running a RX from 7.2V pack

[pldaniels]

Hey there all,

I recently lost my favorite little 4 x 2/3AAA NiMH 300mA pack, so now I'm in a bit of a spot (I'm living in a remote place, cannot get to a battery store in a hurry).

What I do have is plenty of NiMH 150mA "9V" bricks (7.2V), would I be safe to simply use a 7805 or similar regulator and draw 5V from the RX (2 servos)?

Regards.

[Mr.RC-CAM]

The 7805 is a poor choice. Most will not operate correctly if the voltage drops below 7.0V (that is surely to happen in with your 6-cell packs).

You need to use a LDO (Low Drop Out) Vreg. For 1A and under apps, the ancient LM2940T-5.0 is one such choice. But, there are better LDO's out there, so search around.

BTW, some "9V" size packs can be taken apart and reduced in cell count. Have you tried that?

RC-CAM

[Miami Mike]

Yes. For over a year now I've been using a 7805 regulator with an opto-type ESC that I have that doesn't have BEC. It works fine. However, in your case you'd probably be a little better off simply tapping into the pack at the fourth cell.

[pldaniels]

I have considered ripping apart the 9V pack... guess I'm not super keen on trying to re-attach the wires on the tiny NiMH packs. Dang I wish I hadn't lost the original 300mA one :-\

The other method I was thinking was to use a zener, though I'm not sure if I have anything around 4.8~5V.

Paul.

[vintage1]

Any zener below 6v will do. You ned to create a voltage reference that is stable enough, tap off 4.8v with a resistot chain, and apply that to some sort of low saturation buffer. If you regulate the -ve rail you can use an NPN powre transitor in teh line with a PNP complementary driver and get a fairly low sauration 'emitter follower'

[Miami Mike]

... but you definitely can't use the zener alone, if that's what you meant.

You said you have plenty of those little packs, so why not try taking one apart and making a four-cell battery?

[rorywquin]

If he cannot get to the shop for a battery where is he going to get these components from ? :eek:

I would take a pack apart !

[pldaniels]

Aye, it is quite difficult living in outback Australia. I will scrounge around for a solution; perhaps I best just order a complete pack from somewhere and wait a week. It will not be of great pleasure though to be waiting so long before I can fly my Hand-launch-glider (Though I should avoid losing my battery packs into the bush).

Paul.

[pldaniels]

btw, here's a picture of the original battery (the smaller one).

[AndyOne]

Paul,

I use a simple emitter follower circuit when using 2s lithium cells to run the receiver on engine powered models.

The advantage with this circuit is that it does not drop-out, the voltage just drops with the battery voltage below 5.6V. No sudden loss of power.

Andy.

[pldaniels]

andyone,

That's a great little circuit. I like the fall-off voltage effect (below 5V). Very simple, does the job - excellent stuff. Now, I just have to whip one up on a bit of prototype board.

Thanks a bunch!

Paul.

[AndyOne]

Paul,

The resistor is not critical probably anything from 1k to 10k would work and you could use a 5.1V Zener for a lower output voltage. The output voltage is about 0.6 volts lower than the Zener voltage.
Remember to put the regulator on the receiver side of the switch as it takes a few milliamps continuously.

Andy.

[AndyOne]

A quick check shows that a lower value of resistor gives a more stable output, its probably better to use 1k or even 470 Ohm as long as the the power rating of the Zener is not exceeded. The smallest are around 400mW so this is unlikely.

Andy.

[Grimtooth]

How many amps can I pull through this circuit?

[AndyOne]

Grimtooth,

As you might expect there is no simple answer to how much current this circuit can take so here's an example...

Using, say, a TIP29, TO220 power transistor, max. power is 30W. Assuming you have a fully charged 2sLiPo pack at 8.4V and you want 5 volts out, the voltage drop in the regulator is 8.4 - 5 = 3.4V

Max power of 30W / 3.4V = 8.8A but this is assuming you can dissipate all 30W of heat. Using a heat-sink of, say, 1°C/W thermal resistance the temperature rise would be 30°C above ambient. This assumes the heat-sink is mounted correctly to take advantage of convection.

So you see, it depends on the power handling ability of the device and the heat-sinking available.

If you need to run this circuit at much above 2A, it may be necessary to further lower the value of the resistor as the gain of a bi-polar transistor reduces with increase in collector current.

For low currents such as driving a few standard servos no more heat-sinking is required than the device is capable on its own.

Andy.

[Comatose]

Pdaniels, one thing you're likely to find if you rip the battery apart is that the cells are spot welded together with tabs. These tabs are both easy to solder to and relatively hard to break. I agree just rip the battery apart.

Of course, you might find little wires, but I've never ripped apart one that didn't have spot welded tabs.