Adding up voltages question

[Joony]

Greetings,

I bought an old Nokia IP120 Firewall off ebay to turn into a m0n0wall router.

The guy I bought it from sent me the wrong power supply though.

The wrong one outputs the following:
+5V - 3.0 A MAX
+15 V- 1.6 A MAX
-15 V - .5 A MAX

What the IP120 needs is a 5V - 5A input, but I think it can get away with even 3 Amps?

Anyhow, I busted out the ole Volt meter and it seems it outputs the following:

Pin 1 - 15V
Pin 2 - Ground
Pin 3 - Negative 15V
Pin 4 - Ground
Pin 5 - Negative 10V

So there is no 5V output! Could I combine the +15V output with the -10V output to create 5V? Would that be the 5V that's on the label?

If so, how would I combine it? Serially? Hook up 15V and -10V together to make the Positive and use Ground as Negative?

BTW, whenever I plugged the wrong power brick in, it would short out pins 3-5 because that was ground for the IP120 (While Pins 1-2 for 5V). No wonder why there was a burning plastic smell.

Also, they said they would refund me the costs to buy the right PSU, hopefully the IP120 still works.

 

[Aluvus]

Originally posted by: Joony

What the IP120 needs is a 5V - 5A input, but I think it can get away with even 3 Amps?

If Nokia was being cautious and the power supply maker was too, then perhaps. But if they were both being realistic, it may not be safe.

Anyhow, I busted out the ole Volt meter and it seems it outputs the following:

Pin 1 - 15V
Pin 2 - Ground
Pin 3 - Negative 15V
Pin 4 - Ground
Pin 5 - Negative 10V

It's possible you have the terminals of the volt meter the wrong way around. The 10 V may be a poorly regulated 5 V, or the unit may be damaged, or something else may be up.

So there is no 5V output! Could I combine the +15V output with the -10V output to create 5V? Would that be the 5V that's on the label?

If so, how would I combine it? Serially? Hook up 15V and -10V together to make the Positive and use Ground as Negative?

No, that would give you a short circuit between the 15 V and -10 V, which means you have 25 V across a short circuit (a wire). If the supply has good overcurrent protection, this will not be very dramatic (but may kill the supply). If it has bad or no overcurrent protection, that should be fairly entertaining to watch.

Assuming your voltages are measured correctly, you do have a 5 V difference between the -10 V and -15 V terminals, but the power supply is likely to get very unhappy if you try any such shenanigans.


The best advice I can offer is to buy a known-good 5 V supply that can deliver a large amount of current (you might repurpose a cheap ATX supply, if you can find one that will put up with this kind of use).

 

[Jiggz]

Negative and Positive together equals explosion or short circuit! Positive and another positive together equals the difference between the two. Same with two negatives when put together. When a negative and positive put together, results in adding the two voltages together thus maximum amperage or short circuit. You've better off using a regular PSU to get that 5V-3A power. It's not really that much especially if you have like a 450W or higher PSU. Another option is to use a dedicated PSU of smaller rating.

 

[Aluvus]

Originally posted by: Jiggz
Negative and Positive together equals explosion or short circuit! Positive and another positive together equals the difference between the two. Same with two negatives when put together. When a negative and positive put together, results in adding the two voltages together thus maximum amperage or short circuit.

The OP is talking about directly connecting two output voltage pins together; regardless of what those voltages are, that would be a short circuit. The voltage being dropping across that short circuit will be the difference in the voltages of whatever is being connected. If two positive rails are connected, that still well may lead to too much current draw and heat.