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Old 23rd September 2009, 04:52 PM   #1
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Default Help with B1 PSU

Hi all,

I have a schematic of a PSU for the B1 buffer from a B1 tread, though I can't remember by who and when. Please see image 'psu'. I have done the pcb based on the schematic. Please see pdf 'B1 psu pcb'. Now, my transformer is 230Va 28v (it was stated as 24v but the output is 28v) but my out-put is only 13.5v. And when I put in a 230Va-32v transformer, my out-put is only 15.5v. What is the problem and how do I solve it? Can the B1 buffer be driven by 13.5 or 15.5v? I read in the article by NP that 18v-24v is needed. Thanks in advance.

pmchoong
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File Type: png psu.png (32.5 KB, 413 views)
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File Type: pdf B1 PSU pcb.pdf (105.4 KB, 68 views)
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Old 23rd September 2009, 05:40 PM   #2
iko is offline iko  Canada
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Hook up a at the psu output a 1W resistor in the range 470R to 1K. Then measure the voltage drop across R3 and the voltage between R2 and R3 referenced to ground. Come back with those values.
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Old 24th September 2009, 10:21 AM   #3
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I have a 1/2w 470R resistor at the output. I can't find 1w R470 but I have 1w 62k. After putting in the 1/2w 470R, voltage across R3 is 2.75v. Voltage between R3 and Gnd is 18.27v. Voltage between R2 and Gnd is 23.56v. Output is 14.7v. Voltage output before the 1/2w 470R is put in is 15.30v and not 13.5v as mentioned above. The transformer used is a 230V - 28v-0. When I use transformer with a 32v-0 out, the voltage is only 16.5v (without the 1/2w 470R in place).

Last edited by pmchoong; 24th September 2009 at 10:33 AM.
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Old 28th September 2009, 11:14 AM   #4
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Hi

I have attached a drawing of the measurement. Am I doing something wrong here?http://www.diyaudio.com/forums/images/icons/icon5.gif
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Old 28th September 2009, 11:27 AM   #5
AndrewT is offline AndrewT  Scotland
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check Q1. Is it the right way round?
It should have ~600mV across BE and that forces ~600mV across R3. 2.75V is seriously wrong.

What is the voltage drop across R1 and across R2?

What is the load across the output?

I hope you are using just one secondary (24Vac) to power onse side of the supply.

The 33r is setting the pass current of the CCS. this should be ~ 600mV/33r ~18mA.
This is a bit low to supply a B1 and too low to supply two channels of B1.
The 470r resistor. Is it onnected to ground to simulate a load? That will draw ~31mA @ 14.7v and run very hot dissipating ~460mW.

Hang on a minute.
23.xVolts -18.x volts is not 2.75Volts.
You are not measuring correctly.
Start again.
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Last edited by AndrewT; 28th September 2009 at 11:33 AM.
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Old 28th September 2009, 12:12 PM   #6
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1) Q1's BE is measured at 2.74v.
2) Voltage across R1 and R2 = 0.9mV.
3) load across the output = 14.7V
4) Yes, one secondary (28v) and one 0
5) 33r?
6) Yes, the 470R is connected to a piece of metal on the ground
7) R3 referenced to Gnd = 18.35v
8) R2 referenced to Gnd = 23.54v
9) in your opinion, where or which part did I measure wrongly?
10) the pcb layout is attached as pdf file in post#1 above, and I have checked again on the board, the Q1 is soldered on correctly

Thanks

Last edited by pmchoong; 28th September 2009 at 12:21 PM.
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Old 28th September 2009, 12:30 PM   #7
AndrewT is offline AndrewT  Scotland
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what is the point?
I tell you what does not add up and still you repeat the same wrong measurements.
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Old 28th September 2009, 01:05 PM   #8
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Hi Andrew, sorry, I was measuring in DC. I have very limited knowledge on electronics.

1) Q1's BE is measured at ~0.006v
2) Voltage across R1 and R2 = ~0.006v
3) load across the output = 14.7V
7) R3 referenced to Gnd = 18.35v
8) R2 referenced to Gnd = 23.54v

Thank you again. I will check back tomorow. Thank you very much for helping.
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Old 28th September 2009, 03:22 PM   #9
AndrewT is offline AndrewT  Scotland
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2.) 0.006V tells us the current passing into the reg circuit.
I = V/R = 0.006/62r = ~0.1mA.
The regulator is effectively turned off.

1.) 0.006V across BE of Q1 tells us that the regulator CCS is effectively turned off.

8.) - 7.) 23.54 - 18.35 = 5.19V where did 2.75V come from?

Is the 470r connected from the reg output to the reg return?
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Old 28th September 2009, 04:43 PM   #10
iko is offline iko  Canada
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2.75V across R3 tells us that the total current drawn from the power supply is 2.75/33 = 83.3mA. Since the load should draw 14.7/470 = 31.3mA, it means that roughly the difference most likely goes through the shunt transistor, Q3 (2sb716).

I see two problems. First, if you really want 24V out, then make sure the transformer + filter and a load connected to it (100mA to be sure) gives you about 32-34V out. The series mosfet Q2 needs to drop some voltage to give you regulation. Right there's too much voltage drop across R1, R2, and R3 and there's nothing left for regulator to work with. If you can't get a higher voltage transformer right now, do this. Take R1 and R2 out. With a good regulator you don't even need them. You can replace them with some very small value resistor, if you really want to, something like 5R.

While testing the regulator, I strongly suggest you to replace your reference U1 (tl431) with a simple zener diode (two 12V, or two 10V even better, in series). Then you don't second guess the reference.

HTH
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