simple DC electrical question, I hope

[gmason]

OK...I'm tring to convert a 4 "C" battery toy to use a AC/DC transformer. I know the the 4 batteries when connected in series adds up to 6VDC. My question is how do I determine how many amps the toy needs.
I hooked my multimeter up to the 4 batteries and I get around 10amps, but that's the MAX "cold cranking AMPs"/w3tcompact/icons/wink.gif right?
Is there a way to take a 9vDC transformer and drop it to 6vDC?
Another question...I took one C battery and switched my Multimeter to MicroAMPs setting and got a reading of 270microamps.
Was I just maxing out the range for the Micro setting?
When I set it over to the 20amp range, it read 5+amps.
How do I determine with my multimeter how may amps the toy requires?

thanks
gary

 

[Lane]

This is what I would do. Put a milliamp meter in series in the circuit and run the item under max load. This will tell exactly how much ampreage the item pulls. Size the transformer at least 2 times this value, larger is OK. (Put the red meter lead on the + of the battery and the black lead on the red wire to the device to accomplish a series reading). Please note: do not attempt this with amperages larger than your meter is rated for or damage and injury may result. Good luck.

 

[MChalkley]

One way, other than measuring it directly, would be to determine how long the toy will run on batteries. Then, look up the capacity of the batteries, multiply it by the number of batteries, and divide that by how long it runs on them. The result would be the average current draw of the toy. For example, if they're 2000 milliamp batteries, four of them have a capacity of 8000 milliamp-hours. If the toy runs 8 hours, it's drawing 1000 milliamps, or 1 amp. I would size the transformer to provide about 50% more than that, or 1500 milliamps, to allow for surges and "breathing room", though. Now, somebody like Twinkle_Toes will probably come along and correct me, but I think that's right...

You can reduce 9v to 6v, but it's a lot easier, and more efficient, to just buy a 6v transformer.

 

[Anonymous Poster]

<font color=blue>...I hooked my multimeter up to the 4 batteries and I get around 10amps, but that's the MAX "cold cranking AMPs" right?...</font color=blue>

Wow... that's one hefty toy... /w3tcompact/icons/crazy.gif... but I really don't think you measured something right... also it may not be "6 vdc"... maybe "3 vdc" if batteries in parallel/series combo...

What exactly is this toy... what does it do...?

Depending on the type of unit... most toy devices are somewhat forgiven voltage wise... especially after voltage dropped and considering the "el cheapo" ac/dc adapters used...

You can start out with a cheap Radio Shack universal DC adapter 3-12 vdc/800-1000 ma ...~~~$5-8.... adjust the voltage down as low as possible(say start with 3 vdc and work your way up carefully) and select higher voltages and observe unit operation...(also have a milliampmeter in series and measure the current draw as well)...

If the toy is somewhat expensive, I'd be leary of a real cheap dc supply because of poor regulation...( the Chinese 3 volt setting may be 8 volts.../w3tcompact/icons/blush.gif)

 

[TWINKLE_TOES]

Mark,
<font color=blue>Now, somebody like Twinkle_Toes will probably come along and
correct me, but I think that's right...</font color=blue>

/w3tcompact/icons/smile.gif I would never correct anyone that is armed with a leathal weapon like Earth Force, but I might offer some clarification/w3tcompact/icons/wink.gif
If the cells are in series the capacity of the battery is the same as the cell. If the the cells are in parallel the battery capacity is the product of the individual cell capacity times the number of cells.
Al

 

[gmason]

JM3, I got the 10amps reading from lining 4C cells end to end, so + to -, and red lead to the last + end and the black lead to the other end of cells - end.
Of course the leads got a little warm in short time.
This is just a funky wall clock that has bell and whistles for infants to watch. It can be set to "go-off" every 5 minutes to keep the child watching it...
Chew's up batteries, and I've got 6-7 old transformers laying around from old/dead telephones/flashlights/cell phones, that I thought I'd convert one of these (mostly 9Vdc) to use for this instead of paying(out the snoot) for Ni-Cads.
I did test a few of these transformer's voltage reading and some were nearly 20% off the voltage information on the plug transformer.
When it says, 800Ma on the plug, is that the Max current output but if a device needs less the current draw maybe less right?

thanks
gary

 

[MossRoad]

Yes. I think so. You can get a transformer at radio shack that adjusts between 1.5, 3, 4.5, 6, 7.5, 9 volts. I think it is less than $15.00.

