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Author Topic: Cheap power supply?  (Read 949 times)
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Hi
I have seen these cheap led drivers http://www.banggood.com/Duable-300MA-4-5x1W-Watt-High-Power-LED-Light-85-265V-Power-Supply-p-906724.html and tried one. With no load it tests at 18 v dc. As soon as there is a load it is putting out 70 v ac. I have no idea why and would be very grateful for any explanation. I wonder if it is not really the correct supply or if there is a way to make it work.
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which side did you connect the 120VAC to ?
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I dont know this specific power. There should be a current limiter at .3A, which is suitable for high power LEDs.
There can be 4 or 5 in series.
Without a schematic, it difficult to tell how this one works.  There may be some kind of PWM that explains your 'funny' reading.
Try with a scope..
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Did you also measure the DC output ?
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which side did you connect the 120VAC to ?
Hi
The board is marked with ac in live and neutral and dc out v+ and v-. The first image shows the module connected to 240 vac (UK mains voltage) and I can measure 18 v dc on the output leads with my multimeter.  The second image shows the veroboard arduino circuit. There is a L7805 regulator and some smoothing cappies (100uf and 100nf both sides of the L7805). When powered the nano leds flash continuously and I can measure 70vac on the terminal block. If I connect a 9 v battery to the block it works perfectly.


* IMAG1017.jpg (276.66 KB, 2688x1520 - viewed 21 times.)

* IMAG1018.jpg (405.15 KB, 2688x1520 - viewed 18 times.)
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I dont know this specific power. There should be a current limiter at .3A, which is suitable for high power LEDs.
There can be 4 or 5 in series.
Without a schematic, it difficult to tell how this one works.  There may be some kind of PWM that explains your 'funny' reading.
Try with a scope..
Hi
Good point, but I don't have a scope to test it
The circuit appears to be a transformer and rectifier with another mystery component which is an 8 pin ic. See photo. I can't find a schematic or datasheet anywhere.
If it is pwm is there any way to use that output to power the arduino circuit?


* IMAG1020.jpg (76.4 KB, 1215x858 - viewed 14 times.)
« Last Edit: June 22, 2014, 02:56:25 am by bevangg » Logged

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With a 400v rated capacitor onboard it is almost certain to be a switched mode power supply.  Your best bet is to cut your losses, throw the heap of junk away and buy a decent supply rated to do what you wish.
« Last Edit: June 22, 2014, 05:27:57 am by jackrae » Logged

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With a 400v rated capacitor onboard it is almost certain to be a switched mode power supply.  Your best bet is to cut your losses, through the heap of junk away and buy a decent supply rated to do what you wish.
Hi
I don't understand as google shopping search for "arduino mains power supply" comes up with a list of smps wall warts. There are three listed here http://www.embeddedmarket.com/products/SMPS-9-Volts-1Amp-Power-Adapter/ at 5, 9 and 12 volts and they are all smps. I am new to electronics, please could you explain why these modules are no good? I bought them primarily because they are so small and would be ideal for home automation.
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LED power supplies are constant current sources and no use for
anything but a current-driven load.  A current source has a high output
impedance, a voltage source has a very low output impedance.  Pretty
different really...
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I'm not suggesting smps are junk, they are generally excellent at what they do; it's just that the one you have would appear to be.  If a device alters its output from the design DC voltage to a much higher AC one when you load it, then it is obviously of questionable use to power a microprocessor based system.  But at the end of the day, it's your choice.
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It has 70 v AC on the output side ?    If that is what you mean,  it sounds dangerous.

It is strange that the ad for that device doesn't mention the output voltage or current at all,   it's impossible to understand from the ad what you could actually use it for.

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This power supply is meant to supply a current of  300mA  to a string of 4 or 5 LEDs in series.  Under these conditions the output voltage will be  up to around 16V.

If you connect a different load then things will change.
It will try it's best to deliver a current of 300mA, (that is what it is designed to do), and the voltage will increase or decrease as appropriate to maintain that current.

If your load is a 9V battery, then a current of 300mA can flow and the voltage will be a little over 9V.  I would expect the battery to get hot after a while.

However if your load is a 7805 voltage regulator powering an Arduino, that only requires a current of around 50-100mA when powered correctly, then the only way the power supply can try and deliver that 300mA is to raise the output voltage as high as it can.

You are lucky that the 7805 hasn't failed and allowed a high voltage to damage your Arduino.
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It has 70 v AC on the output side ?    If that is what you mean,  it sounds dangerous.

It is strange that the ad for that device doesn't mention the output voltage or current at all,   it's impossible to understand from the ad what you could actually use it for.


It works fine as a led driver, output is 16v dc with a short length of strip


* IMAG1021.jpg (66.11 KB, 1000x565 - viewed 8 times.)
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This power supply is meant to supply a current of  300mA  to a string of 4 or 5 LEDs in series.  Under these conditions the output voltage will be  up to around 16V.

If you connect a different load then things will change.
It will try it's best to deliver a current of 300mA, (that is what it is designed to do), and the voltage will increase or decrease as appropriate to maintain that current.

If your load is a 9V battery, then a current of 300mA can flow and the voltage will be a little over 9V.  I would expect the battery to get hot after a while.

However if your load is a 7805 voltage regulator powering an Arduino, that only requires a current of around 50-100mA when powered correctly, then the only way the power supply can try and deliver that 300mA is to raise the output voltage as high as it can.

You are lucky that the 7805 hasn't failed and allowed a high voltage to damage your Arduino.


Thanks, that explains it perfectly. I still don't understand why I was measuring 70vac on the input and 12vac on the output of the 7805. Fortunately I have got some led strip to use with the drivers and you were spot on with the output voltage.
Do you know of any alternative supply that is ultra-compact? I am hoping to get the power supply, a four channel relay module and a veroboard with a stripboard arduinio, radio etc  into a 80 x 140mm box. You can see from the photo that there will not be much room with the mains wires in place.


* IMAG1022.jpg (63.46 KB, 1000x565 - viewed 8 times.)
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On the basis that you have to plug your mains lead into a wall socket then why not buy a suitable wallwart which outputs 12volts (or whatever) and instal a suitable socket on your case for the wallwart DC lead to plug into.  That offers the advantage of solving your space problem and keeps the mains AC voltage away from your low voltage electronics.
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