same about 24VDC voltage source can cause something that you have not expected. Buck converters take current pulses which all are as strong as the load current at the +5V output. The total current is the sum of the pulses of all regulators. They are not synchronized so in theory there can be moments when every regulator wants current at the same time.

My suggestion: Do a simulation where the feeding lines are realistically simulated and let there be a bunch of current sources which take realistic pulses. Then you can see if the capacitor can be reduced.

Placing an inductor in place of D1 can smooth the input current pulse, but the total effect with cables and 30 regulators must still be simulated.

C2 has manufacturer's suggestion in TI's datasheet, do not omit it.

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a good value for dampening is sqrt( L /C); this produces Q = 2, so has some ringing.

With 10 uh and 1,000 uf, you have sqrt (10/1,000) == sqrt (0.01) = 0.1 ohm.

And you need several times that, to avoid all overshoot.

Plus this assuming 10uH.

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