Converting AC to DC Question

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  #1  
Old 11-11-10, 08:44 AM
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Converting AC to DC Question

I am aware that to convert AC to DC (I'm working with 12v) you need a rectifier and a capacitor and if you want very clean power, a regulator.
I am just using it for powering a couple of motion sensors and some relays, so I think a rectifier and capacitor would work fine for me.

I have seen DC voltage produced by only using a capacitor between + and - off of a transformer and nothing else. No bridge rectifier.

Can I expect usable DC power by doing this?
What are the benefits or draw backs?

Thanks,
Ben
 
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  #2  
Old 11-11-10, 10:21 AM
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You can't produce DC from AC with just a capacitor. It just doesn't work that way. There HAS to be a rectifier. The capacitor is there to smooth out 'ripple' in the wave. Otherwise it serves no function in rectification.
 
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Old 11-11-10, 11:17 AM
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Why not just use a DC power supply? What is the DC load? Even a wall wart might be enough.
 
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Old 11-11-10, 04:58 PM
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Some undercabinet LED light strips use a single diode, a cap, and I believe a zener, and maybe a couple other components to rectify AC.
 
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Old 11-11-10, 05:27 PM
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I measured the power coming off of the transformer with a meter and it was 11.7 VDC and 0 VAC. It does something... Not saying I would use it though.

I would rather not use a wall wort, this is going to be wired in in commercial applications.

You can use a single diode and a larger cap to make up for it but only in low draw applications.
 
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Old 11-11-10, 07:50 PM
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Then there is a rectifier built inside the transformer casing. There is no such thing as a transformer that steps down DC to DC. Nor is there any such thing as an AC to DC transformer. The transformer steps down the AC voltage from 120 to 12, then a rectifier changes it from AC to DC, and the filter capacitor smooths out the ripple. Whether this happens internally or externally is up to the manufacturer. But there is no possible way that a raw transformer can have a DC output.
 
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Old 11-12-10, 04:25 AM
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I will take it apart when I get to the shop. (maybe post some pictures too) But I don't think there is anything inside the transformer. It's just a transformer, no plastic casing or anything. I'll check inside of it, but also I will measure the power coming out of it when I take the capacitor off.
 
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Old 11-12-10, 04:28 AM
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Are you sure the transformer you measured isn't a "Power Supply"?

The label or stamped specs should be on the box somewhere. It will read something to the effect of "Input 120 VAC .2 amp, Output 12 VDC 500ma". That means it has a built-in rectifier.

Nothing says you have to have the rectifier on the transformer. You could use an AC step-down from 120 to 12, then put the rectifier circuit on the device's PC board or inside the chassis. In fact, depending on length of wire run and gauge of the wiring, it may be to your advantage to run 12 VAC from the transformer to the device and convert it to DC at the device end.
 
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Old 11-12-10, 04:40 AM
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I will check when I get to the shop, but I don't think it is.

Basically what I'm making is a safety system for folding gym partitions. The system will all be contained in one enclosure above the partition, and have motion detectors on either side of the gym. When the system is armed, the partition can be run, and if someone moves near it, it will shut off the partition and sound an alarm.
 
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Old 11-12-10, 05:15 AM
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Here are a couple pictures. I did not see anything inside it.

This is one that is obviously not hooked up.

The thing that gets me is there are close to 100 systems out there using this for dc voltage. (powering 12vdc relays and 12vdc motion sensors) And they all work...






Ben
 
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Old 11-12-10, 11:11 AM
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I've seen those before, usually in old portable AC/DC radios and alarm clocks (note the 117v, indicates at least 20-30 years old) The diodes are buried inside near the core. There is no room inside for the smoothing capacitor, which is why it is outside. Unfortunately there is no direct replacement for these. I searched Mouser and Allied and none of the power transformers they carry have rectified DC secondaries. You are going to have to use a regular AC-AC transformer with a bridge rectifier and a cap.
 
  #12  
Old 11-12-10, 12:22 PM
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OK!! That makes sense now! Glad we figured it out!! Thanks for all the help guys!


Ben
 
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Old 11-12-10, 03:14 PM
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If this is a school gym you may want to check rules and codes. You didn't say where you are, but the schools in my area don't allow anything that isn't UL listed to be permanently installed.
 
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Old 11-12-10, 07:39 PM
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I will check before I go ahead with anything. Thanks.
 
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