Need help please

[flancfuster]

Hi everyone!

I've found on this site a power supply supposedly from a Goldmund preamp.
There is no voltage written at the input/output.
Can someone tell if this power supply will work and what should be input/output voltage?
I know that I can build probably different/better supply, but I want to test this one.
Thanks.
pwr_spply.JPG

[sawreyrw]

This is an extremely old design. You can do much better using modern components and ICs. Your first problem is to figure out what output voltages you need.

[AndrewT]

The 36V Zener tells you the voltage available at the output.
Then you find the minimum AC voltage that can ensure no drop out.

But as San says. Start with your requirements and develop a brief. From the brief you expand to a specification and then you get down to the details of the circuits to meet that specification.

[flancfuster]

Thank you all for reply.
The reason that I want to use this circuit is that a friend of mine gave me as a present a PCB which includes this power supply. I've populated the PCB and my first guess was that the output should be 36V, so I tried with 40V and 45V AC input, but the output is not regulated at 36V. The output is the same as the one after the diodes. All parts checked. Anything wrong in the schematic?

[agdr]

Is your transformer 40-45VAC center tapped? The center tap needs to connect to that center "ground" line. In other words your transformer should measure 80VAC to 90VAC end-to-end at the secondary and 40-45VAC from either end to the center tap.

Like AndrewT says just work backwards. You have a 36V reference with the zener for each rail. The differential pair will work to maintain that with a similar input on the other side. So that gives you 36VDC across the 10K resistor, or 3.6mA through the 10K, which also has to go through the 4.7K. Add the drop across that (4.7K * 3.6mA = 16.92VDC) back onto the 36VDC and you get 53VDC at the output.

The TIP35/36 pass transistors would need a minimum of 2V - 4.5V across them, depending upon current flow, from the data sheet [Vce(sat)] so that gives you 55VDC at the output of the diodes. Plugging in some typical transformer 1/2 secondary values gives 42VDC (84VDC center tapped transformer) as about the sweet spot. When you multiply by sqrt(2) to convert RMS to peak stored by the caps = 59.3VDC. Your filter caps are rated at 63VDC so you wouldn't want to go much over that, including allowing 10% or so in either direction for line and trans regulation fluctuations.

[AndrewT]

I would be very wary of using 63Vdc caps with a 42Vac transformer.
Worst case conditions could exceed the capacitor rating.

I would also doubt very much that a 42Vac transformer gives sufficient headroom for a regulated 55Vdc supply.

If you use 63V caps then check thoroughly that you don't overvoltage your caps in worst case conditions.

if you want to use a 42Vac transformer, I'd suggest you aim for 45Vdc upto maybe as high as 50Vdc. Again you will need to check that you have sufficient supply voltage to allow for regulator drop out and supply ripple, particularly if you target 55Vdc as your regulated output.

It is very easy to lower the output voltage. either change the ratio of the output sampling resistors and/or change the Vref Zener.
For fine trimming you can change the Zener current. As you lower the current Vref drops and takes output voltage down with it.

Next you must look at worst case dissipation of the regulating transistors. You need this dissipation to determine heatsink size and ventilation.

Finally, at your apparent level of knowledge of regulators, I'd suggest that you leave this part of the project alone until you know what you are capable of doing/designing/building.
Most projects can work successfully without a regulated PSU.

[flancfuster]

Thank you all for explanation. Like Andrew noticed, my level in electronics is beginner, so I will try to absorb as much as I can. From what I understand I have to change the caps to ones rated at higher voltage(80Vdc), and increase the AC input(50Vac) so I can have sufficient supply voltage in order to get 55Vdc regulated.
Thanks.

[AndrewT]

39V is the common upper limit for many readily available Zeners.
You can get higher but it may take a while to source them.

The LTP needs some voltage to operate with. This requires the output voltage >>Vref
36:55 is OK, but you can vary that by quite a margin. At present you have ~19V difference from Vout:Vref, try as low as 10V and as high as 30V. The higher this difference the harder the LTP has to work to maintain good regulation, without introducing avoidable noise.

50Vac is a good start for a 55Vdc regulated PSU. Byt once again be wary of going high on the AC voltage input. This makes for considerably more heat to be dissipated in the regulating transistors.