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Hi All I am hoping to use a Placid to power a Squeezebox 3, I have measured the power consumption at 950ma running but 1.1A on start up so my target is 5V at 1.2A. I built the kit as supplied + some extra heat sinks for testing and a 25va 7.5v transformer. All worked fine but I could only adjust the current output to a max. of 1.05A and voltage down to a min. of 5.15v. Now begins the fun, after a lot of reading of this and the DIY forum + a bit of guess work, I changed R7 to 4k7 this allowed me to reduce the output voltage well below 5V, next I Added a second green LED in series with BD1, although they were veryvery dim this did allowed me to increase the output current to well over 1.2A great! Ok the rectifiers are now VERY hot, so I changed it for higher current one, turn on and all ok. R1 VERY VERY hot so in with a larger ww one, turn on and.....R2 starts smoking, on checking R1 I find I have put in .05R by mistake, ok change it to 1R and all seems to work. Time check the output with the scope, I find I have 30mv of ripple and bad oscillation. Some settings of current and voltage stop the oscillation (it starts at the bottom of the waveform if that's a clue) however the ripple is always there. Here endeth my knowledge, I have returned the board standard values but no change. Can anyone suggest what I may have killed with the wrong R1? or where to look next? I did try a 9V transformer to rule out a headroom problem. I stupidly didn't scope the output before I modified the circuit. Any help will be much appreciated Many Thanks. Spresto9 Edited by user Friday, January 13, 2012 10:29:21 AM(UTC)
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Well with no load you are very likely completely saturating the shunt driver. You may want to make R10 and R11 1K and R9 75R, but try this last. Do the below first. Where is BD1? Do not use two LEDs here, that is too much drop. Instead reduce R5 to 470R. Leave R1 as 1R but make R2 .1R (or similar low value) and remember to factor this in when calculating shunt current. R2 looks like it is close to toast. Placid in it stock configuration is compensated for a capacitive load. Add 100uf or so to the output along with your dummy load, which should use at least half of you CCS current. You can also experiment with C5 (say 1nf) but you should not need to once you have a proper load. I have not used such high current yet, but I am glad to help you out. Edited by user Friday, January 13, 2012 5:22:27 PM(UTC)
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Also keep R7 100K, and make R12 1K - 1.5K to get down below 5V. Edited by user Friday, January 13, 2012 5:26:50 PM(UTC)
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Hi Russ Thanks for the quick reply, I know how very busy you are. I implemented all the suggestions you made above:- BD1 on bottom of board for now R5 tried 470R but had to reduce to 127R to get 1.2A R1 - 1R, R2 - .05R R7 - 100K R12 - 1K C5 - 100p Load - 100uf + Squeezebox ~950ma No oscillation but still ripple then changed:- R10/11 - 1k R9 - 75R No change, about 50mv of ripple. Thanks again for any help you may be able to give. Spresto9 spresto9 attached the following image(s): IMAG0233.jpg (120kb) downloaded 112 time(s).You cannot view/download attachments. Try to login or register.
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Measure the rectified voltage, very likely you are running out of headroom from your trafo. Also that is a ton of current, you would do good to add some additional capacitance. Say double. I bet if you measure the ripple at the rectifier it is pretty bad.
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Hi Russ
Thanks again for your time, I have changed to a 9V transformer and added an extra 22000uf to C1, this has improved the adjustment range so headroom may have been a problem, ripple has gone down to ~ 8mv (same waveform)
Do you think that this is the best I can expect considering that the Placid was not designed to work with this much current?
Do you think there is anything else to try to improve things?
There are many hundreds of people out here enjoying the fruits of your labour, Please keep up the good work.
Cheers
Spresto9.
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Yes the result can be improved, One thing that may help is increasing C2. Even say 220uf.
Lowering R5 had the effect of making the PSRR lower. You could go back to 1K and that would help. To do that you need to either reduce R1(say two 1R in parallel) and keep this mind when calculating CCS current, and/or raise the BD1 VREF voltage. To do this just replace BD1 with a zener diode. Say 4 - 5V. If you do this keep in mind you will need the extra input voltage headroom. I would say 9-12VAC would be called for. This thing is going to run very very hot. :) This is why personally I would prefer a series reg for this type of situation.
Cheers!
Russ
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Hi Russ Meet Frankenstein’s monster, It lives, I implemented the changes you suggested but the ripple Is still 5mv, so I thought I’d try it anyway……………………well,it’s a big improvement over the original supply, more detailed and musical! But I think you’re right, by the time it’s boxed up George Forman would be proud to use it! Plan B, return Placid to original circuit and find a better use for it and look for a good series regulator design! Thanks again, it’s been fun. Spresto9 spresto9 attached the following image(s): IMAG0234.jpg (91kb) downloaded 75 time(s). sideImage_history.jpg (23kb) downloaded 71 time(s).You cannot view/download attachments. Try to login or register.
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