|
Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
|
|
|
Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
|
In Reply to: Gepetto: Responding to your suggestions/questions posted by rwalton on May 4, 2002 at 20:16:30:
Hello RickHere are some answers to your questions:
>Put a small sense resistor in series with ther transformer lead that connects to one of the AC terminals of your rectifier bridge.What is a sense resistor? Can you specify what I need and where I'd get one?
Answer: You can get a suitable sense resistor from places like Digi-Key, I would pick something like part # 15FR050-ND that you can see on the following pages of their catalog. It will handle 10 A steady state and much larger peak currents, there are lower values as well that will handle even larger currents.
http://info.digikey.com/T022/V5/0639-0642.pdf>Use something like a 50 milliohm, high power resistor. Float your oscilloscope and measure the voltage peak across the resistor when you power up the amp.
I can probably borrow an oscilloscope and I have this digital signal processor that can sample analog input voltages at a rate up to 100 khz, so I should be able to use it to capture the voltage peak/decay. Do I need an oscilloscope or will the DSP do? Also what does it mean to float the oscilloscope?
Answer: What it means is to put a cheater on the 3 pronged AC plug of the oscilloscope so you can make measurements which are not ground referenced easily. There are ways of doing this on a scope without using the cheater but are harder to describe how to do. This is the simplest. After floating the scope and installing the sense resistor just before the bridge rectifier, put th eground clip of the scope on the end of the 50 milliohm sense resistor that is connected to the bridge rectifier and the probe tip to the other side.
If you use the DSP that you have, you will probably have to re-reference the range that the transformer output to that which the DSP will tolerate which can be done but is likely to be a hassle. Try and borrow the oscilloscope if you can.
>Set your trace sweep to something relatively slow so you can see the current decay as your input caps charge up. You should be able to capture the peak currents after turning on numerous times.
What would be relatively slow? Is this just a trial an error process?
Answer: You want to capture multiple line cycles so you can see the initial pulse and subseqent charging cycles as you turn on the amp. Since each 60Hx cycle is 16.6 milliseconds and a 50Hz cycle is 20 msec, I would set the scope to no less than 20 msec/div and more likely to 50 msec/div on the horizontal scale. Yes this is somewhat a trial and error process. The slower trace rate will give you better persistence on the scope screen so you will be able to see the peak current better.
I skipped a couple of your answers in this space, thanks for the info it helps with what follows:
>What is the continuous current rating of your bridge rectifiers? Non-repetitive surge current? And PIV?The diodes are BYQ28E double diodes rated for 5A continuous (per side) and surge current of 55A. What is PIV?
PIV is Peak Inverse Voltage which is 150V for the BYQ28E that you are using. This should be fine given your rectified DC voltage of ~+/-45 VDC which you are probably getting. However if you have used reasonably large wiring on your primary which I suspect you are, you are getting well over 10A peak inrush current to charge up your 9400uF input capacitors. These diodes are only rated for 10A repetitive peak. Even under normal operation you are likely to get that. I suspect your problem is the choice of these diodes. I recommend that you try to get the BYQ30E as soon as possible. These are a direct drop in and give you 16A continuous and repetitive peak and a non repetitive peak of 80A. Still not enough in my opinion but it will make an improvement.
(These diodes you have chosen are intended for high frequency switchmode applications where they see a square wave input from the switching transformer and relatively lower output capacitance than you have chosen. Therefore they see much lower peak to average rectified current ratios than diodes that are used for 50/60 Hz AC line rectification. I know these fast recovery types are in vogue for tweaking amplifiers but are in general not as rugged as conventional bridge rectifiers. I am trying to get you info on more conventional rectifiers but the OnSemi web site is currently down for maintenance)
We can take this to email if you need additional info. It is a hassle cutting and pasting into this dialog box they give you.
Regards
Joe...
G.
Crank it up...
This post is made possible by the generous support of people like you and our sponsors:
Follow Ups
- Some answers... - Gepetto 07:36:44 05/05/02 (1)
- Also try to get the OnSemi MUR1620CT... - Gepetto 08:21:02 05/05/02 (0)
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
