Help needed with power supply rectification and regulation!

Thread Starter

PegasusIII

Joined Nov 12, 2016
18
I'll start by saying I only have a fairly basic understanding of electronics and circuits! So please bare with me!

I have a project which houses several individual circuits in one box. I have a multitap torodial transformer (secondaries of 24-0-24, 12-0-12, 6v) to power them, and I am needing to rectify and regulate this power to use in my circuits.

The outputs i need are as follows: 5VDC, 12VDC, 26-28VDC, AND 12VAC.. I have access to the following voltage regulators: (LM7805 5V 1.5A, MC78T12 12V 3A, LM338 1.2-33V 5A).. I thought these might be useable for my needs.. Also I have some rectifier chips (GBJ1510 15A 600V) I thought I could use.

I have looked at the data sheets but I am really not sure what the correct components I would need to complete this power supply circuit are. (smoothing caps? Etc).

I drew up a quick schematic which shows the transformer with the input voltages I have on the left and the outputs voltages I need on the right.. (obviously it is very incomplete, just for illustrative purposes) :)
POWERSUPPLY~2.jpg

If anybody is able to help me with this I would be very grateful, or if someone could help me with a schematic I would be happy to pay a small fee :) I am sorry, I just don't know where to get help with this!


If there are other Voltage regulators or diodes you think would be better please let me know.

I hope this makes sense. Please let me know if you need anymore info or clarification on anything!
 

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AlbertHall

Joined Jun 4, 2014
11,211
It all depends on what output currents you need (including the capacitor values) - please tell us.
The 7805 and 7812 will supply up to 1A each and the LM338 will supply up to 5A, but it needs a couple of resistors to set the output voltage.
 

Thread Starter

PegasusIII

Joined Nov 12, 2016
18
Thank Mr Chips those articles are helpful in teaching me more about this.

It all depends on what output currents you need (including the capacitor values) - please tell us.
The 7805 and 7812 will supply up to 1A each and the LM338 will supply up to 5A, but it needs a couple of resistors to set the output voltage.
Not sure about the capacitor values, that is what I need help with! :) in terms of the loads, 1a is sufficient for the 5v, the 12v needs about 2 a (I was thinking of using MC78T12 instead as it offers 3a), and the 26v needs about 4.1a (so 5 should be OK? ) .
How do I calculate the resistors for the LM 338 to get an voltage between 26-28?

Thank you for your patience, I am sorry I am slow at this!
 

AlbertHall

Joined Jun 4, 2014
11,211
From the datasheet:
upload_2016-11-12_13-44-46.png
If you choose 120Ω for R1 then Iadj is negligible, so R2 = (Vout-Vref)*R1/Vref. This gives 2.472kΩ. Using the nearest preferred value, 2.4kΩ, would give an output voltage of 26.25V.

The 7805 needs at least 7V input to work correctly. The 6V from the transformer will produce a peak voltage of 8.5V. The bridge rectifier will drop at least 1.4V from that leaving only 0.1V headroom and that's without considering that there will be some ripple voltage on the regulator input.
You could use the LM2940 which has a minimum input voltage of 6V, and so is viable.
The '2940 will need at least 22uF on its output and at least 4700uF on its input
The MC78T12 will need, say 22uF on its output and at least 6800uF on its input.
The LM388 will need 0.1uF on its output and at least 8200uF on its output.

I hope someone checks my arithmetic for all that ;)
 

Thread Starter

PegasusIII

Joined Nov 12, 2016
18
Wow! Thank you very much AlbertHall!
I assume all of these caps should be electrolytic? And what is a safe voltage rating to choose? Or what is the correct way to calculate the voltage rating needed for input and output caps.
Thanks again, and sorry for my n00bness. I have tried searching google for these answers but clearly I am searching for the wrong things as I cannot find the info i need!



From the datasheet:
View attachment 115194
If you choose 120Ω for R1 then Iadj is negligible, so R2 = (Vout-Vref)*R1/Vref. This gives 2.472kΩ. Using the nearest preferred value, 2.4kΩ, would give an output voltage of 26.25V.

The 7805 needs at least 7V input to work correctly. The 6V from the transformer will produce a peak voltage of 8.5V. The bridge rectifier will drop at least 1.4V from that leaving only 0.1V headroom and that's without considering that there will be some ripple voltage on the regulator input.
You could use the LM2940 which has a minimum input voltage of 6V, and so is viable.
The '2940 will need at least 22uF on its output and at least 4700uF on its input
The MC78T12 will need, say 22uF on its output and at least 6800uF on its input.
The LM388 will need 0.1uF on its output and at least 8200uF on its output.

I hope someone checks my arithmetic for all that ;)
 

Thread Starter

PegasusIII

Joined Nov 12, 2016
18
Also, can the LM7805 also come off the 12v tap? As I already have some 7805's lying around. or maybe i should just find some 2940 as you suggest
 

crutschow

Joined Mar 14, 2008
26,968
I see a potential problem in this part of your circuit:
upload_2016-11-12_21-18-25.png

As shown you can't have the 12VAC circuit common with the +12VDC circuit.
If you want those two to have a common ground, then ground the center tap of the transformer and use two diodes in a full-wave rectifier configuration instead of the bridge rectifier, to generate the DC.
 

AlbertHall

Joined Jun 4, 2014
11,211
The capacitors should be electrolytic except the 0.1uF which should be ceramic.
The voltage should be at least one and a half times the transformer secondary voltage but a little higher would be good. The peak voltage of a sine wave is root two (1.414) times the RMS voltage but remember that at low loads the transformer voltage will rise a bit.
You could run a '7805 from the 12V winding on the transformer but there are two possible problems. The voltage across the '7805 will be higher and so it will get hotter and need better heatsinking. The 12V winding will then be feeding three things and it needs to be rated to handle the current. Using a bridge rectifier means that the RMS transformer secondary current is nearly twice the DC output current.
 

Thread Starter

PegasusIII

Joined Nov 12, 2016
18
I ended up putting together this circuit like this:
power_supply-ecfa2b8d0f8f4d26bf1117ccf146f75e.png

And it seems to work perfectly! I get a steady 5v, 12 v and 28v dc output, perfect for my project. :)
20161207_224132.jpg
 
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