Making a 12v 5v power supply

[[XC] 2long4u]

I need a power supply that can power 12vdc fans 5vdc temp display.
Ok I have some questions that I hope somebody here can answer.
I'm looking at a +12v voltage regulator. The brand is NTE part# 933, it is 5 amp's 50 watts. It is basically a PNP transistor. Input voltage and output voltage is positive.
Then the -12v voltage regulator part# 1915 is 1.5 amp 15 watts.
That is basically a NPN transistor. Input voltage and output voltage is negative.
Am I correct that one is for the positive leg and the other is the negative. Also do they use ac input current or dc?
I would think that they would need dc if it is like a regular transistor. :shrug:
EDIT: Here is the pdf http://www.nteinc.com/specs/900to999/pdf/nte933.pdf

[ruffus]

is there a reason that you can not just use computer power supply as it has 12vdc and i do believe that it has a rail that is 5vdc as well correct me if i am wrong

[{.bLanK} GoD]

He wants it for his temp display, so it will need to be independent of the pc psu.
Before the pc starts, there is no voltage on any of the low voltage rails.

I am interested to see if you can make an affordable simple mini psu.

[[XC] 2long4u]

^Yes you are right. Without the computer on you don't have the 5v for the display or the 12v for the fans.
I was going to use an independent power supply inside the phase case but space is limited.
The why has been discussed in other threads. I want to keep this thread as clean as possible as there is no other info at this time on how to build one.

[Polizei]

Just get a transformer to take 120v AC to step it down to 15v AC, get yourself a bridge recitifier and some caps rated for 20v DC. Grab a couple voltage regulators, one that outputs 12v and another that outputs 5v. I'll draw up a simple schematic and post it here in a minute.

[Polizei]

http://i2.photobucket.com/albums/y26/toms918/Misc/simplePSU.jpg

Hooray for crappy MS Paint. Or just an unsteady hand in Photoshop. :rolleyes:

A couple notes:
- The voltage regulators will need to be cooled.
- The voltage regulators have 3 legs. One is input, the middle is ground, the other is output. They will take a voltage modestly higher (probably 13-18v for example) and regulate it down to 12v.
- The caps are in there to minimize AC ripple.
- The bridge rectifier takes the AC sine wave and makes it look like a bunch of hills instead of hills and valleys. Makes all the voltage positive instead of switching positive-negative-positive-negative etc.

http://www.societyofrobots.com/images/sbs_voltage_regulator.JPG

If you have the voltage regulator infront of you like this, the left leg would be input, middle is ground (along with that metal tab), and the right is the output.

I think that is about it. I had a similar project like this last year in a basic electrical engineering course, except my power supply was variable from 18v-0v.

[Polizei]

http://i2.photobucket.com/albums/y26/toms918/Misc/waves.jpg

From TOP to BOTTOM:
- Normal AC sine wave. Fluctuates here in the US from 170v to -170v at 60Hz. RMS value is 120v. ;)
- Rectified sine wave. Converts it to all positive.
- Rectified sine wave with caps as I have drawn before. The caps cut out the valleys in between the waves. The more caps you have in there, the less notiable the dips are between the waves. The flatter the DC line gets.

[n00b 0f l337]

Why not get one of those powersupplies from jabtech I believe, $10, 2a @ 12v and 2a @ 5v.

[[XC] gomeler]

Agreed NoL, there are a ton of very small PSUs on the market that'll satisfy the requirements set out here. I'm talking PSUs the size of a cell-phone, not an ATX psu ;) I can understand the fun in making your own circuits though, just a thought.

[n00b 0f l337]

http://www.jab-tech.com/12v-DC-Power-Supply-with-Single-Molex-Connector-pr-3776.html
Tadaaa!

[bassie]

Just get a transformer to take 120v AC to step it down to 15v AC, get yourself a bridge recitifier and some caps rated for 20v DC. Grab a couple voltage regulators, one that outputs 12v and another that outputs 5v. I'll draw up a simple schematic and post it here in a minute.

