Amp 1 SMPS fluctuation problem

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locky

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Joined 2005
#1
Hi,
I have just built my 2nd Amp 1 from 41hz. I have picked up an SMPS for this which gives me +-24vdc when not hooked up to the amp. The problem I have is that when hooked up the voltage on one rail keeps rising whilst it decreases on the other side. I'm wondering why this is and how I stabilise it. The SMPS I have is a Lambda JWS 600 48 and the specs can be seen here...

http://www.lambda-gb.com/uk/range_overviews/range_id21data.htm

I'm a bit of a novice and would appreciate any thoughts/advice,

Many Thanks!
 
#3
locky said:
Hi,
I have just built my 2nd Amp 1 from 41hz. I have picked up an SMPS for this which gives me +-24vdc when not hooked up to the amp. The problem I have is that when hooked up the voltage on one rail keeps rising whilst it decreases on the other side. I'm wondering why this is and how I stabilise it. The SMPS I have is a Lambda JWS 600 48 and the specs can be seen here...

http://www.lambda-gb.com/uk/range_overviews/range_id21data.htm

I'm a bit of a novice and would appreciate any thoughts/advice,

Many Thanks!
Click to expand...


I have seen this with my SMPS in combination with UcD modules as well. It is caused by the amp pulling more current from one rail than the other under relatively low current load (so relatively large current difference between both rails). In that case, one rail goes up while the other one goes down as the SMPS tries to regulate the total output voltage to stay constant. The way I removed it is very quick and dirty (but it works). I added a couple of power resistors from + rail to GND (in the case of UcD + rail draws less current) to pull about the same current from + and - rail of the SMPS. This solves it.

I wonder what happens with the coldamp SMPS when I hook it up to UcD modules, I hope the resistor trick will not be necessary for their SMPS.

Best regards

Gertjan
 
#4
fredos said:
Add more capacitor to damp the psu output. Commercial power supply likethis is not design to drive that kind of load..

Fredos

www.d-amp.com
Click to expand...


This may help in some cases, however, in my case it made the effect only slower (meaning it took more time for the + rail to go up) it did not solve it. On the other hand, you have to be careful as some SMPS can become unstable when additional caps are added. Adding excessive amounts of caps can also trip current protection circuits in the SMPS (have seen that in my SMPS). Had to slown down the feedback loop and also the Vref (added an RC network to the reference voltage coming from the SG3525 chip) that went to the error amp to make it work with large amount of caps added.

Gertjan
 
#6
I added a couple of power resistors from + rail to GND (in the case of UcD + rail draws less current) to pull about the same current from + and - rail of the SMPS. This solves it.
Click to expand...

Thanks for the response. I tried adding some caps but this made no real difference. I am wondering what value resistors should I be using, considering that I have +-24vdc @ 13A?
 
#7
locky said:


Thanks for the response. I tried adding some caps but this made no real difference. I am wondering what value resistors should I be using, considering that I have +-24vdc @ 13A?
Click to expand...


Can you measure the rail currents (the current the amp is pulling from the SMPS), one rail pulls more current than the other. Then you can calculate what resistor value you should add from the rail with the lowest current to GND to make the current drawn from each rail equal (or at least less different).

In my case with two UcD400 modules, I have added 3 4.7kOhm resistors in parallel from + rail to GND. I used 2Watt resistors for that.

Gertjan
 
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