Simple Full Wave Rectifier with smoothing capacitor question

[lloydi12345]

Okay, so I have an input voltage to my rectifier diodes 1n4007 of 25.10VAC. I'm using this circuit:



My capacitor's value is 1000uF 50v. Strange thing is that the output dc voltage is 34.2VDC. Any explanation you could share with this one? I was expecting I would get 24VDC to 26VDC not this high.

Thanks


lloydi

 

[Diver300]

AC voltages are measured as RMS (root mean square) voltages. If the waveform is sinusoidal, the peak voltage is sqrt(2) times the RMS voltage.

The actual voltage is reduced by the diode voltages and by resistive drops, but it will often be more than the RMS voltage.

 

[lloydi12345]

Thanks Diver300 for that comprehensive answer. I have follow up question though, do you have any suggestion I could add on the circuit to reduce it's voltage to 24vdc aside from placing a voltage regulator?

 

[gilmanli]

Hi,
Why the rectifies output voltage is high than 24V which is that the output voltage use fullwave rectifies should be sqr(2) time the 24V.

 

[crutschow]

Hi,
Why the rectifies output voltage is high than 24V which is that the output voltage use fullwave rectifies should be sqr(2) time the 24V.

What??

 

[crutschow]

Thanks Diver300 for that comprehensive answer. I have follow up question though, do you have any suggestion I could add on the circuit to reduce it's voltage to 24vdc aside from placing a voltage regulator?

A voltage regulator is the usual way to drop the voltage to a desired steady voltage independent of load.

 

[lloydi12345]

Hi,
Why the rectifies output voltage is high than 24V which is that the output voltage use fullwave rectifies should be sqr(2) time the 24V.

Thanks

What??

I think he's trying to explain.

A voltage regulator is the usual way to drop the voltage to a desired steady voltage independent of load.

How about using zener diode for dropping voltages? Will it have good result?

 

[Diver300]

A zener diode will need a series resistor. Also a zener circuit will always use the maximum power of your circuit, so for large powers that is very inefficient, and you have a lot of heat to get rid of.

However, you haven't said how much power your load takes. For small loads, a zener is very simple and robust.

 

[ericgibbs]

hi lloyd,
You should find this pdf helpful.

 

[lloydi12345]

A zener diode will need a series resistor. Also a zener circuit will always use the maximum power of your circuit, so for large powers that is very inefficient, and you have a lot of heat to get rid of.

However, you haven't said how much power your load takes. For small loads, a zener is very simple and robust.

I guess I'll be dissipating much heat if I'll add zener diode. I'm converting the 32Vdc to 5Vdc. Using Emitter Follower Voltage regulator (with the use of TIP31C) 32Vdc turns to 14Vdc and is fed through 7805 turning down the voltage to 5Vdc. The IC heats up even with a big heatsink and same as through with the transistor with heatsink even hotter. The load takes 0.9A making them heat so bad.

hi lloyd,
You should find this pdf helpful.

Thanks for the document! Is it okay to use the Full Wave Bridge Resistive Load? How about the Full Wave Bridge Choke Input Load? I've seen an inductor was used but there is no value.

 

[Diver300]

With a 24 V transformer and a 5 V, 0.9 ohm load you will have around 25 W of heat to get rid of.

If you use a zener regulator, you will also have another 4.5 W to get rid of in the zener diode, but only at no load. At the full load there will be very little extra heat to get rid of.

So either a series or a zener regulator will have a lot of heat.

You can cut the heat down by either using a lower voltage transformer, or by using a buck switching regulator.

 

[ronv]

If your transformer has a center tap you could use that and cut the votage in half.