Its better to use some 30+V caps. DC = 1.4 x AC (if you use a bridge recitifier and a cap) 15x1.4=21. Without load even a bit higher.

In your schematic its better to add a 100nF cap after each regulator, gives a more stable output voltage.


- Rectified sine wave with caps as I have drawn before. The caps cut out the valleys in between the waves. The more caps you have in there, the less notiable the dips are between the waves. The flatter the DC line gets.
The number of caps is not important. The capacity is important. The lower capacity = higher rippel. In most cases 2.5milliFarad per Ampere is enough.

@2long

How much current do you need on the 12 and 5 volt line?

[[XC] 2long4u]

Again, Please don't post about other ways to do this. This is a thread on how to make one. I'm not looking for a power supply like that.
Please keep posts relevant.

[[XC] 2long4u]

A couple notes:
- The voltage regulators will need to be cooled.
- The voltage regulators have 3 legs. One is input, the middle is ground, the other is output. They will take a voltage modestly higher (probably 13-18v for example) and regulate it down to 12v.
- The caps are in there to minimize AC ripple.
- The bridge rectifier takes the AC sine wave and makes it look like a bunch of hills instead of hills and valleys. Makes all the voltage positive instead of switching positive-negative-positive-negative etc.

If you have the voltage regulator infront of you like this, the left leg would be input, middle is ground (along with that metal tab), and the right is the output.

I think that is about it. I had a similar project like this last year in a basic electrical engineering course, except my power supply was variable from 18v-0v.

Correct and not correct at the same time.
In a transistor there are three legs one is the collector one is the base the other is the emitter. Power goes into the collector and out the emitter, the base is the switch.
The base isn't always in the middle. You need to either look on the box, or test it to find the base.
Edit: Each fan is .48A so 3 fans would be 1.5A most of the regulators in the catalog were either 1A, 2A, 3A, or 5A. I think I'm going to go with the 5A.
The ground regulator (or npn) has less amperage rating. Is this because it is the ground circuit and does not carry a load?
Is it ok to use chassis ground for ac and dc?

[n00b 0f l337]

Okay mate just trying to offer my advice.

[Polizei]

2long4u;2870004']Correct and not correct at the same time.
In a transistor there are three legs one is the collector one is the base the other is the emitter. Power goes into the collector and out the emitter, the base is the switch.
The base isn't always in the middle. You need to either look on the box, or test it to find the base.

A voltage regulator is not the same as a transistor.

Its better to use some 30+V caps. DC = 1.4 x AC (if you use a bridge recitifier and a cap) 15x1.4=21. Without load even a bit higher.

In your schematic its better to add a 100nF cap after each regulator, gives a more stable output voltage.


The number of caps is not important. The capacity is important. The lower capacity = higher rippel. In most cases 2.5milliFarad per Ampere is enough.


I was just drawing up a basic schematic hoping someone would come along to improve it. I've only got the basics as I'm only in my second year of five. Thanks though. Learn something new every day. :up: :)

[bassie]

2long4u;2869276']I need a power supply that can power 12vdc fans 5vdc temp display.
Ok I have some questions that I hope somebody here can answer.
I'm looking at a +12v voltage regulator. The brand is NTE part# 933, it is 5 amp's 50 watts. It is basically a PNP transistor. Input voltage and output voltage is positive.
Then the -12v voltage regulator part# 1915 is 1.5 amp 15 watts.
That is basically a NPN transistor. Input voltage and output voltage is negative.
Am I correct that one is for the positive leg and the other is the negative. Also do they use ac input current or dc?
I would think that they would need dc if it is like a regular transistor.


A npn transistor for positive voltages, a pnp for negative.

But this thing isn't a transistor. Its a Voltage regulator.

The NTE933 is a 3 terminal fixed positive voltage regulator in a TO3 type package designed for use in
applications requiring a well regulated positive output voltage. Outstanding features include full power
usage up to 5A of load current, internal current limiting, thermal shutdown, and safe area protection on
the chip, providing protection of the series pass Darlington, under most operating conditions. Hermetically
sealed steel packages are utilized for high reliability and low thermal resistance. A low–noise temperature
stable band–gap reference is the key design factor insuring excellent temperature regulation
of the NTE933. This coupled to a very low output impedance insures superior load regulation.

A transistor doesnt have current limiting, thermal shutdown etc. Also if you look at the last picture in the data sheet, you'll see it has an input output and a ground.

So just look for a lm78xx schematic, swap the lm78xx with your regulator and your ready to go.

Couple of things you need to know. With a brigde rectifier and a cap for stabilizing there are 2 imporant rules.

VDC = VACx1.4 important for cap voltage.
IDC = IACx0.6 so if you want 5A you need a 8A transformer. ( I=Current)

The capacitors capacity depents on your output current, you'll need at least 2milliFarad per Amper.
Capacity = 3x output current.

And get some good cooling for the regulator.


I was just drawing up a basic schematic hoping someone would come along to improve it. I've only got the basics as I'm only in my second year of five. Thanks though. Learn something new every day.

Take a look at This pdf (http://www.hammondmfg.com/pdf/5c007.pdf) important for correct sizing tranformers and caps.

Is the nte933 really $25++? With a lm7812, a lm7805 and 2 2n3055 you can replace the 5 and 12V regulator. I can buy those 4 parts for €4 around here.

[Polizei]

Here's a schematic of the internals of an IC voltage regulator. It's got diodes, transistors, resistors and capacitors within it, but all of the connections are internal and reduced to the input, ground, and output.

http://www.rason.org/Projects/discreg/Image3.gif

[[XC] 2long4u]

A npn transistor for positive voltages, a pnp for negative.

But this thing isn't a transistor. Its a Voltage regulator.


I think you have that backwards.
I will post a pic of how they work but I'm waiting for an admin to approve my profile.
The pic helped me understand how transistors work.
Correct me if I'm wrong but a voltage regulator is a transistor just with different values. A transistor is just a switch put power on one side, power will not flow from the collector to the emitter without ground being applied to the base (in a pnp)
In a npn it would have ground at the collector and when you apply power to the base it would be allowed to flow to the emitter.
This confuses me. With a v-reg like I posted how are you going to put power on the base of the npn when it's body is the base, and it is attached to aluminum?

[Polizei]

2long4u;2870099']
Correct me if I'm wrong but a voltage regulator is a transistor just with different values. A transistor is just a switch put power on one side

A transistor and a voltage regulator are NOT the same thing. A transistor is a very simple device, which is used in conjuction with other things like capacitors and resistors and diodes to make things like voltages regulators and power supplies.

Look at my above diagram. That voltage regulator has 4 transistors within it, Q1, Q2, Q3 and Q4.

A transistor can be used as a switch or as an amplifier.

[bassie]

2long4u;2870099']I think you have that backwards.
Correct me if I'm wrong but a voltage regulator is a transistor just with different values. A transistor is just a switch put power on one side, power will not flow from the collector to the emitter without ground being applied to the base (in a pnp) In a npn it would have ground at the collector and when you apply power to the base it would be allowed to flow to the emitter.

Wrong, look at my attachment. In a npn transistor there is no flow from the collector to the emitter. But if you put a little current on the base you'll get a big flow from the collector to the transistor. The collector emitter flow is base current x Hfe. If you connect your collector to ground it wont work. And a big current on the base wil destroy your transistor because the base is made for regulating.

I think this (http://www.hobby-electronics.info/course/html/index.html?PHPSESSID=64b2c758926e9ce6a7358d32e4b7b c8b) is nice for you to read.


This confuses me. With a v-reg like I posted how are you going to put power on the base of the npn when it's body is the base, and it is attached to aluminum?

1. If your read the datasheet of the nte933 you'll find:
pin 1 = input
pin 2 = output
pin 3 = ground.

2. A transistor and voltage regulator are NOT the same. A transistor is a device that has a flow from the collector to the emitter regulated with the base.

