I am building a gainclone amp based on the xy pcb's and today was testing the power supply with a pure resistor load.
The power supply is this configuration :-
http://store.qkits.com/assets/jpg/QK186b.jpg
Transformer is 225VA 18-0-18-0
Single Bridge Rectifier
2 x 22,000uF then 2x 10,000uF caps (33,000uF per rail)
0.1uF bypass caps soldered directly on each big cap and a mylar 0.15uF bypass at psu output terminals.
centre rail of the psu and the 0-18 centre tx taps joined and go back to a star earth on the chassis which is connected to the mains iec earth.
I put a load of 800R on each rail just to test and get +-28v
I then used a cheap Uni-t DMM on AC to see if I could measure ripple and got a very odd reading of 60V AC on each rail (Not possible with 18-0-18 in). I thought I might have a bridge fault at first so I got out my scope and used that and it showed 27mV of AC ripple on each rail sitting on 28V DC
Questions...
Can you use a basic DMM set to AC to measure ripple at the DC output and why would I get such an odd reading like 60V AC across each rail. Infact thinking about it, it is half mains voltage across the + and - even though the scope shows correct +- 28V DC with some ripple. I just don't get what it was reading ? Maybe a analogue meter would have been better ?
What is an acceptable amount of ripple for the LM3886TF and is it possible to reduce it without regulators ? I know the resistor I used will not be the i/p supply resistance of the 3886 circuit so this maybe a dumb test at this stage since a higher resistor should give less ripple. I guess I need to test on its correct load but have no idea what resistor I should have used just to psu test.
Sorry if the first question seems dumb but I can't get my head round that.
The power supply is this configuration :-
http://store.qkits.com/assets/jpg/QK186b.jpg
Transformer is 225VA 18-0-18-0
Single Bridge Rectifier
2 x 22,000uF then 2x 10,000uF caps (33,000uF per rail)
0.1uF bypass caps soldered directly on each big cap and a mylar 0.15uF bypass at psu output terminals.
centre rail of the psu and the 0-18 centre tx taps joined and go back to a star earth on the chassis which is connected to the mains iec earth.
I put a load of 800R on each rail just to test and get +-28v
I then used a cheap Uni-t DMM on AC to see if I could measure ripple and got a very odd reading of 60V AC on each rail (Not possible with 18-0-18 in). I thought I might have a bridge fault at first so I got out my scope and used that and it showed 27mV of AC ripple on each rail sitting on 28V DC
Questions...
Can you use a basic DMM set to AC to measure ripple at the DC output and why would I get such an odd reading like 60V AC across each rail. Infact thinking about it, it is half mains voltage across the + and - even though the scope shows correct +- 28V DC with some ripple. I just don't get what it was reading ? Maybe a analogue meter would have been better ?
What is an acceptable amount of ripple for the LM3886TF and is it possible to reduce it without regulators ? I know the resistor I used will not be the i/p supply resistance of the 3886 circuit so this maybe a dumb test at this stage since a higher resistor should give less ripple. I guess I need to test on its correct load but have no idea what resistor I should have used just to psu test.
Sorry if the first question seems dumb but I can't get my head round that.
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