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Old 27th November 2004, 11:11 AM   #21
lopan is offline lopan  United States
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Quote:
Originally posted by peranders
To be more precise: If the smoothing caps are small and you for some reason has additional smoothing on the regulated side...and the discharge time is longer in the regulated side than the unregulated, then you need these diodes but normally you don't, espcelly if you have known loads but if you are going to make a general power supply you should have these diodes, just in case. They are for protection purposes only.

I don't quite understand what you are saying. Doesn't one always have more capacitance on the regulated side. I know I will. Wouold you mind explaining this again.

Thanks
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Old 27th November 2004, 06:14 PM   #22
maxw is offline maxw  United Kingdom
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Quote:
Originally posted by lopan



I don't quite understand what you are saying. Doesn't one always have more capacitance on the regulated side. I know I will. Wouold you mind explaining this again.

Thanks
You always have lots of capacitance on the pre-regulator side but I think peranders is talking about having capacitance the post regulator side, ie 470uF on my schematic and these diodes protect the regs from the discharge from these caps?
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Old 27th November 2004, 06:41 PM   #23
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The point is, if you switch off the primary side, that side may fall faster than on the regulated side. That means that the output voltage could actually become higher than the input voltage, iow the reg is reverse biased, and it may be damaged. The reverse diodes make sure that the reverse voltage stays below 1V. What effectively happens is that the output caps spill over through the diodes into the input caps.
Whether this actually happens depends on the relative capacitances, the load and a couple of other things that are difficult to predict, so it is like an insurance.

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Old 27th November 2004, 07:54 PM   #24
lopan is offline lopan  United States
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Thank you. clear concise explanation
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Old 27th November 2004, 08:22 PM   #25
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Quote:
Originally posted by lopan
I don't quite understand what you are saying. Doesn't one always have more capacitance on the regulated side. I know I will. Wouold you mind explaining this again.
Example: rectifier bridge + 10000 uF + LM317 + 22-100 uF This will never create any problems.

rectifier bridge + 470 uF + LM317 + 2200 uF This _may_ create any problems.

So what Jan means is that if you don't have control over your load (know how it is), use always proetcion diodes.
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Old 27th November 2004, 08:38 PM   #26
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Quote:
Originally posted by peranders

Example: rectifier bridge + 10000 uF + LM317 + 22-100 uF This will never create any problems.[snip]

Pure speculation Per. If you have a problem in your equipment so it doesn't work, for instance, and you switch it off to do fault-finding, the sec voltage will stay up long after the input caps have discharged and Boom!


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Old 27th November 2004, 08:42 PM   #27
maxw is offline maxw  United Kingdom
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My last schematic gives +/- 13.75 BTW, I'll probably change it to +/-12 so I can use it with AD op amps though.
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Old 1st December 2004, 08:13 AM   #28
maxw is offline maxw  United Kingdom
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Finished!!!

I changed the layout to include a rectifier and screw terminals as this will be for testing opamps so I needed easy connectivity.

Click the image to open in full size.

Custom mirror PCB

Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

Costs:
transformer $26
Both Regs $8
PCB etc about $4
Caps, resisters, connectors and diodes about $17
Total approx: $55NZ = $39US

Now I just gotta make something for it to power
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Old 13th December 2004, 11:31 AM   #29
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hope those nasty yellow caps are not tantalum... otherwise try some starget/ panasonic instead....... good work ...
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i am currently using 60,000 uF of capacitance in my preamplifier..
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Old 13th December 2004, 12:37 PM   #30
maxw is offline maxw  United Kingdom
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Quote:
Originally posted by costiss
hope those nasty yellow caps are not tantalum... otherwise try some starget/ panasonic instead....... good work ...
They are tantalum as specified here:
http://tangentsoft.net/elec/teps/pguide.html
I dont know enough to know what is better but I know that starget would be overkill given how much they are in New Zealand


So you think small electrolytics would be better? how come?
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