NerdKits - electronics education for a digital generation

You are not logged in. [log in]

NEW: Learning electronics? Ask your questions on the new Electronics Questions & Answers site hosted by CircuitLab.

Basic Electronics » How to convert 6V rechargeable NiCd to 5V regulated to power NerdKit

July 17, 2009
by rajabalu21
rajabalu21's Avatar

I have some 6V rechargeable NiCd battery packs (used in RC toy). Is there a way to convert 6V to 5V regulated to power NerdKit?

I understand that with a silicon diode in series there will be a voltage drop of about 0.7V. So theoritically with one silicon and one germanium diode is series I could possibly achieve around 5V. But I would like to get regulated voltage so that A/D conversions are not affected by battery drain.

When I measure the voltage of my supposedly 6V battery pack, under no load it gives about 6.4V and with a lamp load it drops to 6.2V.

L7805's datasheet states that the dropout voltage is between 2V to 2.5V so in order to use that I need to have an input supply voltage > 7.5V.

Any ideas for a simple circuitry to achieve my objective 6V to 5V regulated? Thanks

July 17, 2009
by mrobbins
(NerdKits Staff)

mrobbins's Avatar

Hi there,

You are on the right track. The key word to look for is low dropout or LDO regulator. For example see this digikey part.

Hope that helps!

Mike

July 18, 2009
by rajabalu21
rajabalu21's Avatar

Thanks Mike. Yes. It helps. Much appreciated.

February 21, 2010
by peabo
peabo's Avatar

This will work when the battery is fully charged, but won't take full advantage of the battery capacity because as the battery discharges, the battery voltage will fall below the LDO minimum voltage.

A better solution is to use a switching regulator chip that can convert a range like 4.0 to 6.5 V to a regulated 5 V.

An example of such a chip is the LTC1751, which can convert 2.7-5.0 V to 5V regulated at 100 mA. It requires two capacitors to store charge. This is probably not the chip you want, but looking at a datasheet will be helpful before you go looking for a chip better suited to your needs.

February 23, 2010
by rajabalu21
rajabalu21's Avatar

Peabo,

When Mike came up with a part number for me to experiment, I was able to understand that VDO is the key for this part selection. I went back to TI's site and looked up the characteristics for UCC383T-5 and the VDO was 0.4 V.

My NiCd battery was a 5 cell x 1.2 V = 6 V pack. I also looked up NiCd characteristics (courtesy google) and found an interesting graph Page#2 'Discharging Characteristics'. This gave me some additional information on NiCd batteries in general. When their voltage drops below 1.2 V, they are well about 85% of their rated mAh and that a rapid descend to 1.1 V is expected. After that, when the voltage reaches 1.1 V the descend takes place at an exponential rate and the batteries are not useful anymore and it is time to recharge them.

So practical limit on the battery was not to go below 1.1 V. So for a 5 pack that would be 5 x 1.1 V = 5.5 V. Since the regulator's VDO was 0.4 V, it would still deliver 5 V regulated. Of course all these are typical numbers and in a practical scenario things might be slightly different. But, based on the above the selection of the regulator theoretically satisfied my requirement.

Now from the practical standpoint, my modified power supply (UCC383T-5 in place of a LM7805) works fine till date.

Now coming back to your proposal, Yes, a switching regulator might be a more efficient solution but I could not find any part number that satisfied my requirements. I did look up several alternatives from LT/Maxim in fact before posting the question in this forum. I was looking for a inductor-less, DIP/SOT (something that could be inserted to the breadboard) packaging and I was surprised that there was not a single regulator that could satisfy my requirement based on my limited study. I did not want to handle the SOP packaged component as part of the breadboard model and so stayed with the UCC383T-5.

I wanted to document these just in case fellow members have similar requirements.

Thanks

-Raja

Post a Reply

Please log in to post a reply.

Did you know that a flyback diode is important when driving a motor or any inductive load? Learn more...