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-   -   What size resistor for 250 ma check?? (http://www.rcuniverse.com/forum/batteries-chargers-84/1607602-what-size-resistor-250-ma-check.html)

flipstart 03-09-2004 04:02 PM

What size resistor for 250 ma check??
 
What size resistor should be used, in series, to similate a 250 ma load for checking a 4.8 volt, 4 pack nicd receiver pack? I came up with 20 ohms but this size resistor does not give the same voltage reading as my built in 250ma check on my hobbico meter. Flipstart

azvmax 03-09-2004 04:27 PM

RE: What size resistor for 250 ma check??
 
OHM's Law E= Voltage I= Current R=Resistance This only applies for D.C.

E
I R

So E=I*R
I=E/R
R=E/I
In your case R=4.8/.250
R= 19.2 ohms

Also when batt. fully charged 5.2/.250=20.8 ohms


So 20ohms shd work good.
You need to hook it up in parallel not series, across the + - then read the voltage across the + - while the battery is being loaded.
Voltage is always read in parallel, current is read in series.
Explain "with 20 ohms but this size resistor does not give the same voltage reading as my built in 250ma check on my hobbico meter."

I used 15 ohm myself for approx 300ma.
I used a DP DT swt
Double Pole Double Throw
I wired it so I had 15 ohms for RX batt then when the swt went the other way I had 30 ohms
This Gave me the option to load 4.8 RX and 9.6 TX Resulting in a load for both TX and RX
I rarely use The Tx part because most modern Tx'x have a battery meter built in which is being loaded while on.

1x 2x

3x 4x

5x 6x

6 pins on swt 1-6

connect 15 ohm across pins 1 and 2
then connect 15 ohm across 2 and 6
then jumper(solid wire) across 1 and 5

Battery is connected across 3 and 4
As well as volt meter acrss 3 and 4
When swt connects 1 and 2 to 3and 4 you are loading your battery with the 15 ohm resistor so RX load test
When swt connects 5 and 6 to 3 and 4 you are load your battery with 30 ohms So TX load test.

JNorton 03-09-2004 04:32 PM

RE: What size resistor for 250 ma check??
 
Wattage = Current times voltage - so 0.25 X 5 = 1.25 Watt double it for a safety factor. You'll need a 20 ohm 2 watt resistor.
John

JNorton 03-09-2004 04:38 PM

RE: What size resistor for 250 ma check??
 
Easiest way to buy one. Radio Shack sell (2) 10 ohm 10 watt resistors for $1.69 Part #271-132 Wire them in series and you have a vey good load, and at 20 watts of power disipation probably won't even get hot.
John

azvmax 03-09-2004 04:48 PM

RE: What size resistor for 250 ma check??
 
Doh I forgot the wattage ratting Thx.

flipstart 03-09-2004 07:56 PM

RE: What size resistor for 250 ma check??
 
Thanks guys for the prompt reply-I ordered 20 ohm, 2 watt resistors from Allelectronics, 2 resistors for $1.00, they seem to have good prices. Just wanted to be sure Ohms Law had not changed, nice to have confirmation. Flipstart

Red Scholefield 03-10-2004 03:34 AM

RE: What size resistor for 250 ma check??
 
Quote:

ORIGINAL: flipstart

What size resistor should be used, in series, to similate a 250 ma load for checking a 4.8 volt, 4 pack nicd receiver pack? I came up with 20 ohms but this size resistor does not give the same voltage reading as my built in 250ma check on my hobbico meter. Flipstart
Might I suggest a #47 light bulb. It will give approximately a 250 mA load. The advantage of a light bulb is that you can actually see that a load is being applied. No light - No load.

flipstart 03-10-2004 08:27 AM

RE: What size resistor for 250 ma check??
 
Thanks Red-will give the bulb a try-better idea than the resistor and failsafe in case of short-have seen your schematic on your site since I started to setup a separate battery check. Flip

JoeAirPort 03-10-2004 12:47 PM

RE: What size resistor for 250 ma check??
 
I did this last summer, works awesome. I used a pack of 100 ohm 1/2 watt resistors from radio Shack and soldered 5 in parallel. That's 20 ohms total at 2 1/2 watts. Get a female servo extension plug with the other end cut off. From the cut off end, solder one of the wires and one end of the resistors to an allegator clip. Do the same to the other side. The female servo extension plug goes to your male battery pack or fuse test plug. Each of allegator clips go to your volt meter. Just plug in the servo extension jack into the battery check plug on the side of your plane. Then clip each allegator clip to your voltmeter leads. Set your volt meter to DC volts and you have a cheap battery checker. Get a $10 volt meter from anywhere and use it with your battery tester. The volt meter can be used for many other things on your radio gear.

You will read DC volts on the voltmeter. It may read plus or minus depending how connectoed your allegator clips but the negative sign doesn't matter. You just want to know what your loaded voltage level is.
It will load your batter pack to 240ma at 4.8 volts (needs charging) and 270ma at 5.4 (freshly charged).

Joe

Regardless of whether you buy one or make your own, a battery checker with a voltage read out helps you get familiar with how your batteries behave while you're out at the field. If you grab a battery that's around 4 volts and connect it to your tester, you can see the level drop. You can tell that the thing is fading. Same with a bad battery out at the field. If you watch it for 5 seconds, it better hold the voltage at 5.4 or only drop to about 5.38 at the lowest. Then you know the pack is strong.

Hangtime 03-10-2004 02:24 PM

RE: What size resistor for 250 ma check??
 
Here's something you folks might like..

http://www.hangtimes.com/sitebuilder...s/DSCN4636.jpg

Just plug it in between your charge port/pack and your DVM.

Low load, 4 cell: 250ma
High load, 4 cell: 400ma

Low load 5 cell. 300ma
High load, 5 cell 500ma

It's fuse protected (guess why. ;)) has 4mm piggyback bananna plugs and comes with a FJ/JR/Hitec/AirtZ 18 gauge female conn to jack it into your pack or charge port.

Sells for $19.95, available on the website.

gunny11 03-17-2004 12:11 AM

RE: What size resistor for 250 ma check??
 
I got their digital version. Smaller and fits easily in the flight box. I booted the bulky hobbico one into the shop room.


gunz

Mitty 03-23-2004 07:22 PM

RE: What size resistor for 250 ma check??
 
I used two motorcycle light bulbs in series. It gives me 270 ma.Thats the one I discharge my 1400 ma/h rx pack.
As for TX I just leave it on and put something against the stick to imitate a load..Not sure if in central position batarey drains the same way but just do it anyways...

Mitty


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