Hello.

I have made a transformerless power supply for a small 24V DC fan.



The 330 ohm resistor is to simulate the fan. The question is I don´t understand why the filter capacitor moves down the voltage

Wihtout capacitor


With capacitor


Maybe it is a silly question.

Thank you.

 

Why the capacitor in series with the bridge?

 

Why the capacitor in series with the bridge?

Because it's a transformerless power supply (capacitive dropper)

 

Hello.

I have made a transformerless power supply for a small 24V DC fan.



The 330 ohm resistor is to simulate the fan. The question is I don´t understand why the filter capacitor moves down the voltage

Wihtout capacitor

With capacitor


Maybe it is a silly question.

Thank you.

Because the ultimate limit to the current is set by the reactance of the series capacitor. The average DC voltage should be the same in each case, but your meter is probably reading the RMS voltage, which will change with the shape of the waveform.

 

Looking at those two images, I would say that the cap did raise the voltage.

 

Thanks for your answers.

Yes, the 1,17 if capacitor is to limit the current instead using a resistor which would dissipate a lot of power.

 

What are you using for a 1.17µF capacitor?

why the filter capacitor moves down the voltage

It's load dependent.
If you reduce the load current, the capacitor voltage will rise.

 

The filter capacitor changed to voltage that the meter was showing because every reactive component in that circuit affacts the voltages. a series capacitor will limit the current depending alsovon every other item in the circuit.
Where did you find this circuit, and how much of an explanation was provided as to how it works?

 

Will the fan start up reliably with that supply configuration is the question.

 

Thanks for your answers. Yes, the fan works nice, but with the filter capacitor drains 63mA and when I remove it the capacitor drains 68mA. It is important to maintain this capacitor? It is a 24V 70mA brushless motor.

What are you using for a 1.17µF capacitor?
It's load dependent.
If you reduce the load current, the capacitor voltage will rise.

First I use a 440 ohm resistor in series, and the fan behaves as a load of 24/0,07 = 343 ohm,



The result is a 1,12 uf and the closer I have is 1,17uf. And it works, or almost. The fan should drain 70 mA and it drains 68 without filter and 63mA with filter

 

Is that the rms current or the average DC current?

 

The difference in current readings stated in post #10 is not a big deal, given the tolerance of the components in use. And what sort of device is the TS using to measure current? In an AC powered system there is usually a fair difference between average values and peak values, and without instruments to observe the waveform an exact measurement is not likely.
So the correct question is "how close to the fan's required performance is the actual performance?"
Also, be advised that using the circuit shown in post #1, there exists a potential shock hazard from every section of the circuit. That means that contact with any portion of the conductors should be avoided.

 

Is the insulation breakdown voltage of the fan specified? It must be >2.5kV, and the wiring to it must be double insulated, and there must be no exposed connections (as there sometimes are on small DC fans).

 

Thank you for all your help and advice. The current is measured with a cheap ammeter.

Yes I know the risks of a transformerless power supply so everything is going to be carefully insulated

So can I get rid of the filter capacitor or it is important for the fan?

 

Thank you for all your help and advice. The current is measured with a cheap ammeter.

Yes I know the risks of a transformerless power supply so everything is going to be carefully insulated

So can I get rid of the filter capacitor or it is important for the fan?

Keep it. The fan probably has a UVLO, so it will start when the voltage reaches a certain threshold. At that point it’s going to need extra energy to start, so some stored in a capacitor would be useful.

 

Probably the filter capacitor helps run the fan much closer to the intended conditions and specifications. The "cheap ammeter" is giving the average value, probably, if it is a mechanical meter. If it is an electronic digital mutimeter, then it may be closer to the peak value,if you are using a DC current setting.

 

Thank you! I´ll keep it