Hi guys I'm working on a Biomedical project, in this project I need -5V, 5V and 3.3V, and I'm using two 18659 batteries connected on serial, so together they gives 8,4V when they're fully charged.

to get the 5V, I used a 7805 regulator, it worked fine, and to get the 3.3V I used another regulator and it worked, now the problem is with the -5V, I tried to generate a minus voltage with an NE555 ( it worked I get -7,5V) and then used 7905 to low it to -5, once I use 7905, the voltage drop directly to -1.5V for no reason.

I would like to know how I can fix this, and if there's another idea to slove this I will be happy to get it.

PS: I use wh-2s5a circuit to charge the batteries.

 

The 7905 alone is going to take 2-6mA quiescent current.
Try replacing both the NE555 and 7905 with an LMC7660.

 

Ready made +/- 5V from 9V: http://www.mouser.com/ProductDetail...GAEpiMZZMvGsmoEFRKS8KL%2b4IeMiqIOrrjIa704cwY=
Add for 3.3V: http://www.st.com/content/ccc/resource/technical/document/datasheet/15/55/e5/aa/23/5b/43/fd/CD00000446.pdf/files/CD00000446.pdf/jcr:content/translations/en.CD00000446.pdf

Ken

 

Another option is to use a voltage converter, such as the ICL7660S, or the LTC1046. It's rather easy to configure it as a -5V source from 5V. It's only drawback is that it can only supply a limited amount of current.

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The 7905 alone is going to take 2-6mA quiescent current.
Try replacing both the NE555 and 7905 with an LMC7660.

yes that is true using these will waste energy unnecessarily

 

Hi guys I'm working on a Biomedical project, in this project I need -5V, 5V and 3.3V, and I'm using two 18659 batteries connected on serial, so together they gives 8,4V when they're fully charged.

to get the 5V, I used a 7805 regulator, it worked fine, and to get the 3.3V I used another regulator and it worked, now the problem is with the -5V, I tried to generate a minus voltage with an NE555 ( it worked I get -7,5V) and then used 7905 to low it to -5, once I use 7905, the voltage drop directly to -1.5V for no reason.

I would like to know how I can fix this, and if there's another idea to slove this I will be happy to get it.

PS: I use wh-2s5a circuit to charge the batteries.

The basic problem with your approach is that a charge pump circuit can only supply so much current -- and generally not a great deal unless they are design with significant care. So while you 555 timer-based charge pumps works, it can't supply much current. You should be able to estimate how much average current it can supply to a load. We can't, because you haven't shown the circuit you tried.

The other suggestions -- the ones that use chips specifically design to be DC-DC converters, are a much better option.

 

+/- 5V and 3.3V. Sounds awfully familiar. Oh, wait, that's the Arduino Uno output. If your appliances don't draw too much current, you can just use an Arduino Uno. Plus you can generate PWM and do coding stuff.

 

Why not use 4 batteries and put your 0v point between them?