Newbie - PSU Building

[lesabb]

I'm building a +24V and +48V PSU to power up a mic pre. The schemo is below. However, I built it in 2 section: 24V and 48 V sections and just connected the 2 as per the schematic from transformer to the second bridge at the LM317. The problem is that I'm only getting +34V at the 317 output. How should I wire the trimpot (pin 1,2,3)? It's like there is no doubling. Transformer is 24VAC. The 7824 reads +24V at the output. Any thoughts? Thanks

L

[void_error]

First thing - the maximum input-to-output voltage for the LM317 is 40V so don't short the output (+48V to 0V) or you'll have a fried LM317. The LM317 solution doesn't seem like a good idea to me... check the datasheet.

Second thing - add a resistor between pin 3 of R4 and adj, I'll leave the math to you, just make sure you don't have more than 40V across the LM317, also pin 1 & pin 2 of R4 should be shorted together, although I'd be against using a LM317, see above.

Third thing - use a voltage doubler like the one in figure 4 in place of your two bridges, it's much simpler than using two bridge rectifiers and good enough for the low current the mic preamps will probably draw.

[electros6]

What is the need for c1 & c2

[Jay_Diddy_B]

Hi,
There is a small error in the doubler circuit. I have constructed the original circuit and a modified version in LTspice to illustrate the issue:




In the original circuit the second rectifier has its negative terminal grounded. We essential have two full wave rectifiers, the only difference is that the input on the top one is capacitive coupled to the transformer. We get two similar output voltages:



In the modified circuit the rectifiers are stacked. In this configuration the voltage on the second rectifier is added to the output of the bottom one. This gives the desired result:



This just shows how to configure the rectifiers, it does not help with the overvoltage condition on the LM317 if the 48V output is connected to ground.

Regards,

Jay_Diddy_B

[lesabb]

Thanks for your time Jay! This is exactly what I was wondering about after a lot of browsing. Would the new schemo be better? I'm still wondering about the trimpot connection. The wiper (PIN2) should connect to the ADJ pin and ground Pin1 or 3? Thanks a bunch!

L

[Jay_Diddy_B]

Hi,

You should connect pin 2 to pin 3 on the potentiometer. This will give you increasing output voltage as you turn the pot clockwise (normal convention).

You should consider using a fixed a resistor in series with pot. This will reduce the sensitivity. You could arrange the adjustment to be 43 - 53 Volts for example.

You could even use 1% fixed resistors and you will get reasonable accuracy.

Regards,

Jay_Diddy_B

[Jay_Diddy_B]

Hi,
Lesabb asked me if I could help with the regulation of the 48V supply. Here is an idea that use some high voltage bipolar transistor to build a low current, 15mA, voltage regulator. The LM317 is not suitable for this application.

Here is an 'old skuul' design using a long-tailed pair. The output of the 24V supply is used as the reference for the 48V supply:



The performance is reasonably good:







The voltage is regulated and the ripple is reduced by a factor of 20.



Regards,

Jay_Diddy_B

I have attached the LTspice model
 

[Jay_Diddy_B]

Hi group,
 I realized that I do a lot better than the design that I posted earlier if I change the output transistor to a PNP transistor.
This is the new LTspice model:



And the results:



The new version has much lower ripple:



I modified the LTspice model so that I perform ac analysis to check for the control loop stability:




The ac analysis shows reasonable bandwidth and good phase margin:




I have attached a zip file with the models.

Regards,

Jay_Diddy_B