Converting ATX Power Supply for Workshop: Diode Resistors, Parallel Connections & 12V Line Load

Hi!
I am at the stage of converting the ATX power supply into a workshop.
There are many instructions on the Internet, but I don't understand everything. Therefore, I have a few questions for you.


1. The diodes in my project are powered by 5V line. I know what resistors I should match when it comes to the Ohm parameter, but I don't really know what about W. On websites, everyone explains how to make a good resistor by matching the ohm value. Is a 0.25W resistor enough for a 5W line, or maybe 0.5W ... or maybe it must be 5W?

2. How do I connect 2 0.25W resistors in parallel, I will have 0.5W? Is that how it works?

3. With relatively new ATX power supplies, it is said that the + 12V line must be artificially loaded to a minimum of 22 Ohm for the power supply to work stably. I also read that in this case you should use resistors of at least 10W. Is it true? Is it possible to somehow use 5W resistors so that they do not burn? I have already asked in 5 local stores and the largest resistors they have are 5W.
Or maybe in my case it is enough to load the + 5V line?

4. It's also nice if someone was able to recommend me a good book, some basics of electronics (but written in such a way that the child understands, because in this area not everything is easily absorbed for me).

You can find answers to some of your questions at this link
https://majsterkowo.pl/zasilacz-warsztatowy-ze-starego-zasilacza-atx/

1) How do you calculate DC current power? I * V ..... you choose the resistor power with reserve to the obtained result.

2) It depends on the resistance value of these resistors. These values must be perfectly the same so that each has the same power. But in simple terms, yes, this is how it works.

3) Similar to the first point. Given the voltage and resistance, you calculate the current. Given the current and voltage, you calculate the power and select resistors in reserve.

4) There is so much that everyone will recommend something different ...

helazbyś wrote:

You can find answers to some of your questions at this link
https://majsterkowo.pl/zasilacz-warsztatowy-ze-starego-zasilacza-atx/

I won't find answers to these questions here. I have been to this site before.

zster wrote:

1) How do you calculate DC current power? I * V ..... you choose the resistor power with reserve to the obtained result.
3) Similar to the first point. Given the voltage and resistance, you calculate the current. Given the current and voltage, you calculate the power and select resistors in reserve.

It's black magic to me.
Could you show me how this is calculated with an example?
a) I want to connect the diode to e.g. + 5V 2A, I found out that the resistor should be 300 Ohm, how many "W" is enough?
b) I want to artificially load the + 12V 17A line with a 22 Ohm resistor

12V ^ 2 / 22? = 6.55W; but you can take, for example, 2 resistors 43? / 5W connected in parallel (each will have 12V ^ 2 / 43? = 3.35W), or 3 resistors 68? / 3W ...

Do you want to power high-power LEDs from it? But how did you get 300??

_jta_ wrote:

12V ^ 2 / 22? = 6.55W; but you can take, for example, 2 resistors 43? / 5W connected in parallel (each will have 12V ^ 2 / 43? = 3.35W), or 3 resistors 68? / 3W ...

Do you want to power high-power LEDs from it? But how did you get 300??

I don't know how I calculated it. That's what I have in my notes. Now as I use the calculator
for the green diode I get 100 Ohm
(Green LED Uf = 3.1V, If = 20mA,)
and for the red diode (Uf = 2.03V, If = 20mA,) 148.5?
hmm can you give more resistance without side effects? because in total I have soldered to green instead of 100 I have 220, and with red instead of 150 I have 300. They shine well

as for these artificial load resistors, it means that I can safely use 5W in parallel? even if I needed 20W (e.g. 4x5W in parallel)?

Thank you for your understanding

mathun wrote:

hmm can you give more resistance without side effects?

Of course you can. 20mA is the upper limit of the current for this diode, but it can be powered and 10mA and it will shine well.

mathun wrote:

does it mean that I can use 5W in parallel? even if I needed 20W (e.g. 4x5W in parallel)?

You can. You can also experimentally check whether the power supply will turn on at higher load resistances. The greater the resistance, the less power goes to "heating the resistors".

ATX power supply usually has a power counted in hundreds of W (however there are also smaller ones, e.g. 90W for computers in Small Factor housing) - do you have enough LEDs for power supply that you need such power? If so, then maybe it is worth thinking about how to better manage it - for example, make a current source powered from 12V, and connect a pair of LEDs in series so that over 80% of the power taken from the power supply goes to power the LEDs, not heating the resistors serial.

_jta_ wrote:

Do you have enough LEDs for power supply that you need that much power?

This is to be a workshop power supply. LEDs, i.e. in Polish, the LEDs are in my project only to make them shine nicely 1. The red LED is to indicate that it is connected to the network, 2. the green LED that it is running / ready to work. This is just such a good thing, and the power supply itself will not be used to power other LEDs than those in the housing.

The + 5V 2A output is probably Stand-By, so the voltage is there even with the power supply turned off (or rather its main part is turned off), so it may be possible to adjust the voltage for the main part and keep the + 5V to power the LEDs.

I don't want to play regolation. Not yet. I also have a second power supply. If it goes well, maybe I will do another one with regulation.

Further questions:

I have 6 yellow + 12V cables, how many should I connect the artificial load to and how many should I leave? I have 11 wires at my disposal for grounding.
I have a set of 3 resistors connected in parallel 68 Ohm 5W.

update
The power supply is ready. I just don't know why
at + 3.3V it is 3.46V
at + 5V it is 5.46V
at + 12V it is 11.08V
and at -12V it is only -10.29V




Update 3

Everything is fine. I connected two + 12V lines to 3 68 Ohm resistors. I equipped the resistors with heat sinks. Nevertheless, with the power supply turned on empty, the resistors get hot. You can touch it, but not longer than a few seconds because they burn.

What is the power rating of those resistors that get hot?

,, I have a set of 3 resistors connected in parallel 68 Ohm 5W. ''

Maybe the power you give off on these resistors is greater than their rated power and therefore they heat up?

There is 12V, and to achieve a power of 5W on a 68? resistor, you need over 18V - at 12V it will be 2.1W.

The resistors heat up because this is their role, they dissipate power in the form of heat, in the case of wire in ceramic housings, these are temperature-resistant elements and it would be a pity not to use it They are designed so that at nominal power the surface temperature can reach from 150 ° C to 300 ° C depending on the model and manufacturer, the smaller the heat dissipation area and the higher the power, the higher the temperature. While the resistor itself could work at such high temperatures, it would rather not be in the vicinity of other elements or the laminate, which means that PCBs usually have to limit the power loss below half the nominal value, and you can get burned anyway.

Below are some examples from the datasheet

Thanks, I'm just learning the basics of electronics and such knowledge will definitely be useful to me.