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Let's say I have a power source of 4.2V/1A. I know how to reduce the voltage using resistors, but if I would like to be able to 'divide' current in half for example, how can I do that using standard components?

My component is a battery, which should not be able to draw more than 150mA from the power source.

  • I know I can buy already built components for this, but I want to understand how to do this using only the basic components.
  • I know that charging battery is dangerous, but I do this daily by hand (from bench power supply with current limiter enabled), but battery is just an example. All I'm trying to do is to reduce the 'max current potential' from the power supply.

I'm familiar with 'voltage divider', I want to do exactly that, but only with current. Can I do this purely with basic components, such as diodes? Thanks!

0x29a
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  • If you are actually using this to charge a lipo battery -- a resistor in between the power supply and your battery is the simplest way to do it, but it's not active current limiting. Your current will be the highest when the battery is at the lowest voltage. – Bryan Boettcher Jan 04 '17 at 17:13

3 Answers3

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My component is a battery, which should not be able to draw more than 150mA from the power source.

You aren't looking for a current divider but rather a current limiter and this is an active device because, in principle, it doesn't restrict current into (or out of) a device until the "limit value" is reached.

So, trying to make one from passive components or diodes isn't going to be effective as a protection against over charging with too high a current.

Andy aka
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  • Thanks! I gave battery as an example (since it draws more than it needs in case of charging). How can I make a not-so-effective current limiter? Which component actually does the 'current trimming' in current limiter? or does it simply 'pulsate' the signal to reduce current once the limit is reached? – 0x29a Jan 04 '17 at 12:04
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    @0x29a it depends really on how efficient you want it to be. You can make quite a simple and easily made current source with a transistor or linear IC such as a LM317 (google the datasheet for ideas). But they are linear and are not very efficient. Otherwise there are switching limiters which 'pulsate' as you say. Slighly more complex but if you are willing to try you might find it interesting. – crowie Jan 04 '17 at 12:29
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    The simplest component is a resistor but this limits current by linearly reducing voltage as more current is taken so it can't really be described as an effective current limiter. A transistor and three resistors can provide reasonable current limiting but you will always lose open circuit voltage with this method. Using an op-amp, three resistors and a transistor can reduce voltage loss down to about 0.3 volts open circuit. – Andy aka Jan 04 '17 at 12:37
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Well, as others in this thread have already pointed out, a (decent) current limiter requires some active circuitry. A very simple current limiter using only one basic component, a JFET, is the following:

schematic

simulate this circuit – Schematic created using CircuitLab

That circuit limits the current into the load to the value of \$I_{Dss}\$ of the JFET. Just select a JFET having an \$I_{Dss}\$ of 150mA and you'll be fine.

There are a couple of drawbacks, though:

  • Current limiting is not very stable: due to its simplicity there will be small variations of the current with varying supply voltage.

  • Idss is not well specified and has wild tolerance: usually a datasheet will give you a (broad) minimum and maximum value, or sometimes just only a minimum. For example, the J107 has a specified minimum \$I_{Dss}\$ value of 100mA, and no maximum (see image below). So, in the end, you will end up selecting a specific specimen by hand using trial and error.

enter image description here

  • Power dissipation: JFETs are usually low power devices (<1W); if the voltage drop across the JFET multiplied by the current is higher than that (assuming a reasonable safety margin), you are out of luck.
LorenzoDonati4Ukraine-OnStrike
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2

There are two ways to provide a current-limited supply to charge a battery.

a) The current limiter way. Use an active current limiter. The simplest of these, if you have the voltage headroom, is an LM317, which maintains 1.2v between its output and adjust terminals. If you connect (for instance) 12ohms between them, it will limit at 100mA. Choose different value resistors to get between 5mA and 1A.

b) The poor-man's limiter, designed for battery charging. As your battery voltage doesn't change quickly, and as power supplies are often adjustable, a resistor of an appropriate value will limit current from a supply to the battery. As the battery voltage rises, the drop across the resistor will fall. This will reduce the current, unless you are monitoring it every few minutes and adjusting the supply up to compensate. Quick to do as a one-off, very tedious if you have to do it more than twice!

Neil_UK
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