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what is the best type of voltage regulation for my application?

what is the best type of voltage regulation for my application?

(OP)
I need a voltage regulator to send a consistent 4.2 voltage to the load, even as the battery voltage drops (from 4.2 V to 3.7 V) as it get discharged. The load resistance can vary from 0.5 Ohm to 1.5 Ohm.

There is a mechanical switch between the battery and voltage regulator, which the user turn on/off. When the switch is off, there is no current. When the switch is on, I need to voltage regulator to output a specific voltage regardless of battery voltage is at the time.

Is this possible? In another word, the voltage regulator need to be turned "on" the instant the mechanical switch is on and goes "off" the instant the switch is off. No delay or anything like that. Nor do i want to have to "turn on" the voltage regulator first before it'll start working (as if it's separate unit). This is because the voltage regulator itself is not very accessible and the only switch available to the user is the mechanical switch that connects the battery to the regulator.

The user shall swap out the battery once the battery voltage is at 3.7V, at which point the regulator will flash a signal light, and give off no more output to the load.
Ideally, the voltage regulator, the load, and the battery share the common ground. thx

The device works the best at 4.2 V, hence the need for a regulator. thanks

RE: what is the best type of voltage regulation for my application?

Define "no delay".

From a typical user viewpoint, flipping the switch on the wall shows no delay in turning on the ceiling light... yet high-speed SDRAM lines would fail miserably over those distances as it looks like a million miles.

Delay is relative...

Dan - Owner
http://www.Hi-TecDesigns.com

RE: what is the best type of voltage regulation for my application?

(OP)
anything under half a second, or is "instantaneous" by human sensitivity standard.

RE: what is the best type of voltage regulation for my application?

What you may need is a voltage doubler, to store charge in a capacitor at twice whatever the battery voltage happens to be, followed by a regulator to get your constant 4.2 volts.
Because of the AC operation of the voltage doubler, and the energy stored in the capacitor, it will take extra complication to make the circuit appear 'instantaneous'.
On the other hand, with a big capacitor, the warning light can stay on for a while after the battery is removed.
All of this stuff gets heavier and more expensive as the current supplied to your magic device, so far unspecified, goes up. You will also need some filtering to isolate the noise from the doubler and the regulator from your device.

In short, you need a real EE, in person.



Mike Halloran
Pembroke Pines, FL, USA

RE: what is the best type of voltage regulation for my application?

There are what are called buck-boost converters that can convert from both higher and lower input voltages. Beyond that you should get an EE, as suggested.

TTFN
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Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

RE: what is the best type of voltage regulation for my application?

just need a buck-boost setup..

RE: what is the best type of voltage regulation for my application?

eBay offers plenty of DC-DC Converters, and some of them can achieve this sort of current level (~8A). You might need to have an intermediate voltage (e.g. 5 or 12), and then a 2nd converter to regulate this down to 4.2v. An advantage is that it would be easy to buy and easy to integrate, assuming this is a one-off.

A problem I can foresee is that such a system (being 8A capable) would occupy more volume than simply using a 2nd (18650?) Li-ion cell and regulate down to 4.2v.

RE: what is the best type of voltage regulation for my application?

Couldn't this be done by a typical boost converter, maybe the LTC1872? The converter will simply shutdown when the power is switched off. When it's switched on you'll get a very short blip of current as the input capacitors charge, then the converter will power up very shortly afterwards. At first the current will pass through the converter with no switching, supplying power to the load. The converter will then make its first switching cycle, boosting the voltage until it reaches 4.2V. Even if the battery voltage is 4.2V there will be a small drop across the Schottky diode, so the boost converter will still operate at 10% duty or so.

The way you typically "turn on" a boost converter is simply by supplying it power. I don't see how this scenario is fundamentally different. Am I missing something?

RE: what is the best type of voltage regulation for my application?

You better start off with something better than a 4.2V battery if the draw will be 8A.

RE: what is the best type of voltage regulation for my application?

8A isn't that bad, IF there's enough ampacity, or there's a very short duration. A typical AA cell can dump 10A for up to about 15 minutes.

TTFN
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Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

RE: what is the best type of voltage regulation for my application?

Quote (IRstuff)

A typical AA cell can dump 10A for up to about 15 minutes.
I know I'm pretty tired right now, but I find this very difficult to believe.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: what is the best type of voltage regulation for my application?

I didn't believe the first time I heard it, which was in the context of a motorized toy. But, I have shorted an AA with my DVM in the 10A mode and the battery pegs out the meter.

TTFN
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Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers

Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

RE: what is the best type of voltage regulation for my application?

It obviously depends on the AA cell (internal resistance) and on the resistance of the 10A circuit through the leads and meter. With alkaline AA cells under the same test dead short through a DVOM, I've seen about 2A max and I've also seen closer to 10A. It's a case of YMMV. I think in general it's gone up through the years as AA cells have improved.

RE: what is the best type of voltage regulation for my application?

Sure, it will GIVE 10A... but for 15 minutes? I think not... typical power draw given in datasheets is only a few Watts at 50mA. Crank that up to 10A and I just can't imagine there's enough storage capacity for that length of time.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: what is the best type of voltage regulation for my application?

It may be a better solution to revisit the original design and select a battery:load combination based on the end of discharge voltage rather than the full charge voltage of the battery. Then you may use a simple off the shelf voltage regulator.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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