Electronics > Beginners

lab power supply

<< < (3/15) > >>

glinjik:
can anyone tell why there is caps on the rectifier part or is this just simbolising the internals of the bridge rectifier?

george.b:

--- Quote from: glinjik on December 14, 2019, 07:35:48 pm ---after experimenying with it im not gonna continue it seems flawed by voltage variations for input changes and output changes so lookang at this next one looks more stable although im not sure about the qulity of the 317 i have here 5 of them and they all give different output on same power supply  :palm:

--- End quote ---

I would only use an LM317 that came from a reputable source. Had my share of woes from counterfeit LM317s. :--


--- Quote from: glinjik on December 14, 2019, 08:11:44 pm ---can anyone tell why there is caps on the rectifier part or is this just simbolising the internals of the bridge rectifier?

--- End quote ---

To reduce noise when the diodes switch. It's not absolutely necessary.

glinjik:
thank you for your replies unfortunately here its difficult and expensive to get parts from a reputable dealer even they sell fake parts for a 317 from like rs it can take several weeks to get it, just for me to fry the thing

MarkF:

--- Quote from: glinjik on December 14, 2019, 07:35:48 pm ---after experimenying with it im not gonna continue it seems flawed by voltage variations for input changes and output changes so lookang at this next one looks more stable although im not sure about the qulity of the 317 i have here 5 of them and they all give different output on same power supply  :palm:

--- End quote ---



If you are not doing any current limiting, why are you NOT using the suggested high current configuration from the datasheet?

David Hess:

--- Quote from: george.b on December 14, 2019, 08:34:02 pm ---
--- Quote from: glinjik on December 14, 2019, 08:11:44 pm ---can anyone tell why there is caps on the rectifier part or is this just simbolising the internals of the bridge rectifier?
--- End quote ---

To reduce noise when the diodes switch. It's not absolutely necessary.
--- End quote ---

Whether they are needed depends on the diodes and application.  Some rectifiers have a very fast and hard reverse recovery like a step recovery diode which produces broadband RF spikes at the power line frequency.  This varies by type, manufacturer, and even lot but usually occurs with standard recovery rectifiers which are often constructed with a PIN structure like a step recovery diode.

It is a major problem with audio and instrumentation circuits.  Solutions to prevent it include adding bypass capacitors directly across the diodes, adding EMI suppression beads in series with the diodes, selecting standard recovery diodes which do not have the problem, and using faster diodes which have a controlled recovery characteristic.  Trying to filter the noise at the input and output of the rectifier is a lost cause because it will already be radiating, parasitic capacitance will allow it blow by the filters, and because it will already have become common mode noise through the transformer's primary to secondary capacitance.

I first ran across the problem with some home built 7815, 7915, 7805 power supplies which only had to be powered on to cause problems with audio circuits anywhere in the room and to show up on oscilloscopes.  Tests determined that swamping the junction capacitance of the diodes with roughly 10 times the value prevented the interference so that is what I did.  Later I noticed capacitors across rectifiers in many service schematics and sometimes EMI suppression beads as well.

Below is the lab power supply schematic that I would emulate from National Semiconductor linear brief 28.  Instead of using expensive LM395 integrated power transistors, the power pass element could be replaced with suitable integrated regulators or bare transistors or some combination of the two.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod