qestion about home made power supply

Thread Starter

Teljkon

Joined Jan 24, 2019
267
Hello,

new to the hobby but I have started to play with circuits. Saw a cool little pcb that turns an old computer power supply into a lab bench power supply. so I had a go at it. Pictured below. here is the funny thing. When you hook up to a battery you have a positive and a negative but the PSU board only has a -12v. Is it a fair assumption that the negative 12v post can be used for all three voltages. Or should I be completing my circuit to ground?

 

MrChips

Joined Oct 2, 2009
30,711
A battery has a + and - terminal.
In reality, the markings are relative, in other words, they just mean that one terminal has a higher potential than another.

The -12V on the PC PSU means something else. All voltages are with respect to GND.

12V means it is 12V higher than GND.

-12V means it is 12V lower than GND.

All voltages, 12V, 5V, 3V3, and -12V need a return path back to the PSU.
That path is the GND connection, not the -12V connection.
 

crutschow

Joined Mar 14, 2008
34,283
Is it a fair assumption that the negative 12v post can be used for all three voltages.
No.
The GND post is the common connection for all four voltages including the -12V output.
GND is the negative connection for all the positive voltages, and the positive connection for the negative voltage.
 

Thread Starter

Teljkon

Joined Jan 24, 2019
267
A battery has a + and - terminal.
In reality, the markings are relative, in other words, they just mean that one terminal has a higher potential than another.

The -12V on the PC PSU means something else. All voltages are with respect to GND.

12V means it is 12V higher than GND.

-12V means it is 12V lower than GND.

All voltages, 12V, 5V, 3V3, and -12V need a return path back to the PSU.
That path is the GND connection, not the -12V connection.
so wait your telling me that voltages exist on a negative number line Grnd being zero. That kinda doesn't make sense to me for some reason. if its a -12v its drawing 12v away from the circuit all the time. that would mean the energy is coming from the grnd having a visual spatial mind with this stuff really dose not help. This brings up strange questions like why don't we just use all negative voltages all the time that way the electric company can just pay me! because then i would be the source of the energy right?

LoL

Seriously though can you point me to some reading that explains this.
 

WBahn

Joined Mar 31, 2012
29,978
so wait your telling me that voltages exist on a negative number line Grnd being zero. That kinda doesn't make sense to me for some reason. if its a -12v its drawing 12v away from the circuit all the time. that would mean the energy is coming from the grnd having a visual spatial mind with this stuff really dose not help. This brings up strange questions like why don't we just use all negative voltages all the time that way the electric company can just pay me! because then i would be the source of the energy right?

LoL

Seriously though can you point me to some reading that explains this.
All voltages are measures of potential energy differences between two points (unless the field is nonconservative, but let's not go there). If you put a 100 Ω resistor between the +12 V post and GND, then a current of 120 mA will flow from the +12 V terminal, through the resistor, and to the GND terminal (the physical electrons move the other direction). If you connect a 100 Ω resistor between the -12 V post and GND, then a current of 120 mA will flow from the GND terminal, through the resistor, and to the -12 V terminal. In both cases current is flowing from the higher potential to the lower potential and so energy is being converted from electrical energy to some other form (heat, in this case). The resistor only "knows" that one end of it is 12 V higher than the other end. It would not be able to tell the difference between this situation and a situation where it was connected between the positive terminal of a 24 V battery and the positive terminal of a 12 V battery, provided the negative terminals were connected together.
 

AnalogKid

Joined Aug 1, 2013
10,986
so wait your telling me that voltages exist on a negative number line Grnd being zero.
Yes. *Some* voltages are negative with respect to GND, some are positive, some alternate.
if its a -12v its drawing 12v away from the circuit all the time. that would mean the energy is coming from the grnd
Not quite. It means that for negative voltage sources the direction of electron flow is in opposite direction compared to the direction of electron flow with positive sources.

Note - electric current (electron flow) is not always exclusively one direction or the other. Sometimes the flow direction alternates between the two directions. This is where the term "alternating current" comes from. Two very common examples of AC are electric power distribution and audio.

ak
 
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Reloadron

Joined Jan 15, 2015
7,501
Seriously though can you point me to some reading that explains this.
It's all a matter of your point of reference:
How to Obtain Negative Voltage from a DC Power Supply or Battery

Lets say I place two 12 volt batteries or power supplies in series.
+ - 12 VDC Power.png

With a reference to the point I named Ground or Common I have -12 and 12 with respect or in reference to my ground or common terminal. Now if I made the bottom -12 my ground or reference I could have a 12 and 24 volt supply. If I make the top my reference I can have a -12 and -24 volt supply.

