# Circuits

Discussion in 'General Electronics Chat' started by mrscrewdriver, Feb 7, 2010.

1. ### mrscrewdriver Thread Starter New Member

Dec 2, 2007
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HI everybody! i have some question and need some help. i have built 4 lamps circuit with 12v supply. Each of them 21w and 12v, except one ,it's rated 10w. here is my setup: i connect lamps in parallel, then set the adjustable power supply to 12V range. Then switch on my circuit, somehow it's drop to 10V and read 4.33A. Before connect my circuit i do some math.

for ohm's law R=1.972Ω I=6.085 P=73.008W then connect my clamp on meter it's read 7.5A is this normal, and another question why drop from the 12V to 10V sorry my bad english

thank's

2. ### Audioguru New Member

Dec 20, 2007
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896
Simple arithmatic:
1) The 21W/12V bulbs each draw 21W/12V= 1.75A. Three bulbs draw 3 x 1.75A= 5.25A.
2) The 10W/12V bulb draws 10W/12V= 0.833A.
3) The total current is 5.25A + 0.833A= 6.08A unless the bulbs were made in China.

If your 12V dropped to only 10V then its voltage regulation is poor. Maybe it is overloaded or maybe it was made in China.

3. ### cumesoftware Senior Member

Apr 27, 2007
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What is the amp rating of your power supply? Do you have a power supply with current limiting (adjustable or fixed)? This might explain the voltage drop. Again, your power supply might be overloaded, as Audioguru suggested (or it was made in China).

4. ### mrscrewdriver Thread Starter New Member

Dec 2, 2007
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it is rated 30V 5Amps and current is adjustable. i built another circuit with 6 resistors each of them 5.1Ω then connect in parallel with 12V supply but this time voltage source is ATX power supply. before measurement i do math.

R=0.85Ω I=14.118A P=169.416W almost 170W. Then i measure total current with my clamp-on meter it's read 12.30A then measure DMM it's read 6A
and i try another load with two dc fan with parallel each of them 12V .5A i measure DMM it's read 1A OK. again i measure clamp-on meter it doesn't read anything clamp-on meter ranges are 600.0A with .1A resolution and 1000.0A with 1A resolution is this normal or one of my meter gonna fail?

thank's

5. ### Audioguru New Member

Dec 20, 2007
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896
I have never used a clamp-on current meter. I thought they only measure AC current.

6. ### jans123 New Member

Jan 30, 2010
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Nope, DC-types exists. A little more expensive... but not much

7. ### cumesoftware Senior Member

Apr 27, 2007
1,330
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It is only natural something like this happens. If your PSU has current limiting (adjustable or not) and if your circuit requires 6.085A to work with 12V, that means that your PSU will reduce the voltage in order to limit the current. That is how current limiting works. Using your example:

I = 10V / 1.972Ohm = 5.071A (it makes sense, since your PSU is limited to 5A)

Now, your clamp meter must be using the wrong settings, or it is badly calibrated. Doesn't your PSU have an ammeter? If you have one, you should trust that reading. Mine is just like yours: adjustable voltage up to 30V and adjustable current up to 5A, and it has a voltmeter and ammeter. Almost all PSUs have.

Dec 2, 2007
8
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