heaveno,
Is possible use a BC556B instead of BC556C?
Best Regards
Yes.
Hi Per-Anders,
I'm currently debugging my PSU (filtering), I have 2 SSR boards in a +-15V configuration. Regulators are working as expected. When powering up the device with a variac, I can observe that I get different voltage drops (output voltages) before the regulators start regulating, e.g. 13V vs -17V. Now there are two possible reasons I could think of. First, the difference is caused by the transistor selection quality of Q1 and Q3. I use BC456C (hfe roughly 600) and BC556B (hfe roughly 290) for the positive and negative voltage boards. Other than that I use almost identical parts. The other reason would be that my load (TI evaluation board) is broken, although it seems to function properly. Do you think the difference might come from the different selections for Q1/Q3?
Thanks for your help.
Cheers,
Dan
Does ot work OK in steady state? Is it just the start up you are puzzled about?
Hi Per-Anders,
it works as expected, but I am not able to apply the same amount of filtering to +15 and -15V, right before the regulator boards. Too much voltage drop on the +15V side (BC556B). Another thing that I see is that the AD (the +-15V are only for feeding the TI opamps for analogue) has "excessive" noise below 3Hz.
How much raw voltage do you have? You should have at least 19 volts in for 15 volts out.
How much noise do you have?
The raw voltage depends on the amount of filtering I use. I would like to filter a bit more thank what I have. I understand that I need 19V which is easy to achieve on the -15V side with/including the additional filtering. The +15V side seems to drop more voltage so I cannot add the same amount of filtering like on the negative side. I think it's at least 4V lower raw voltage on the +15side. I have even built a second +15V board to see if I have any mistakes, but it behaves virtually the same. The only real difference between the two boards are the BC556B/BC546C. About the noise I cannot tell you exactly now. What I know is that the AD Board is a bit noisy (in the computer fft roughly -108) but only below 3hz. Specs of the board say it must be better than that.
Noise performance is dependent of opamp choice. -108dB is pretty good at 3 Hz. How much noise does your sound card have?
This thing is my soundcard :-). TI states that the 4222 has -124 noisefloor. I've seen other noise-plots of the same chip/board and if I compare it with mine, then mine is quite some bit noisier but only regarding this bump below 3Hz. Whatever that might be... so I thought whether I would need some better filtering, but I cannot apply because of the greater voltage drop on the positive side. But there is always a chance that my measurements are wrong. :?
It takes a lot to measure -124 dB. It's nearly 500 nV.
Quote from: peranders on February 14, 2011, 07:45:25 PM
It takes a lot to measure -124 dB. It's nearly 500 nV.
That's the main reason why I need very good! filtering :-).
Except for Q1,Q3, the circuits for +15V and -15V are identical. I suspect that the load (AD) sees different Ri for +15V and -15V. Do you think this could be due to the difference in the transistor selection? Or what is the effect of different hFe for Q1,Q3? As soon as the regulators start working I get +-15.6V which is perfectly good. But, why do I get a such a huge difference in "raw" voltage before the regulators start working?
I appreciate your help!