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Old 8th April 2009, 04:47 PM   #311
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Tripmaster
I have just built a couple of adjustable regulated power supplies for a pair of Pedja's discrete buffers. I have read somewhere that I can put a low value resistor after a single reservoir capacitor to help filter the supply.
that creates a CRC filter.
Find your excess voltage when worst conditions apply.
That is the maximum voltage you can drop across the added R of the CRC. use a resistor combined with your output current that drops <=your excess voltage.

If your excess voltage at worst case conditions is very low or negative (i.e. dropout @ low mains voltage and maximum current draw) then R<=0r0.
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Old 13th April 2009, 05:21 PM   #312
andreja is offline andreja  Serbia
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I have transformer with output of ~45 V and I want to use it to built an amplifier with tda1514 but I need symmetric power supply +-30V How to make a ground connector if I haven't one?
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Old 13th April 2009, 05:51 PM   #313
AndrewT is offline AndrewT  Scotland
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either adopt a single polarity chipamp schematic or buy a dual secondary transformer.
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Old 13th April 2009, 06:07 PM   #314
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Quote:
Originally posted by andreja
I have transformer with output of ~45 V and I want to use it to built an amplifier with tda1514 but I need symmetric power supply +-30V How to make a ground connector if I haven't one?
There was some talk about it here: http://www.diyaudio.com/forums/showt...12#post1799612
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Old 18th April 2009, 06:07 AM   #315
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Hi

I purchased several 18vac encapsulated transformers a while ago as they were only 70p each! I have just come to use a pair of them and the output from both secondaries are 23vac. When I have measured other transformers in the past the voltage normally exceeds the stated amount by one or two volts.

Click the image to open in full size.

Click the image to open in full size.

Secondary output 23vac
Rectified 20.2vdc (not connected to power supply)
Rectified 30.7vdc (reading taken from power supply input)

I am using KBU8G rectifiers.

Any ideas?

Thanks

Richard
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Old 18th April 2009, 08:27 AM   #316
AndrewT is offline AndrewT  Scotland
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small transformers have much higher regulation that big transformers.
It not unusual for regulation to exceed 15% in the smaller sizes and even approach 30%.
23Vac is<28% regulation But I see you have a 230Vac transformer running on a 240Vac UK supply.
That takes off a further 4% giving <23% regulation.
This is all quite normal.

Now to specifics.
What is the mains input voltage when you measure the secondary open circuit output voltage?
That will let you calculate the turns ratios and the regulation.

Now add a 180r or 220r Power resistor to each secondary and repeat the voltage measurements.
Now add a second pair of 180r or 220r Power resistors in parallel to the first pair and repeat the measurements.

The output voltage should be getting close to what the manufacturer specified for a 230Vac mains supply.
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Old 18th April 2009, 09:12 AM   #317
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Quote:
Originally posted by AndrewT
small transformers have much higher regulation that big transformers.
It not unusual for regulation to exceed 15% in the smaller sizes and even approach 30%.
23Vac is<28% regulation But I see you have a 230Vac transformer running on a 240Vac UK supply.
That takes off a further 4% giving <23% regulation.
This is all quite normal.

Now to specifics.
What is the mains input voltage when you measure the secondary open circuit output voltage?
That will let you calculate the turns ratios and the regulation.

Now add a 180r or 220r Power resistor to each secondary and repeat the voltage measurements.
Now add a second pair of 180r or 220r Power resistors in parallel to the first pair and repeat the measurements.

The output voltage should be getting close to what the manufacturer specified for a 230Vac mains supply.

Thanks Andrew

I donít think I have any power resistors in stock at the moment (at the office whilst typing this) so I will call into Maplin at some point.

I have built a couple +&- power supplies using LM317T and 337T regulators. I am currently dropping 10.7vdc between input and output, this is then regulated again on a buffer from 20vdc to 15vdc using LM7815 and LM7915.

Will the LM317T - 337T run hot without a heatsink?

Thanks

Richard
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Old 18th April 2009, 11:00 AM   #318
Nuuk is offline Nuuk  United Kingdom
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Quote:
Will the LM317T - 337T run hot without a heatsink?
Probably. I reckon you would be better to drop around 7.5 volts on both stages of regulation and then use small heatsinks to be on the safe side.
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Old 18th April 2009, 11:09 AM   #319
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Quote:
Originally posted by Nuuk


Probably. I reckon you would be better to drop around 7.5 volts on both stages of regulation and then use small heatsinks to be on the safe side.

Ok, thanks
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Old 20th April 2009, 01:50 PM   #320
keaster is offline keaster  Canada
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I recently built a LM3886 gainclone amp and what Iíve noticed is that every circuit as a switch before the transformer. After testing and playing around with the amp I found that after switching the switch off the amp still runs for about 10 seconds drawing voltage from the caps. So everytime the amp is turned back on the caps would charge back up, does this but any stress on the caps and/or transformer? Is there a better solution to this like a relay between the psu and amp board because Id like to find a way to have the amp just turn off when the switch is turned off

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