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Author Topic: How to get a 3.6V supply from 5V?  (Read 2540 times)
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Valencia, Spain
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I'm trying to add a device to a project that needs a 3.6V power supply at about 80-100mA. Is there an easy way to get 3.6V from 5V?

I've tried putting 2 diodes together which should drop 1.4V in theory but in practice it was all over the place, anywhere between 4.5V and 3.0V depending on what the device was doing.

3.6V regulators seem to be thin on the ground and an adjustable buck regulator from eBay seems overkill (and will take a while to arrive).

Is there a quick hack with transistors/diodes/etc...?

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Hi

A simple Zener diode voltage regulator might do the trick.  I punched the maximum values you mentioned into this online calculator and it appears a 14 Ohm resistor (so you'd use a 15R) along with the 3.6V Zener diode will cover it off nicely.  A very cheap solution too.

I'm not certain what will happen inside the 2V headroom they suggest for the voltage, so need to do more reading.  If that's a problem perhaps a potential divider with just two resistors will be more stable?

Cheers ! Geoff
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The  "voltage regulator" circuit. 
Instead of using a zener, assuming the 5V supply is regulated, you could use a divider to establish VB (the values chosen will effect IB.)
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A LM317?


http://www.reuk.co.uk/LM317-Voltage-Calculator.htm
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First are you sure that the project needs exactly 3.6V?  Maybe 3.3V is OK - but you've neglected to provide
any hard information about this project like a link to tech-specs or a datasheet - please do so.
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I've tried putting 2 diodes together which should drop 1.4V in theory but in practice it was all over the place, anywhere between 4.5V and 3.0V depending on what the device was doing.

Wow, this device sure has variable current needs.

Before anyone can give you truly meaningful advice, we'd need to know a little more.  Can you tell us:

1) What the device is (link to spec sheet would be nice)?
2) Voltage tolerance of the device?
3) Current requirements of eh device?
4) Any other hard facts that might be pertinent to your use of the device?
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A simple Zener diode voltage regulator might do the trick.  I punched the maximum values you mentioned into this online calculator and it appears a 14 Ohm resistor (so you'd use a 15R) along with the 3.6V Zener diode will cover it off nicely.  A very cheap solution too.

OK, I tried that. It works pretty well, although I got 3.4V from a diode labelled 3.6V.

The only worrying thing was that the load can drop down to zero and when that happens the diode gets quite warm. If I go that route I might have to add a PNP transistor to be able to power the whole thing off when it's not being used.

First are you sure that the project needs exactly 3.6V?  Maybe 3.3V is OK - but you've neglected to provide
any hard information about this project like a link to tech-specs or a datasheet - please do so.

Yep. It has an SD card reader on it. The SD card spec allows cards up to 3.6V, 3.3V doesn't work for many cards.

The device is this: http://www.ebay.com/sch/i.html?_nkw=WTV020-SD-16P

You can read about all the problems it causes in the "audio" forum: http://arduino.cc/forum/index.php/topic,117009.0.html

I was playing with one yesterday and I found out that all the problems vanish if you raise the voltage a bit. I'm not designing a PCB or anything fancy like that, I just want to stick one in a toy I'm making for a nephew. It's probably going to be driven by an ATtiny84 with a regulated 5V wall-wart.

I've tried putting 2 diodes together which should drop 1.4V in theory but in practice it was all over the place, anywhere between 4.5V and 3.0V depending on what the device was doing.

Wow, this device sure has variable current needs.

Yep. It can drop down to almost zero when it's idle and when it's driving a speaker it can vary quite a bit.

Before anyone can give you truly meaningful advice, we'd need to know a little more.  Can you tell us:

1) What the device is (link to spec sheet would be nice)?
2) Voltage tolerance of the device?
3) Current requirements of eh device?
4) Any other hard facts that might be pertinent to your use of the device?

1) See above

2) The chip is a WTV020 - datasheet here
Operating voltage: 2.5-2.6V
Maximum rating VCC-GND: -0.5~4.5V

3) Current can vary between 2uA in standby up to 150mA max (depending on the speaker). I put a big capacitor on the supply line and measured a fairly steady 60mA consumption with my speaker.

My module does nothing at all with the SD cards I have here when it's connected to the 3.3V output of an Arduino. It works perfectly with a 3xAAA battery pack (about 3.8V measured) or using the Zener diode trick above (around 3.4V measured).
« Last Edit: April 27, 2013, 03:23:56 pm by fungus » Logged

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This looks like the way to go, although the dropout voltage is right on the limit. I guess he'll have to wait a few days until I can get my hands on some.
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With the zener and a 15 ohm resistor you'd be burning off nearly a watt of power.  I'd strongly suggest the voltage regulator circuit (first one) that runaway pancake suggested.  Use 4.3V-4.4V zener.  The rest of the values look good but you could use any resistor from about 500 to 2K and the cap could be almost any value (10uF - 330uF, whatever).
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With the zener and a 15 ohm resistor you'd be burning off nearly a watt of power.

My heat-testing finger never lies!

I'd strongly suggest the voltage regulator circuit (first one) that runaway pancake suggested.  Use 4.3V-4.4V zener.  The rest of the values look good but you could use any resistor from about 500 to 2K and the cap could be almost any value (10uF - 330uF, whatever).

I'd love to be able to reach into my extensive parts collection and pull out a 4.3 and 4.4V Zener to experiment with but it doesn't exist so it'll have to wait until Monday. On Monday I'll pop down to the local shop and see if they have any LM317s, if not I'll see if they have any 4.3V Zeners and try that circuit.
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So, my suggestion (reply #2) is just a bridge too far then?
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IN400X datasheet

use 2 pcs  IN4001 will give you ~3.4V output at current 80-100mA.

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The  "voltage regulator" circuit. 
Instead of using a zener, assuming the 5V supply is regulated, you could use a divider to establish VB (the values chosen will effect IB.)


I've always been curious what would happen if you switched (for the constant voltage circuit, 1st one of the 3) the zener diode for
a resistor/voltage divider?  would the transistor hold the voltage? i'll try that out tomorrow.
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So, my suggestion (reply #2) is just a bridge too far then?

No, at least not for me.
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This is what I refer to.

However, i've not put in any caps or resistors or capacitors (like will in the real world i would) .

More importantly, could this method be used reliably on a low voltage circuit?


* VDTransistor.png (121.29 KB, 1828x946 - viewed 55 times.)
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