 

[gws]

You cant measure the current of a battery without a load. connecting a multi meter to a battery shows you the current going through the meter, not the current the battery is capable. 4C batteries is about an amp of power. You can get a 6v 1amp power supply from radio shack.

 

[Charlie_Iliff]

Gary:
Be careful putting a meter across any battery on an amps setting. It is easy to fry many meters, since the amp setting is merely a shunt, designed to go in series with the circuit being measured. Some meters have fuses on low amperage circuits. Putting a meter across a 12v lead acid storage battery, however, may fry the meter and the fuse. The ranges are for the amperage you think you will see, not the limit of the meter in most cases.

 

[JATO_RaT]

But GWS, does not a amp meter have virtually Zero resistence thereby providing virtually a dead short and therefore the greatest current draw from its source?

On the other hand, a good voltmeter will have almost infinite resistence.

Rat...

 

[kcbarnes]

All you need to do is put a dc ammeter in series with the battery and toy to find out the amp/milliamp draw is and then go to ripshack and get an ac to dc converter rated as large or larger than your toy draws.

 

[TWINKLE_TOES]

Gary,

A couple of items in your post caught my eye. The first <font color=blue>10amps reading from lining 4C cells </font color=blue> The second <font color=blue>paying(out the snoot) for Ni-Cads.</font color=blue>
Ten amps short circuit current is pretty high for an alkaline battery. Paying out the snoot for Ni-Cads is a little strange for the toy application which doesn't appear to have a charger. The 10 amp reading and low capacity is consistent with Ni-Cad batteries. Alkaline batteries of the same size will have 3-4 times the capacity of the equivalent size NiCad. Are you perhaps using NiCad batteries as throw away when dead?

<font color=blue>some were nearly 20% off the voltage information </font color=blue> This is not unusual.
Most DC wall transformers are transformer/rectifier/filter capacitor configuration (no voltage regulation in most) and the open circuit (no load) voltage will be up to 40% higher than the voltage at rated current. A 9 volt DC rated wall transformer @ 500mA would read ~ 12.6 V with no load. At a load of 500mA the voltage should be 9 volts. Tell me a little about the toy. I suspect it has a little motor that runs infrequently (200-300ma) something smart that tells the little motor when to run (10's of micro amps) a whistle/bell/light flashes intermittently(a few milliamps). For this kind of operation you should get good life from alkaline cells. Have your hopes been dashed for the simple DC electrical question? Nothing on this board is simple, most, after they log off, go to bed and red line the bible./w3tcompact/icons/wink.gif
Al

 

[RMulkey]

Gary,

Finally I can use something I learnt (sic) in EE school.

Your measurement method is actually measuring the resistance of the meter you are using to make the measurements. Anyway,to get an accurate mesurement of the current the toy is consuming, this measurement method will work:

1) Connect the original batteries using WIRES to the toy power connections.

2) Cut one of the battery wires and insert a LOW resistance resistor (100_Ohm). Measure the resistance of the resistor by itself with your Ohm Meter (before you hook it up).

3) Hook up the toy and start it up.

4) Measure the VOLTAGE drop across the resistor using a your volt Meter.

5) Using the formula: Vdrop=I*R, I=V/R, where
Vdrop= Measured Voltage drop across the resistor (in Volts)
R = the Measured value of the Resistor, (Ohms)
I = the Measured current in the wire. (in Amps, not milli or micro)

6) The key to success is to not use too high a value of resistance that the toy does not run and always use the measured voltage and resistance values. If the toy does not run, lower the resistor value (like 10_Ohms).

Hope this helps,
-Roger