View Full Version : Power Supply Queries

06-19-2007, 08:28 AM
I am making a power supply that will give a range of voltages ( 5V - 25V) at 5A current output level. Following are my queries:

1 ) I have got a 6 A 12-0-12 transformer that also has 6-0-6 output capability. I need 12V on the secondary side, so can I connect between the 0 and 12?

2 ) I have got a metal can type LM338 and it's heatsink. Please advise so as to how to connect the heatsink to the LM338?

3 ) Datasheet of LM338 says that the metal can type uses the can as the output, so if the heatsink is connected then can I connect the output to the heatsink?


06-19-2007, 08:47 AM
1. Your 6A transformer may not give you 5A, unless it is a top class designed transformer (meaning 'expensive). Try a 10 A transformer if you consider continuous duty cycle.

2. Are the 6V and 12V windings separate or just tapped o/p from a single winding?

3. Get a TO-5 mounting kit. It has a mica isolating sheet, 2 nylon bushes and apair of bolts. This is used to isolate the body of the 338 from the box and chassis. you take tthe o/p from a washer tag in direct contact with the 338 body. use Heatsink paste between the can and heatsink.

Do not mount the 338 touching the heatsink.

06-19-2007, 11:00 AM
Thank you Docel for your replies.
Yes the transformer is tapped 6V and 12V from a single winding.
Do you think Rs.490.00 translates into expensive (meaning good quality) in Calcutta?? :?: :D

I have yet another query.................if a transformer says 240V/12V 150VA then what does it say about the current output of the transformer??

06-19-2007, 06:12 PM

1. The cost will mean anything unless from a reputed company. The cost is OK but not sure if justified.
On a general note, the cubic size of a transformer is an indication of current, along with the secondary wire size.

2. VA=Watts (Power)
You want a 12v 5A supply. That means 12Vx5A=60VA or 60 watts.
150VA is good. ie, 12Vx12.5A
To check the label claim, get a 12v50W car headlamp bulb, connect to the 0-12 ( I SAID 0-12 wire!!!) wire and monitor the heat ( of the trans0 after 1 hour. If it just warms up it is good. If it starts smoking......

06-19-2007, 07:01 PM
Thank you Docel for your reply.
It really helped.
I will put the transformer through the test..........if it stands up alright then :D otherwise :(
Does the power factor not play a part in the calculation of the VA rating of the transformer. I googled it up to see that the general PF is 0.6. But as far as I know PF varies widely with the type of load. Is there any direct way of measuring PF?

06-19-2007, 10:49 PM
just a doubt.... for the output to be 12v(say).. the input voltage must be atleast 3v greater ie 15v.....am i wrong here ....so to get 12v i think u have to connect 12 and -12v to the bridge rectifer then to capacitor input filter and to the regulator....do see the schmatics....

06-20-2007, 12:05 AM
Good observation Biscuit :!: :!:
The AC voltage is deceptive. When we say 220V for our Mains , we are looking at the sine wave RMS value: ~312v(the Vrms)=Vpeak/sq root of 2=220VAC
. The peak o peak value of a 220VAC is +/- ~624VAC :!: :!:

Power factor is neglected for fractional HPs. But, It becomes increasingly important as the Power value increases ( in multiples of HP)

06-21-2007, 01:08 AM
thanks for the reply
but i have few contradictions
the 12v is available at the output it means rms value is 12v
now the peak value would be~17v
now the output dc value after ther bridge rectifer would be 2*(vp-1.4)/PI
ie is 2*(17-1.4)\3.14 would be ~10v
so when it is fed to the regulator the maximum op would be 7v

principles of electronics albert paul malvino

06-21-2007, 09:53 AM
Hey biscuit!
Nice to see you reffering and THINKING at the same time!!!
You are right, upto the rectifier....
But the DC Power supply starts after the Smoothing Filter capacitor. You missed the other page:

In reality, transformers are rated for the maximum load condiition - if they are of quality. The open terminal voltage will be greater than the specified Voltage/curent rating and will approach this values at the maximum ratings. Ie., your Tr is rated 240AC/12VAC 150VA. When measured open load, you'll see 17 to 20VAC. On full load of 12.5A ( 150VA=12Vx12.5A) the terminal voltage will be 12VAC.
:arrow: These are the ones with serial no. printed on them, costly and come with a replacement warranty. 450INR does'nt ratify this!!!

:idea: for hi-current units, It is better to put 2 or more capacitors in parallel than one single capacitor rated for twice the input V for the filter.

:arrow: Nobody bothers about these things , really!
Thats why, the practical method is to use twice the needed output voltage rating for the transformer, keep the overrated capacitors outside the box, bleed the PwrSup with a constant load resistor after the VReg IC and build a great big heatsink with a fan.... :lol: :lol:

....Its easier to use a Battery.... :wink:

06-22-2007, 10:47 PM
i now understood my mistake ... the mistake i made was the that i calculated the maximum dc op of the bridge rectifier instead of the capacitor input filter... in other words the op of the filter would be equal to the peak outputy value of the bridge rectifier
i learnt a lot from this thread
thanks once again

06-23-2007, 08:42 AM
Good to hear that Biscuit!!
You seem to be through with what you do and have the patience to learn and apply. That must be appreciated.
Keep it up buddy :!:
And Best wishes for whatever you want to do!!

:idea: p.s: it will be nice if you can post details of the completed supply. maybe, also some VxI graphs ....?

06-24-2007, 11:15 PM
here are pictures of the simulation

as for the photos sorry i dont have a digital camera
thanks for the help