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I'm working on a project in which we are using a bank of inexpensive 100 μF caps (up to 20 of them) to keep a microcontroller powered up long enough to do housekeeping for shutdown when the main power is disconnected, things like close SD card files, etc. Charging these capacitors is a challenge though, for a few reasons:

  • The device can accept an input voltage of 12-60 VDC
  • The device can be on a very very very long wire or not. For testing it's only on say 6 feet of 22 ga wire, but when in use we could be on the end of nearly 6000 ft of wire, the wire becoming a serious resistor in the circuit.

Currently my design has the capacitors storing the raw incoming voltage (12-60 VDC) and then all voltage regulation after that so we can get the maximum potential runtime of the microcontroller after the main power has been lost. I need to current limit the charging of the capacitors, but then not limit the current available to run the device or keep a current limiting resistor in circuit just burning away power as heat. I've done a number of simulations and haven't found a solution I like. system diagram

Simple Current Limiting Resistor

Sure, this would do the job, but it generates a lot of heat and does so especially when the rest of the device is doing tasks with high current demands. enter image description here Edit Schematic

MOSFET time delay

mosfet time delay circuit Edit Schematic

This also works, but not great for this application as the input voltage range varies so widely that the time constant that we need to set changes by an order of magnitude. The idea is to charge through the resistor, then after the time delay the MOSFET bypasses it.

Comparator delay

Probably the best option so far - set a ratio at which the comparator turns a MOSFET on to bypass the current limiter. Might need to add some hysteresis in here, but the basic idea is the same. enter image description here Edit Schematic

The question

So the question is - what am I missing here? Though the problem is complicated by a wide input voltage range and potentially high resistance from the wire, it seems like the solution should be a bit less complex. In general I expect the device to draw about 1-1.5 W during operation with peaks of another 0.5-1 W when some active things are happening.

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  • \$\begingroup\$ You need to show a diagram of the worst case cable scenario so that it's easier to understand; words won't do. \$\endgroup\$
    – Andy aka
    Commented Apr 10 at 21:29
  • \$\begingroup\$ Tip: when you use the editor toolbar CircuitLab button to draw your schematic you can use the Save and Insert button to insert a PNG of the schematic into the post along with an edit link. There's no need for a CircuitLab account. No screengrab. No grid. It also means that others can copy and paste your schematic for further editing. \$\endgroup\$
    – Transistor
    Commented Apr 10 at 21:31
  • \$\begingroup\$ Good suggestions! Added a system diagram and made the schematics editable incase someone wants to do so. \$\endgroup\$
    – geo_leeman
    Commented Apr 17 at 17:22

1 Answer 1

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I think a current limiter would be best for your application, there are many many different circuits, some based off of NPN's and inline: Help dimensioning resistors in a two NPN transistor current limiter circuit

But that doesn't help the resistor of the wire.

There are also ways to do current limiting with volatge regulators, but you have to 'burn' the power up resistively which probably won't work for your application.

Another way to do this is to use a DC-DC with current limiting that has a wide output range and control the voltage going into the caps, but then you lose power storage ability.

The best way would be to reduce the resistance of the wire. If that can't be done you could use a voltage sense power supply to negate the resistance of the wires: Source: https://www.pickeringtest.com/de-de/kb/hardware-topics/pxi-instrument-topics/using-the-voltage-sense-connections-on-a-power-supply

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  • \$\begingroup\$ We cannot limit the wire length or guarantee what it will be. 4 wire would require a lot of extra wire. We're not really concerned with what voltage gets to the bottom of the power lead, hence the wide supply range. The main thing is I want to current limit the cap charging, but once they are charged, remove whatever limiting is in place and let the device run (wire would be the largest current limiter then), unless the bank is later discharged. \$\endgroup\$
    – geo_leeman
    Commented Apr 17 at 21:09
  • \$\begingroup\$ There are designs for fet based current limters. \$\endgroup\$
    – Voltage Spike ♦
    Commented Apr 19 at 18:56

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