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Building a Power Supply

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Musicmanager

Well-Known Member
Hi Guys

I've got a 0 - 30v Power Supply kit which I got from Banggood a while ago.
I going to build the kit into a good sized enclosure and add some discrete 5vdc and 12vdc bananas which are completely separate from the kit and to this end I've designed my very first circuit.
I'd appreciate your expert view of the attached when you've got up off the floor from laughing !!!

Thanks
S
 

Attachments

  • Power Supply Scheme.pdf
    50.7 KB · Views: 617
15VAC has a peak of 21V and the rectifier bridge drops it to 19VDC. Then if you draw much current the regulators will get very hot, especially the 5V ones.
 
A quick question, the secondary of the transformer, is it 15-0-15 volts ?

JimB
 
Hi Jim

Haven't bought the transformer yet, but I had in mind a 240/ 2 x 15 in series to create a centre tap - nice and cheap at Maplins right now !
Why? Have I missed a good trick ?

Audioguru - I'm intending heatsinks on the regulators and there will be a fan in the case. What do you access as 'much current ' ? The 0 - 30 kit is rated to 3 amps so I imagine the likely load on the discreet taps would be quite small, Mamps ? - for testing only maybe.

I appreciate the guidance

S
 
Sorry I've just read back what I've written - complete rubbish !! Watch this space !!
 
The transformer can only input AC and output AC. Noticed a minor error on the left side of the transformer.
 
Hi

Transformer is 20VA 240v/ 0 - 9v; 0 - 9v 2 windings
I thought I could join in parallel to give 18vac with centre tap ?

S
 
I would remove the switch. Leave all the regulators on all the time.
You need heat sinks. The 78xx have the case connected to ground. The 79xx have the case connected to Vin. You can mount the 78xx on the same heat sink.
If you use insulators then they can all be on the case or a single heat sink.

For the 7x05s, can you get a 6-0-6 volt transformer?
 
Transformer is 20VA 240v/ 0 - 9v; 0 - 9v 2 windings
I thought I could join in parallel to give 18vac with centre tap ?
Now I don't understand. If 9-0-9 then 9*1.414=12.7 12.7-0.7 V =12V Will not work with the 7x12 but will work with the 7805/7905.
 
Hi Ron
Thanks for your help.
I was switching between the two sets because I thought best to share the load on the transformer. The main case is ABS so I'd have to use heatsinks which was already in the plan.
For the 7x05s, can you get a 6-0-6 volt transformer?
I don't know, but I guess so. however, I'm confused because in my reading I've noted that input voltage to 78** and 79** should be at least 3 volts greater than regulation ? Have I got that wrong ?

S
 
Oh heck !
I've found this transformer technology very difficult to grasp ! I thought I'd got it sussed but .. ..

If the secondary winding is 0 - 9vac; 0 - 9vac .. .. (not 9-0-9) then linking the 'middle pair' , that is the 9vac of one winding with the 0vac of the second winding gives 18vac off the other pair with a centre tap ?? No ??

If the answer is NO then I'm off to read again

Thanks Guys

S
 
Oh heck !
I've found this transformer technology very difficult to grasp ! I thought I'd got it sussed but .. ..

If the secondary winding is 0 - 9vac; 0 - 9vac .. .. (not 9-0-9) then linking the 'middle pair' , that is the 9vac of one winding with the 0vac of the second winding gives 18vac off the other pair with a centre tap ?? No ??

If the answer is NO then I'm off to read again

Thanks Guys

S
 
Yes you need 3 volts of head room for the regulators. (depends on load)

Start at transformer: The diodes will cause the DC to be at the peak voltage of the transformer. If you AC meter measures 9 volts then the peak will be close to 12V. Transformers are usually measured at RMS or average voltage. Example 110/220 volts from the wall has a peak of 1.414x that.

If the load is small the Dc will be 1.414x what the transformer has stamped on it. (-0.7 or -1.4V depending on how you diodes are connected)
As the load goes up two thinks happen.
1) the transformer voltage will drop some. (resistance inside the transformer)
2) There is ripple on the filter capacitors. The peak voltage will be about the same but the valley voltage will drop with load.
Words=not clear. Do you need pictures?
 
bridge_rectifier_with_capacitor_filter.jpg
 
Hi Ron

Thanks for that. Eureka I understand it too !
I've realised I'm the architect of my own confusion because what I'm talking about is not what I've drawn !! I'm a complete novice trying to run before I can walk !!
I'm just in the process of revising my schematic so it's actually telling the truth then it'll make sense to all of us and I'll ask the question again.

I assured you I never make big misteaks !

S
 
Right

I think the attached is a bit more truthful !
Sorry .. ..
 

Attachments

  • Power Supply Scheme.pdf
    50.8 KB · Views: 545
I don't think you have enough voltage for the 12V regulators.
Do you have the transformers? Can you measure the transformer just to be certain?

If you are not using a regulator they do not pull much current.
 
Hi Ron

I haven't bought the transformer yet. No problem to make it a bigger one. Is there not an issue that if I beef up for the 12vdc I have more of a heat problem with the 5vdc circuit ? What do you suggest ? 2 transformers ? I could be getting into space in the enclosure problems .. .. what do you suggest ?

S
 
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