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I built a power supply for a micro-controller board that requires 5VDC and peak of 1A (typically sits at 500mA). The power supply has an input of 24VAC. I used a bridge rectifier and an LM2575-5 switching regulator to get 5V 1A. All good, worked well.

Problem is I've now found out that the 24VAC input transformer must to shared with some external devices in the same system (controls equipment), and they use a 24VAC transformer with the secondary neutral grounded. This made my bridge rectifier go bang!

I don't want to have to use a separate 24VAC transformer, so I was thinking of using a half-wave rectifier. I've found that the LM2575 regulator will accept about 8V to 40V input and is about 75% efficient. To overcome the ripple from the half-wave rectifier, the smoothing capacitor would need to be huge.

Surely there's a better way to do this! Maybe a half-wave rectifier isn't the right way to go about this? I'd really appreciate some advice and suggestions.

Disclaimer: I'm not an EE, just a tinkerer... In case that isn't already obvious ;-)

Edit: Here's a circuit diagram.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Please add a schematic of what your existing design is, and the actual deployed design schematic, including the grounded transformer. In other words, please edit the question to state what the actual problem is, rather than your intended solution. This will make it easier to interpret the problem statement, and also to suggest a workable way forward. \$\endgroup\$ – Anindo Ghosh Jun 20 '13 at 5:36
  • \$\begingroup\$ Yup, much clearer now. Question: Why does your circuit ground need to be connected to the supply ground? Presumably the supply has just two wires coming from it, carrying an AC voltage. Where is that ground connection coming from? A cable shield? In which case, can you simply not connect the cable shield to the circuit ground? \$\endgroup\$ – Anindo Ghosh Jun 20 '13 at 6:35
  • \$\begingroup\$ The supply ground is chassis-ground/earth (devices are mounted in a panel). My circuit has some IOs and serial communications that interface with other devices in this panel, which are referenced to the chassis ground/earth. AFAIK, unless I isolate everything with optoisolators or something, I have to use this common ground. Previously I used an isolated transformer for my circuit, so the bridge rectifier was fine, but I now have to use the existing transformer with the secondary grounded. \$\endgroup\$ – Darian Jun 20 '13 at 7:12
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    \$\begingroup\$ Yup, then you'll just have to use a half-bridge and a bigger capacitor. The full bridge causes two of the diodes to short the peak voltage to GND at each half cycle. \$\endgroup\$ – Anindo Ghosh Jun 20 '13 at 7:15
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To overcome the ripple from the half-wave rectifier, the smoothing capacitor would need to be huge.

For your current circuit you have a 47uF capacitor and if it were half wave rectified your capacitor would need to increase in value but probably only needing to double in capacitance. I'd consider doing this because it's your easiest solution.

If your output transformer produces 24VRMS then the peak voltage it will generate before and after the extra ground would be exactly the same at 34V. With a full wave rectifier your resulting peak of DC would be about 32.5V and for the half wave solution it would be about 33.2V.

With 47uF fitted the bridge ripple would only be about twice that of the half wave solution so I would say, for it to dip close down to the 8V lower operating area of the switcher, you probably haven't got enough capacitance to start of with.

It's also worth remembering that if the ripple is kept lower, the switcher input voltage will be on average, higher and more significantly the current it takes from the rectified supply (half wave or full) will be lower because IT is a power converter and when the net voltage is higher it draws less current - make use of this benefit and increase the capacitance to say 330uF, 50V.

Panasonic Series: FK Type: V 330uF, 50V is 12.5mm diameter and 13.5mm in length. Is this really huge?

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  • \$\begingroup\$ I tested this in CircuitLab, and the simulation shows the half-wave rectifier with a 330uF capacitor output a voltage range of 33V to 18V, which is perfect. Here's the test circuit: CircuitLab link \$\endgroup\$ – Darian Jun 20 '13 at 8:53

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