How to get a 3.6V supply from 5V?

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...?

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

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.)

A LM317?

http://www.reuk.co.uk/LM317-Voltage-Calculator.htm

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.

fungus:
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?

strykeroz:
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.

MarkT:
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: WTV020-SD-16P for sale | eBay

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.

DirtBiker:

fungus:
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.

DirtBiker:
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).

Erni:
A LM317?

LM317 Voltage Calculator | REUK.co.uk

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.

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).

DirtBiker:
With the zener and a 15 ohm resistor you'd be burning off nearly a watt of power.

My heat-testing finger never lies!

DirtBiker:
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.

So, my suggestion (reply #2) is just a bridge too far then?

IN400X datasheet

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

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.

No, at least not for me.

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?

No, it works...I even tried it (see reply #7). The only thing I didn't like is that the Zener gets very hot when the sound module is idle (Not drawing current).

If the current draw was constant and I could fine-tune the resistor then I'd probably use that method - simplicity is good.

sonnyyu:

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

Yep. The thing is that the module current is variable and the voltage goes up to about 5V when it's drawing no current. 5V is beyond the chip specs. 5V might not damage that chip when there's no current passing but it means I can't put in a decoupling capacitor. A big capacitor would supply 5V for a while when the chip powers up - risky for both the chip and the SD card!

cjdelphi:
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.

The Zener is there to allow for variable input voltages (eg. batteries). If the input voltage is constant then it should work.

I might give it a try tomorrow, too. Any voltage from 3.5-3.6V should work so I ought to be able to find a pair of resistors that works (I'll start with a pot attached to the transistor base and see if I can 'dial' an output voltage).

OK
I thought in Reply #7 that you'd breadboarded strykeroz's zener circuit with the 15?. That circuit will blow 100mA all the time, some of the current will be diverted to the circuit placed in parallel with the zener (the "output").

When using a zener to establish VB in the transistor voltage regulator, nothing like 15? would be necessary, maybe 5mA (IB) at most for 100mA (max) out.

I went to the local shop and they had LM317s and a 4.3V Zener diode. It's the first time they had everything on my list. I'll have a play later.

I just wanted to post this:

It's the inside of an LM317. How on earth does anybody design something like that?