How to down the voltage from 5V to 4.2V?

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Hello. I have a problem. MCU works on 5V, but my GSM module (Quectel M95) works only on 3.3V - 4.6V (4.0 recommend by datasheet). On 3.3V module works not stable, but on 4V - 4.6V it works great. BTW, when I connect battery through 2 parallely diodes, it powers down, I don't know why.

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GSM modules always loves a 2200uF after the PSU (diodes) , and superclose to the modules psu pins.

They draw a lot of pulsed power

/Bingo

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OP, what do you mean by

Quote:
when I connect battery through 2 parallely diodes,

With diodes, there is no such concept of using them in parallel.

Einstein was right: "Two things are unlimited: the universe and the human stupidity. But i'm not quite sure about the former..."

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I think this link can help you:

Voltage divider

"One's value is inherent; money is not inherent"

 

Chuck, you are in my heart!

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A voltage divider is not the best choice in this situation.

I think the OP wants the two diodes in series. But you either want two schottky diodes in series or one silicon diode in series and a big electrolytic cap.

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2 or 3 diodes in series.

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Diodes would work, but aren't very precise, so if you want 4.2 V exactly, you'll have to choose something else. (The voltage dropped by a diode will vary with load current.) Low-dropout regulators probably won't be able to supply the necessary current at a measly 0.8 V dropout. You'd get the best results with a switched-mode regulator (buck converter or SEPIC), but that may be overkill. I'd give diodes a try first, in any case. As others have said, use a big capacitor to deal with any transients.

If you're going to interface 5 V with 4.2 V, make sure the logic voltages are correct. The GSM module probably won't like it when the input signal voltage exceeds its power rail voltage. Likewise, you have to make sure the GSM module's high level output voltage consistently registers as high with your MCU. For MCU output to GSM input, you could use a voltage divider if the direction of the signal doesn't change. (GSM output to MCU input is more likely to work out of the box, but trickier to get working if it doesn't. Mixed-direction lines may get quite nasty.) If it's possible, it would be best to run the MCU at 4.2 V as well so you can avoid the logic voltage mismatch altogether.

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what is the power usage of your module?

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Where's the 5V coming from? Can you replace the 5v regulator with an adjustable one and generate 4.2V instead?

Don

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Simply use potential divider by using this formula;
Vx=(Rx/Rs+Rx)*Vs
where,
Vx= 4.2v as per your requirement
Vs= supply volts = 5v
Rs= Resistor in series
Rx= parallel res, parallel to the module which you would have to drive at 4.2.
Just put Rs = 1K in above formula and get the value of Rx.
Try that.

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Quote:
Simply use potential divider by using this formula;

Isn't there some relationship between current and voltage?

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uhf, what is the current use of your module? asking the second time!

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From my experience with Quectel M95 modules and SIM900A modules, the SIM900 works perfect with a diode dropped (two diodes in parallel connected in series with 5V supply to the module's power pins) power supply, even without the charge reservoir capacitor, though I would recommend the large capacitor.

 

In M95 modules, placing the 2200uF cap has helped me some times, but not always. I contacted the Quectel representative regarding this and they only recommend a dedicated switched mode regulator, which I would also agree is an overkill. 

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Just wont work. The module wont even power up. Voltage divider at 4.2V with a 1K resistor with the module's power pin will drop the voltage well below 4.2V due to the 1A burst current this module draw while attempting connection with the network. Diode drop or a dedicated regulator are the only hope i see.

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Potential divider makes the implicit assumption that device to be connected does not eat any current at all (an infinite impedance) ;

more accurate (and ... realistic..) values can be found out, if one makes the assumption device under interst has a constant consumption (but seems unconsistent with the fact there is a big current surge at start) .https://en.wikipedia.org/wiki/Vo...

 

Remain diode in series: parallel diodes are not likely to be useful : current will flow through the diode with the smalles drop voltage (they are not that constant), making the other unuseful. If current is high, one should use diodes meant for this current...

 

 

Edited : well, this thread is rather an old one, isnot it?

Last Edited: Tue. Jul 4, 2017 - 01:22 PM
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dbrion0606 wrote:

 

Remain diode in series: parallel diodes are not likely to be useful : current will flow through the diode with the smalles drop voltage (they are not that constant), making the other unuseful. If current is high, one should use diodes meant for this current...

 

 

I agree that voltage drops of two diodes wont be the same. But as current increases through the diode of lesser drop, wont the forward drop of that diode increase, making the current flow through the diode(s) in parallel and eventually resulting in a lesser effective voltage drop?

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Well, it is safer to use one diode only, with the right current rating (suppose the working diode overheats?) -less complex, anyway-

 

... and it would be wise, too, to look at the date of the beginning of this  thread.... is very old

 

I had a look at https://electronics.stackexchang... (partial quote)

 

 

The issue with putting diodes in parallel is that as they heat up their resistance decreases. As a result, that diode ends up taking on more current then the other diode, resulting it in heating up even more. As you can probably see this cycle will cause thermal run away causing the diode to eventually burn if you give it enough current.

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Why is your uC running on 5V ???

 

Lots of avr's run perfectly fine down to 3V or less and some of them down to less than 2V with reduced spec's.

So throw a LM317 or another adjustable voltage regulator in you circuit and run everything from the same voltage.

What hardware are you using? What voltage regulator is on it? Can you easily modify / replace it to output a voltage compatilbe with your whole circuit?

 

Damn. the revived old thread bug has bitten me again.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

Last Edited: Wed. Jul 5, 2017 - 01:53 PM
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Well, being an old thread has took it to the top five results in google search, which I believe would help the public most in finding a quick solution to the problem, like I just did.!!! Hehe!! So it's wise for me to respond for an old thread, as long as it helps!!.

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One or two schottky diode and one silicon diode in series.