# how to transform 3,5V to 4.5V?

Discussion in 'General Electronics Chat' started by aaimi, Aug 31, 2011.

1. ### aaimi Thread Starter New Member

Aug 30, 2011
4
0
I need to transform a 3,5V DC (generated by 3 LiMh batteries) into a 4,5V DC, but I can't use a different battery pack o another supply voltag.
Does anyone have a suggestion to solve this problem?

aaimi

2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,729
What current (how much) do you need at 4.5v?

3. ### aaimi Thread Starter New Member

Aug 30, 2011
4
0
I need a current of 50 mA, in ordert to supply a led, a pic, and some other components.

aaimi

4. ### PRS Well-Known Member

Aug 24, 2008
989
35
Does a signal ride upon the dc voltage? If so, what is the peak to peak value of the signal? But if all you're doing is changing 3.5 v dc to 4.5 volt dc, then make a common emitter amplifier with a BJT capable of your load current plus 1 ma. The latter is necessary to keep the bias right. Don't forget to subtract the 2mv of saturation voltage when calculating your resistors. The gain of this stage is (4.5v-.2mv)/3.5v which is about 1.3 v/v. Use an emitter resistor and a collector resistor. No resistors are needed at the base, since the battery supplies a fixed 3.5 volts.

This is simplistic. It does not include a range of input voltages, which is what you need in practical battery-driven circuits.

Last edited: Aug 31, 2011
5. ### CraigHB Member

Aug 12, 2011
127
15
I'd use a DC-DC boost converter. There's tons and tons of different PWM controllers out there for that purpose. You can also use a POL module which is basically just a drop in DC-DC converter. They're mostly buck, but there's a few boost out there. The TI PTN04050C is one example.

6. ### SgtWookie Expert

Jul 17, 2007
22,183
1,729
Well, you CAN operate a PIC from 3.5v or less - did you know that?

You could also likely power an LED from a voltage that low. So, that's two things that probably don't need to run from 5v. What else do you need to power?

The fewer things you have to power from a DC-DC boost inverter, the longer your batteries will last.

7. ### CraigHB Member

Aug 12, 2011
127
15
I was thinking that as well. The OP said he wanted higher voltage. But yea, these days, it's common to see parts with an operating voltage down to 2.7V. Many of the PICs will operate even lower. The PIC24 I'm working with on my current project hits BOR at 2.0V. I run it on a 3V supply.

8. ### Audioguru New Member

Dec 20, 2007
9,411
896
A single older lithium battery cell is 4.2V when fully charged then its voltage slowly drops to 3.0V when it needs to be re-charged (their average voltage is 3.6V). Some newer lithium cells (LiFE) have a max charging voltage of only about 4.0V (Maybe their average voltage is only 3.5V).

9. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
If you use a newer PIC, and an LED that isn't blue or white, you should be able to run from a 3.5V Supply without issue, just keep the voltage range during discharge in mind when choosing current limiting resistors.