On another note I am not even sure any newer ATX 12 V power supplies even have the -12 Volt output and if they do do not really expect much current capability. This is the 2013 Design Guide and you can see while the -12 existed the current was very limited.

I see there have been additional post as I muddled along. :) They covered it.

Ron
 

Thread Starter

Teljkon

Joined Jan 24, 2019
267
Thanks for all the replies guys. Very interesting thing to learn about electronics. I have a led that i hooked up to the -12v and ground with a 100K ohm resistor (at least I think that is what it was, it was what they had me use in my micro controller coarse on udemy). An interestingly this salvaged LED is directional the bent lead has to be towards the cathode or it wont work. I discovered this oddity of the led some time ago. Sure enough I had to flip it to make it work with the -12v. thanks again.



Bread crumbs
https://www.av8n.com/physics/anode-cathode.htm
 

WBahn

Joined Mar 31, 2012
29,978
What oddity? ALL LEDs are directional. The D in LED stands for "diode". A diode will (normally) only conduct in one direction and that is when the anode is at a sufficiently higher (more positive) potential than the cathode.

There are times when you intentionally operate a diode in "reverse breakdown", such as with zener diodes, but it is seldom a good idea to do that with an LED.
 

AnalogKid

Joined Aug 1, 2013
10,986
Because an LED is directional, there almost always is some kind of marker to indicate which end is which. For most standard 3mm or 5mm round LEDs, the anode lead is longer than the other. Also, if it has a lip around the base there might be a flat spot near the cathode lead.



ak
 
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Thread Starter

Teljkon

Joined Jan 24, 2019
267
What oddity? ALL LEDs are directional. The D in LED stands for "diode". A diode will (normally) only conduct in one direction and that is when the anode is at a sufficiently higher (more positive) potential than the cathode.

There are times when you intentionally operate a diode in "reverse breakdown", such as with zener diodes, but it is seldom a good idea to do that with an LED.

Thats funny I thought that newer Led's were not directional. I wonder where i got such a bad idea in my head. Thanks for the correction.
 

MrChips

Joined Oct 2, 2009
30,711
Some LEDs are bidirectional. Connect it one way and it emits GREEN light. Connect it the other way and it emits RED light (or some other pair of colors).
 

WBahn

Joined Mar 31, 2012
29,978
Thats funny I thought that newer Led's were not directional. I wonder where i got such a bad idea in my head. Thanks for the correction.
Some packages have more than one LED connected in what is known as "anti-parallel", meaning anode of one is connected to the cathode of the other and vice-versa. Whichever direction you apply voltage (if sufficiently high, of course), one of them will turn on and the other will be off. By using two different colors you can make a single device that can emit two different colors from the same device depending on the polarity of the applied signal. By switching back and forth rapidly, you can also get a seemingly continuous set of shades going from one color to the other.
 

MisterBill2

Joined Jan 23, 2018
18,176
Many LEDs have a rather limited reverse voltage rating so connecting them, even with a series resistor, backwards to 12 volts may destroy them. At least, I have done that. Possibly they are more robust now, but I would not bet on it.
And one thing about that computer power supply is that the negative 12 volts is a much lower current rating than the positive 12 volts. AND, some bad news is that with every computer power supply I have ever tested, there had to be a load of several amps on the 5 volt supply before it would even work. Possibly the kit included such a load.
 

Reloadron

Joined Jan 15, 2015
7,501
Some packages have more than one LED connected in what is known as "anti-parallel", meaning anode of one is connected to the cathode of the other and vice-versa. Whichever direction you apply voltage (if sufficiently high, of course), one of them will turn on and the other will be off. By using two different colors you can make a single device that can emit two different colors from the same device depending on the polarity of the applied signal. By switching back and forth rapidly, you can also get a seemingly continuous set of shades going from one color to the other.
Sort of drifting off topic but here is an example of a Bi Color LED:
LED BI Color.png

Both LEDs are on a single die with a two wire package. When the control is Low D2 Green is on, when control is High D1 Red is on. Now, if you apply a real fast control signal and you can see Yellow.

Ron
 

MisterBill2

Joined Jan 23, 2018
18,176
Sort of drifting off topic but here is an example of a Bi Color LED:
View attachment 169283

Both LEDs are on a single die with a two wire package. When the control is Low D2 Green is on, when control is High D1 Red is on. Now, if you apply a real fast control signal and you can see Yellow.

Ron
Also, in the circuit shown, each LED serves to protect the opposite LED from excessive reverse voltage. That is a good secondary effect of the configuration.
 
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