I am trying to find a way to build a very small voltage regulator assembly that can accomodate a moderate draw (>1.5A) without shutting down. I am an R/C Heli/Car/Truck person that dislikes NiMH or NiCAD batteries and am trying to swith to all LiPo cells. The issue is that each cell is 3.7V, and come pre-packaged in 1 - 6 cells (the 2cell 7.4V and the 3cell 11.1V are the most common). All R/C electronics (servos, receiver packs, telemetry devices etc.) are made to work off of either 4.8V or 6V, depending on the application. There are companies that offer small voltage regulator packs for converting these, but a) they are very expensive and b) they are fairly unreliable and come with no warranty.

In my bench tests using components that I picked up from Radio Shack, I was able to create a working and consistent voltage regulator that would work and provided a consistent 5V using a 7805 and a 6V version using a LM317T. However, when I began to add a load to this, the behavior quickly changed. Regardless of whether I used a 13V or a 7.4V power supply for this, when I began applying any load greater than .5A, the unit began to heat up VERY quickly. With a consistent .4A load, the max temp of the package was about 120degrees, at .7A it jumped to 155degrees, and at 1A (documented rated capacity) it jumped up to 185degrees. The maximum load that I will be pulling will be about 1.8A for my largest vehicle, so obviously, this configuration won't work, either with or without an external heatsink.

Any ideas? Suggestions? How about the LM350T? Will that do any better since it is rated for 3A?

Thank you for any time or suggestions that you can give me.

 

The voltage across a linear voltage regulator times the current in it equals the amount of power it dissipates in heat. All linear regulators will be the same. You need a pretty big heatsink and maybe a fan to remove the heat.

A switching voltage regulator is cooler but might cause radio interference.

 

Your problem isn't the amount of current that the linear regulator can
supply it is the amount of power you are trying to dissipate in the package.
You should be able to get 3A and 5A regulators but you need to calculate
the power dissipation and provide the appropriate cooling.

What package are you using for the 7805 and 317? Do you have a heatsink
or forced convection cooling?

The simplest circuit (which is the most inefficient) would be a linear regulator with
a large heatsink (and possible a fan). At 7.4V in and 5V out you are wasting 1/3 of
your battery on heat.

You could look at a switching supply or DC-DC converter. You can easily get 80-90%
efficiency but it is more complicated to build and more expensive.

(* jcl *)

 

Use the LM350.

 

audioguru and john were right.
the common practice in r/c application is energy conservation. (weight conservation too? a heatsink is heavy enough don't you think so?)
you know better.
every voltage converter are switching ones, just like every the motor drivers, they're all pwm.
maybe you can use lm3488.

 

There is something I do not understand here :

... and come pre-packaged in 1 - 6 cells (the 2cell 7.4V and the 3cell 11.1V are the most common).

If the Lithium batteries are pre-packaged in more than 1 cell in series, the pack SHOULD have its own controller built in ; and you need only to supply slightly above 4.20V times the number of cells in series to recharge them.
A Li series pack battery should not be recharged without monitoring/controlling EACH cell in it.
A spare PC power supply (yellow wire) can supply 12 V for 2 or 3 in series with built-in controller and will easily provide the current.

For charging a single cell, if there is an adjustable potentiometer in the PC power supply, bring the 5V (red wire) down to ~4.8V and put a 5A 0.7V silicon diode in series to get the 4.20 V maximum a cell has to be charged with. It can provide plenty of amperes too.

PC power supplies are a dime a dozen, probably you have one behind spider webs. Hope it is an alternative.

Miguel

 

far as i know, the r/c world has got all the equipments needed for those things.
a special charger for r/c models batteries is common for r/c enthusiasts. li-po, li-ion, nicad, nimh, and all. the industry seems to overleap the commercial toys and electronics industry.

 

I belive that in some of the datasheet for regulator chip, there is a circuit known as current boosting circuit. check that out. it make use of a BJT transitor to boost the output current.