V regulator: +ve, -ve and Ground

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Hi

Can anyone please tell me how to build a V regulator that outputs not only +ve and ground but +ve, -ve and ground; the supply is DC 12V. I would like to use one or more 7805 regulators. Is this possible?

Alternatively I can used a different V-regulator- can someone please recommend one?

Thank you in advance.
a.

 

[ericgibbs]

Hi

Can anyone please tell me how to build a V regulator that outputs not only +ve and ground but +ve, -ve and ground; the supply is DC 12V. I would like to use one or more 7805 regulators. Is this possible?

Alternatively I can used a different V-regulator- can someone please recommend one?

Thank you in advance.
a.

hi,

Its not easy to get a dual supply from a single supply using fixed linear regulators,
IF you require a reasonable current output from both the +V and -V regulators.

For reference there is a 7905 wich is a -5V version of the 7805.

If you only required a low current output from the -Vsupply its possible to use
a capacitive switcher ic, eg: a 7660 would give -5V from a +5V supply, but would only be able to supply a few milliamps.

For higher -V supply currents you need a high current convertor.

What are using the -5V for???

 

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Hi Eric

Thank you for your reply.

I am trying to build a battery recharger that will recharge each battery separately.
I will need about 0.5A to flow through each regulator.
(I have implemented a single battery recharger successfully).

I have thought about this problem some more and perhaps this will be a solution, any comments that you might have would be most welcome:

Have two V regulators:
7812
7805

Input V to both 15V.
The output will then be: 0V, 5V and 12V
Connect 0 and 5V to first recharging circuit (Circuit1)
Connect 5 and 12V to second recharging circuit (Circuit2).

Circuit1 will then see 5V, Circuit2 7V.
I can then connect Circuit1 to Circuit2 at the 5V (i.e. Circuit2 "ground" will be +5V)

What do you think of this?
All the best
a.

Now connect

 

[erosennin]

Hmm... personally I think it's sliiightly dodgy.
Look into the MAX712, simple circuit as well - even provided in the datasheet.
Maxim will even free sample out to you.

 

[ericgibbs]

Hi Eric

Thank you for your reply.

I am trying to build a battery recharger that will recharge each battery separately.
I will need about 0.5A to flow through each regulator.
(I have implemented a single battery recharger successfully).

Tell me what type/voltage of batteries you are wanting to charge??

 

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Hi Eric

Thank you.
I am trying to charge 2 Lithium polymer batteries in series.
They are quite small 170mAhrs.

I baught a commercial recharger which charger LiPo packs and has only two inputs. I tried to charge my battereis, and they went to 8.59V where it stabilized. I am worried about this- since most LiPo notes say do not over-charge batteries. (2*3.7=7.4V or maximum 2*4.2=8.4V so we overcharged it by 0.19V).
The recharger also warned not to charge batteries with a capacity less than 400mAhrs. I did not adhere to this requirement (my batteries are 170mAhrs each); so I do not know if it overcharged due to this minimum not being met.

Ideally I would like to use 2 wires to charge the battery pack (2 batteries).

Any suggestions that you might have would be greately appreciated.

All the best
a.

 

[Speakerguy]

Yep, you overcharged them - I would get new batteries. You charged them at 2.5C, and I think most folks say to charge them at 1C as a guideline. Maybe newer batteries charge faster though, discharge rates are certainly getting better over the last few years. They sell some nice programmable chargers that run down to 50mAH, and I think I have a cheapie around here that does down to 150mAH.

 

[ericgibbs]

hi arrow,

Is this what you are describing??

 

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Hi Speakerguy79

I am assumign that if I have 2 batteries each 3.7V LiPo and each 170mAhrs, when I place them in series the V=3.7*2=7.4V and the capacity is still 170mAhrs.

What I am finding is that these batteries are last a LONG time after that massive recharge. Should I get new batteries none-the-less?

Also can you direct me to a web site which sells 7.4V rechargers that deal with 170mAhr battery capacity?

Thank you
a.

 

[ericgibbs]

hi

Look here: **broken link removed**

http://www.modelpower.co.uk/acatalog/Lithium_Polymer_Safety_Information.html

Google for 'lithium polymer charger'........

 

[Speakerguy]

The two in series should still deliver 170mAH., just at twice the voltage. If they are acting differently than any normally charged 170mAH series pair would, I would get new ones. Lipo's can fail spectacularly when they do.

How much money do you want to spend on the charger?

 

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Hi Eric

Your schematic is not exaclty what I had in mind. Each 7805 would supply an independent MAX1555 (which I got to work before).
However, I have just figured out that this will not work!

Back to the drawing board
Thank you for all your help.
a.

PS I am going to try and buy an appropriate charger which is commercially available- hopefully Speakerguy79 will direct me to a place where I can get these.

 

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Hi Speakerguy79

I dont have an idea of the range of prices. But most probalby one that works well and reliably, has only 2 inputs (ground and 2nd lipo's +ve).

Can you please tell me several web sites I can look at? To get a feel of the lowest and highest cost.

Thank you
a.

 

[Speakerguy]

**broken link removed**

I believe it's the Graupner 12 that I have, and it charges at as low as 50mAH. I use it with my 150mAH lithium polymers with no problem.

 

[Speakerguy]

**broken link removed**

 

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Hi Eric and Speakerguy79

Thank you for the links you sent me.

Please can you explain to me how 50mAhr enters into the the recharging picture?
If the recharger I have specifies that the battery capacity must be at least 400mAhr- why should that matter?

I notice that some rechargers do not specify the mAhrs- what should I do then?

And finally: if the charger says minimum capacity 50mAhrs does that mean that I can charge a larger (say 1000mAhr) batteries with it? Will it charge longer?

Once again- I appreciate your help.
Best Regards
a.

 

[erosennin]

Ohhhh... don't listen to what I said before about MAX712 - they don't like LiPo batteries...

The reason the minimum capacity matters is because, that is the minimum rate the charger is able to supply to the battery(ies) being charged. Hence, if your charger Imin is 400mA and your batteries are 170mAh as said before, that is a charging rate of 2.4C which will kill your batteries - as it has.
Also the minimum capacity has to do with the charging cutoff detection as well in some cases - depending on how it detects.

 

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Hi Erosennin

Thank you for your reply. I am completely ignorant in this field, so please can I ask the following questions:
(a) you obtain 2.4C = 400mA/170mAhr. Is this right? In general do I obtain this value by dividng the charging current by the capacity of the battery?
(b) What does the C stand for? and what is its meaning?
(c) What is an acceptable value for charging?

Finally

After over-charging my batteries they seem to be holding the charge longer than before!

Regards
a

 

[ericgibbs]

hi arrow,

Look at these links, they have your answers.

http://www.mpoweruk.com/chargers.htm

**broken link removed**

 

[erosennin]

a) Yes. If you are charging more than one battery it becomes
(Charging Current) / (Battery Capacity * Number of Batteries)

b) C - The symbol C is coulomb, but in the case it just means a multiple of the battery capacity.

c) I personally believe a maximum charging rate should be 1C

After over-charging my batteries they seem to be holding the charge longer than before!

Lol, that may be the case in the short term - but I would assume that the current characteristics of the battery would have been damaged. I.e. it might roll off suddenly half way through what would have been its original life.
This said however, if it ain't broken - don't fix it.

Edit: Look at the links ericgibbs has posted up for you - they will be quite useful as he has stated.