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Hi. I need to convert AC to DC, and I was thinking of using a bridge rectifier using four diodes. I was wondering if there should be any way to approach this. I have four diodes, V 90SQ045, which I pulled from an old circuit board, and I also have a 22000μF capacitor I pulled from the same board, and I was going to add that smooth the DC output. The leads on the diodes arn't that big, +/- 1/2inch, so can I just solder them together?

I know what AC is and how it works, but I haven't worked with it before, so I appreciate any help.
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On wikipedia is a photo how you can solder them together.
https://en.wikipedia.org/wiki/Diode_bridge

I don't know how much current the transformer is, but if you have connected something wrong, a diode or the capacitor might blow.

The 22000uF seems very large.
Rule of thumb is to use 4700uF per Ampere.
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Looking at my power supply/transformer, here are the specs: Primary: 120VAC 60Hz 50W, Secondary: 24VAC 40VA. VA, is that just Amps? My load it 0.125A - 0.350A @ 24VDC.
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You need to advise what AC voltage you are working with, the voltage and current rating of the diodes, the intended load current  and the DC rated voltage of the capacitor.


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The 24AC will produce around 36DC at the capacitor.  The somewhat large capacitor value should be OK but the inrush charge current will be quite high so you might want to fit a load limiting resistor in the transformer output, say 10ohm rated at 3 watts.
« Last Edit: June 24, 2013, 04:25:27 pm by jackrae » Logged

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Looking up the part number of the diode, it looks to be a low forward voltage (Less voltage loss is how I understand it (0.48V loss to be exact)) and it looks like it should work. I think the board I took it from was actually using it as a bridge rectifier, but I'll have to check on that.
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Looking at the circuit board, the diodes were used as a bridge rectifier. What the board was used for, I don't know. It looks like it has 18VAC going through a 5A fuse before it goes in to the rectifier.
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A drawback of the classic four-diode rectifier bridge is the unavoidable forward voltage drop (Vf) of two diodes when current is flowing.  With conventional silicon diodes, this could typically amount to 1.5 volts or more.  The result of this is wasted power and reduced efficiency in power supply applications.

We could eliminates this drawback by replacing the diodes with MOSFETS. This will be useful when we work on high current.


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A drawback of the classic four-diode rectifier bridge is the unavoidable forward voltage drop (Vf) of two diodes when current is flowing.

In my case, it shouldn't be a problem. The two diode together would be about 1V loss. I metered the power supply unloaded, and it was about 25.6VAC, and I need 24VDC, +/- 10% at 0.125A-0.35A.

I think I am going to solder my four diodes together and hook it up to AC, and then figure out the capacitor from there. 22000μF does seem like it is to much, so I will try to find a diode with maybe less than 4000.
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It works great, and it outputs exactly 24VDC according to my meter. Now I need to find a capacitor.
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I just attached a 2200μF 50V capacitor on to it, and now it is saying it is outputing just a little less than 40VDC. Does anyone know what it going on here, and if it is safe to attach my 24VDC device to it? (Solenoid and relay)
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just attached a 2200μF 50V capacitor on to it, and now it is saying it is outputing just a little less than 40VDC. Does anyone know what it going on here, and if it is safe to attach my 24VDC device to it? (Solenoid and relay)
For a solenoid & relay, don't use the capacitor.

Two things are going on...   The peak voltage of an AC waveform is about 1.4 times the RMS.  The capacitor charges-up to the peak.   Also, transformers are rated at some load.  40VA @ 24VAC is 1.67 Amps.   With a smaller load (higher resistance or no resistance) you'll get a slightly higher voltage.  And, there's some tolerance in that voltage.   (It's one of the main reasons we like to use voltage regulators.)

The RMS voltage is something like an average, and it turns-out that 24VAC RMS (will generate the same power as 24VDC.   For example, here in the U.S. where our line voltage is 120V, the peak is 160V. And if you connect a 100W light bulb to 120VDC, it will glow with the same brightness as 120VAC.     

And, when you rectify AC (ignoring the diode drop) you get the same RMS value.

There is a small difference with a solenoid or relay coil, since they both have inductive reactance, but at 50 or 60Hz, they will usually work fine with rectified AC.
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just attached a 2200μF 50V capacitor on to it, and now it is saying it is outputing just a little less than 40VDC. Does anyone know what it going on here, and if it is safe to attach my 24VDC device to it? (Solenoid and relay)
For a solenoid & relay, don't use the capacitor.

Two things are going on...   The peak voltage of an AC waveform is about 1.4 times the RMS.  The capacitor charges-up to the peak.   Also, transformers are rated at some load.  40VA @ 24VAC is 1.67 Amps.   With a smaller load (higher resistance or no resistance) you'll get a slightly higher voltage.  And, there's some tolerance in that voltage.   (It's one of the main reasons we like to use voltage regulators.)

The RMS voltage is something like an average, and it turns-out that 24VAC RMS (will generate the same power as 24VDC.   For example, here in the U.S. where our line voltage is 120V, the peak is 160V. And if you connect a 100W light bulb to 120VDC, it will glow with the same brightness as 120VAC.     

And, when you rectify AC (ignoring the diode drop) you get the same RMS value.

There is a small difference with a solenoid or relay coil, since they both have inductive reactance, but at 50 or 60Hz, they will usually work fine with rectified AC.

Alright, good to know. I will cut the capacitor off and try it, thanks.
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24VAC 40VA. VA, is that just Amps?
"VA" is approximately Watts.  So a 40VA 24V transformer is probably good for about 1.5A
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I did advise you'd get around 36 volts.  Since your measurement of the DC output was taken with no load, the capacitor was charging to the peak voltage.  (RMS x 1.414) You can still use the 22,000 capacitor if that's all you've got (it's much like using a truck battery to run a radio  -  over-kill but still works).  However if you want 24DC then I'd suggest using a voltage regulator chip.

Using "raw" rectified AC  (viz unsmoothed DC) on a DC solenoid it could result in buzzing or overheating due to 100Hz cycling
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I'm only pulsing the solenoid for 50 milliseconds, so I would think it would be fine.
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