A fixed voltage regulator like the one your showing is use to get a stable output voltage with a wide varity of input voltages. The nte933 has a maximum input voltage of 30V a minimum of 15V. The output voltage is always around 12V (11.05-13.05) as long as the input voltage is between 15-30V.

[bassie]

Forgot the attachment

[[XC] 2long4u]

75011
I thought all transistors were like the circled one.
I know how to test them.

[[XC] 2long4u]

Does there need to be separate negatives or can there be one negative for 12v and 5v?

[bassie]

2long4u;2870600']75011
I thought all transistors were like the circled one.
I know how to test them.

NPN is for positive voltages, PNP for negative. You can find enough about them on the internet, my english is not good enough to explain it.


Does there need to be separate negatives or can there be one negative for 12v and 5v?

Do you mean 1 ground/mass for both?

Are you still planning to use the nte voltage regulator? If you give me some info a probably can find a cheaper solution.

You need 12 & 5V right? Do you also need negative voltages?
How much is the current draw on the 5&12V line?

[[XC] 2long4u]

Could you have multiple 12v legs and just put in a resistor to get the 5v?
Or would you have to put in another voltage regulator?

[Polizei]

I wouldn't do that. I don't think a single 12v voltage regulator could handle the power draw put on it.

[bassie]

2long4u;2871080']Could you have multiple 12v legs and just put in a resistor to get the 5v?
Or would you have to put in another voltage regulator?

A lm7805 + 100nF cap will only cost you $2 or so, its a lot better then a resistor.

If you tell me what current you need i will make a schematic for you.

[[XC] 2long4u]

For the 5V? I only need it to power the temp display. I don't know it's power requirements, as I don't have it yet.

[bassie]

2long4u;2871760']For the 5V? I only need it to power the temp display. I don't know it's power requirements, as I don't have it yet.

I mean the 12V line, 1A should be enough for the 5V line.nl

[[XC] 2long4u]

The fans are .48A each and the LED's will be 140mA.

[bassie]

How much fans are you going to use?

[bassie]

L78S12 data (http://www.st.com/stonline/books/pdf/docs/2148.pdf)
LM7805 data (http://www.fairchildsemi.com/ds/LM/LM7805.pdf)
rectifying rules (http://www.hammondmfg.com/pdf/5c007.pdf)
Part list

TR1 = 12V 5A transformer.
B1 = Bridge Rectifier 7A (or more) 30V (or more)
C1,C2 = 100nF capacitor (normal, not a electrolytic)*
C3= 2x4.700microFarad electrolytic capacitor 30V (or more) wired parallel
IC1= LM78S12**
IC2= LM7805

*You need to place the 100nF caps as close to the ic's as possible to reduce the oscillation a much as possible.
**Be sure you get a S version thats the 2A version the normal 7812 is 1A, see datasheets.

Get a big enough heatsink for the IC's. The trafo is around 15V unloaded, VDC=VACx1.4, 15x1.4=21(peak) The regulator makes 12 of it with a maximum output of 2A. P=UxI, P=(21-12)*2 P=18W, worst case (short circuit).

For the 5V its P=(21-5)x1, P=16W. Your coolblock needs to be big enough to cool it.

If you want less heat output, you can use a 7V 2A transformer for the lm7805, the transformer for the lm78s12 can be reduced to 3.5A in that case.

As an extra safety device you can add an led on the output of the regulator. If you short circuit the regulator it goes in protection, when it goes in protection the output voltage drops to 1V or so. A led doesn't burn on 1 V, so when the led is out you know you short circuit.

Resistor for led formula is:

R=(Up-Uf)/I
R= resistance
Up= the psu voltage (12 and 5 v in this case
Uf= voltage drop across the led (depents on the led collor)
I= power use by the led in ampere, normal 0.02 A

[Polizei]

Ok, so I was close. ;)

I like the LED idea.

[[XC] 2long4u]

How much fans are you going to use?

2 for a condenser sandwich, and one more for ventilation.