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# Why does the voltage drop in my small power supply?

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#### AinaLaw

##### Newbie level 6
Voltage Drop

Dear All,

I have a small power supply rated 12V 1000mA.
The difrence between + and gnd is 11.68V (with no load).
But when I connect it to my circuit ( resistor with 3 leds in a series),
the voltage between + and gnd is 9.8 V only.
Why is the voltage drops ?

Thank you

Aina Law

Voltage Drop

Hi

it depends on the circuit of your power supply, it is possible your power supply doesnt have have a stabilizer circuit, or there is some malfunction or just it doesnt provide the 1000 mA it says.

Sal

Re: Voltage Drop

I think the internal resistance of the power supply is probably higher than you think.
According to what you said, there is an internal voltage drop of 1.88V across this "internal" resistance (11.68-9.8V).

What is your external resistor that is in series with your LEDs. Assuming the LEDs have a forward drop of 1.5V each you would roughly have 5.3V (9.8-4.5) across this external series resistor. 5.3V/Rexternal would give you your current. I think 5.3V/Rexternal is << 1000mA so I don't think you are reaching the power supply output current limits.

Best regards,
v_c

Re: Voltage Drop

I think u have a power supply wiyh high internal resistance u can measure its value by connecting known value resistance(assume its value is R) across the output then measure the voltage drop across it assume its valu is V and the current passing the resistor assume its value I by dividing V by I (V/I) u get the total resistance assume its valu is Rt
then u can get the internal resistance as below
internal resistance = Rt -R
good luck

Re: Voltage Drop

Hi All.

My circuit draw only 30 to 40mA .

It's one line of resistor with 3 leds only.

When I connec it to two lines (in paralel),
voltage drops more, this time is the voltage is less than
9V.

Thank you

Aina

Re: Voltage Drop

I think you might have a bad power supply. Can you provide any details on the power supply itself? Manufacturer, model number, if it has been calibrated, how old it is etc. You might want to connect some other loads besides your LEDs to see if it is giving you anything near the open circuit voltage..

Best regards,
v_c

Re: Voltage Drop

most porbably you got a very bad supply and its rating parameter is not the exact value.

Re: Voltage Drop

Hi
sometimes the ckt has its loading effect on some power supplies, especially if the supply is not very good. i had a similar problem with a power supply in my university lab ( university labs r supposed to have good power supplies but that particular one wasn't). i was trying to drive a high power stepper. i solved my problem by using a power supply from an old computer of mine. works like a gem. u can get 5V and 12 volts from those and they have a high enough rating for most purposes.

ta ta

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