Ac to dc adapter connected to resistor

[Adhith]

Haii everyone...
I have an ac to dc adapter that gives 15 v(500mA) as output. I need 9v from it, but when i connected resistor with the adapter i still got 15 v as output. Is it because the current from the adapter automatically adjust to give a constant voltage ?? Or else what's the reason?? 

[mariush]

Cheap AC-DC adapters will often output higher voltage when there's no load or little load applied on them, either due to the design (it makes it cheaper) or to account for voltage drops in the cables between the adapter and the device.

A resistor in series won't do much, at the very least if you only need mA or so, have a look at voltage dividers. For more than that, resort to proper linear regulators .

[danadak]

Most/many adapters now have regulators in them, so they are doing
their job when you put a varying load on them and their V stays the
same. Note not that long ago the regulators in some adapters had a min
load requirement to stay in regulation. So, for example, if you had a 12 v
adapter but it had no load on it it might measure 14 - 16 V with a high Z
voltmeter. But as soon as you put a few mA of load on it its control loop
would come into regulation and you would get its rated output V.


Regards, Dana.

[Adhith]

Thankyou very much for your replies..
I have made a simple water level indicator with transistor bc 548,220 ohm resistors & LEDs which requires 9v input. If iam using a 9v adapter, what additional setup is required for the proper working??

[Seekonk]

From that description, it will likely work with higher voltage.  I would increase that 220 resistor to at least 560 ohms to limit LED current.

[Adhith]

i used a 9.1 v 500mA zener diode & a 150 ohm(500mW) resistor in series to reduce from 12v(500mA) to 9v. but the zener diode heats up. what is wrong with the setup??

[Monkeh]

i used a 9.1 v 500mA zener diode & a 150 ohm(500mW) resistor in series to reduce from 12v(500mA) to 9v. but the zener diode heats up. what is wrong with the setup??

Nothing..

You're passing current through the zener. It dissipates the energy as heat.

[MosherIV]

i used a 9.1 v 500mA zener diode & a 150 ohm(500mW) resistor in series to reduce from 12v(500mA) to 9v. but the zener diode heats up. what is wrong with the setup??

Do you get 9V?
Is the Zener in the right way round?

If correct, the 150 Ohm give 40mA of current through the Zener, you could reduce it down to 5mA (about 1.2K)

[rstofer]

Everybody is suggesting you drop the LED current.  If it is a normal LED, it will drop about 2V at 20 mA.  Worst case, if your power supply delivers 12V, you need to drop 10V across the resistor.  If you really want the entire 20 mA (and there is no reason you should), you want a 500 Ohm resistor.  If you consider this to be your worst case voltage, fine.  I would probably calculate for 10 mA at 12V and use a 1k resistor.  At 9V I would still have 7 mA through the LED and this may still be bright enough.  Experiment with different resistors and see how it works out.  Higher resistance results in less current.

Try some other values.  Any way you do it, you dissipate heat equal to the voltage drop times the current.  The LED dissipates heat related to its voltage drop and the current.

P = I^2 * R
P = E^2 / R
P = I * E

Values in amps, volts and ohms...  It might be fun to look at options in a spreadsheet!

[Adhith]

i didn't checked the output voltage , the moment i switched on the supply  the zener  starts to heats up and could easily burn my finger. so its not normal right..??.

[Audioguru]

A 500mW zener diode will be hot enough to burn you if it dissipates about 250mW. For a 9.1V zener diode to dissipate 250mW then its current must be 250mW/9.1V= 27.5mA or more. With 150 ohms in series then the voltage across the 150 ohms to produce 27.5mA must be 150 x 27.5milli= 4.13V. So the 12V must actually be 9.1V + 4.13V= 13.23V or more. The zener diode will become cooler if it gives some of its current to a load. If its current drops too low as shown on its datasheet then it will not regulate the voltage anymore.   

[Adhith]

thankyou everyone for helping me ..
i just replaced zener diode with regulating ic 7809 & the problem is solved.