2×AA/USB: OK!

Whereas an inner, rechargeable lithium battery is normally the very best answer for moveable equipment these days, there are nonetheless occasions when utilizing replaceable cells with an exterior energy possibility, in all probability from a USB supply, is extra applicable. This DI exhibits methods of optimizing this.

Wow the engineering world along with your distinctive design: Design Concepts Submission Information

The standard method of mixing energy sources is to parallel them, with a collection diode for every. That’s positive if the voltages match and a few lack of efficient battery capability, owing to a diode’s voltage drop, could be tolerated. Let’s assume the equipment in query is one thing small and hand-held or pocketable, in all probability utilizing a microcontroller like a PIC, with a battery comprising two AA cells, the choice of an exterior 5 V provide, and a step-up converter producing a 3.3 V inner energy rail. Easy steering diodes used right here would give a voltage mismatch for the exterior energy whereas losing 10 or 20% of the battery’s capability.

Determine 1 exhibits a significantly better method of implementing issues. The exterior energy is pre-regulated to keep away from the mismatch, whereas energetic switching minimizes battery losses. I’ve used this scheme in each one-offs and manufacturing items, and at all times to good impact.

Determine 1 Pre-regulation of an exterior provide is mixed with an virtually lossless swap in collection with the battery, which maximizes its life.

The battery feed is managed by Q1, which is a reversed p-MOSFET. U1 drops any incoming voltage down to three.3 V. With out exterior energy, Q1’s gate is extra unfavourable than its supply, so it’s firmly on, and (virtually) the total battery voltage seems throughout C3 to feed the increase converter. Q2’s emitter–base diode stops any present flowing again into U1. Other than the inner drain–supply or physique diode, MOSFETs are virtually symmetrical of their foremost traits, which permits this reversed operation.

When exterior energy is current, Q1.G will probably be biased to three.3 V, switching it off and successfully disconnecting the battery. Q2 is now pushed into saturation connecting U1’s 3.3 V output, much less Q2’s saturated ahead voltage of 100–200 mV, to the increase converter. (The 2N2222, as proven, has a decrease V_SAT than many different varieties.) Observe that Q2’s base present isn’t wasted, however simply provides to the increase converter’s energy feed. Utilizing a diode to isolate U1 would incur a higher voltage drop, which might trigger issues: new, top-quality AA manganese alkaline (MnAlk) cells can have an off-load voltage effectively over 1.6 V, and if the voltage throughout C3 have been a lot lower than 3 V, they might discharge by the MOSFET’s inherent drain–supply or physique diode. This association avoids any such issues.

Reversed MOSFETs have been used to present battery-reversal safety for a few years, and naturally such safety is inherent in these circuits. The physique diode additionally offers a secondary path for present from the battery if Q1 is just not totally on, as within the few microseconds after exterior energy is disconnected.

Determine 1 exhibits U1 as an LM1117-3.3 or comparable kind, however many extra trendy regulators permit a greater answer as a result of their outputs seem as open circuits when they’re unpowered, somewhat than permitting reverse present to circulate from their outputs to floor. Determine 2 exhibits this implementation.

Determine 2 Utilizing newer designs of regulator implies that Q2 is not mandatory.

Now the regulator’s output could be linked on to C3 and the increase converter. Some gadgets even have an inner swap which utterly isolates the output, and D1 can then be omitted. Regulators like these might in precept feed into the ultimate 3.3mV rail straight, however this may truly complicate issues as a result of the increase converter would then additionally have to be reverse-proof and would possibly itself have to be turned off. R2 is now used to bias Q1 off when exterior energy is current.

If we assume that the equipment makes use of a microcontroller, we will simply monitor the PSU’s operation. R5—included purely for security’s sake—lets the microcontroller test for the presence of exterior energy, whereas R3 and R4 permit it to measure the battery voltage precisely. Their values, calculated on the idea that we use an 8-bit A–D conversion with a 3.3 V reference, give a decision of 10 mV/rely, or 5 mV per cell. Putting them straight throughout the battery masses it with ~5–6 µA, which might drain typical cells in about 50 years; we will stay with that. The resistor ratio chosen is near 1%-accurate.

Many elements haven’t any values assigned as a result of they may rely in your alternative of regulator and increase converter. With its LM1117-3.3, the circuit of Determine 1 can deal with inputs of as much as 15 V, although a TO-220 model then will get somewhat heat with load currents approaching 80 mA (~1 W, its sensible energy restrict with out heatsinking).

I’ve additionally used Determine 2 with Microchip’s MCP1824T-3302 feeding a Maxim MAX1674 step-up converter, with an IRLML6402 for Q1, which should have a low on-resistance. Many different, and newer, gadgets will probably be appropriate, and also you in all probability have your individual favorites.

Whereas the exterior energy enter is proven as being bare, it’s possible you’ll need to dress it with some filtering and safety resembling a poly-fuse and an acceptable Zener or TVS. Equally, no connector is specified, however USBs and barrel jacks each have their locations.

Whereas that is proven for nominal 3V/5V provides, it may be used at larger voltages topic to gate–supply voltage limitations owing to the MOSFET’s enter safety diodes, the breakdown voltages of which may vary from 6 V to twenty V, so test your gadget’s information sheet.

—Nick Cornford constructed his first crystal set at 10, and since then has designed skilled audio gear, many datacomm merchandise, and technical safety equipment. He has ultimately retired. Principally. Kind of.

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