What is the -12v DC output on an atx power supply? If I connect the -12v and +12v outputs together will it make 24v?

Cheers,
Mike.

 

Yes, it uses the same common.
Did you measure It?
Max.

 

What is the -12v DC output on an atx power supply?

It's 12 volts but with the opposite polarity of the +12 volt output-- that is, negative instead of positive.

If I connect the -12v and +12v outputs together will it make 24v?

No.

 

I thought you meant use the two, not connect them!
Max.

 

what is the idea of having reverse polarity. what would it be used for?

 

I believe it was/is used originally for the RS232 port for one of the reasons.
Max.

 

What is the -12v DC output on an atx power supply? If I connect the -12v and +12v outputs together will it make 24v?

Cheers,
Mike.

On output is at a voltage that is 12 V higher than the common reference and the other is at 12 V lower than the common reference (which should be the neutral coming into the supply). The voltage difference across them is 24 V. If you connect them together you've just shorted out a 24 V supply.

If you want to use those two as a 24 V supply, then you can provided you are careful about a few things. Your two power rails are 12 V above and 12 V below the common reference. If what you are powering is also referenced to the neutral, then you will be shorting one of the 12 V supplies to the neutral when you connect it. Also, the current delivery capabilities of the -12 V supply is very limited.

 

what is the idea of having reverse polarity. what would it be used for?

Some of the components needed bipolar supplies -- in particular the RS-232 UARTs. They needed very little current and so the -12 V output is very lightweight. On most modern equipment the need for "odd" voltages is often generated on-chip, something that wasn't practical in the old days.

 

The -12V was also used for the 3.5in floppy drive it's only 1 amp rated.

 

What is the -12v DC output on an atx power supply?

Sometimes you want to attract electrons. That is what the +12VDC does.
Sometimes you want to repel electrons. That is what the -12VDC does.

If I connect the -12v and +12v outputs together will it make 24v?

If you connect -12V and +12V you will let out the magic smoke.

 

Some of the components needed bipolar supplies -- in particular the RS-232 UARTs. They needed very little current and so the -12 V output is very lightweight. On most modern equipment the need for "odd" voltages is often generated on-chip, something that wasn't practical in the old days.

I'm confused... The 8250. 16450, 16550 UARTs are +5VDC Vdd? Op Amps and PNPs use negative biasing and the PC PS supplies -12VDC but I honestly don't know where or how it is used?

 

I'm confused... The 8250. 16450, 16550 UARTs are +5VDC Vdd? Op Amps and PNPs use negative biasing and the PC PS supplies -12VDC but I honestly don't know where or how it is used?

The actual UART chips interface to a line driver chip (such as the DS1488) which translates the logic levels to RS-232 levels. It's the driver chips that actually needed the +12 V and -12 V. I believe that, typically, this was the only thing that the -12 V was used for originally. There was also a -5 V supply that was provided to the I/O bus intended for use on stuff like sound cards.

 

The -12V was also used for the 3.5in floppy drive it's only 1 amp rated.

Are you sure? I never had to deal with that so I don't know. I tried tracking down some info but couldn't find anything definitive. But I didn't find anything that even hinted at -12 V being used for the floppy drives. The floppy drive cable pinouts that I found has +5 V and +12 V (plus the common for each). But I couldn't tell if these were used for the 3.5" drives or the 5.25" drives and I don't remember whether they used the same power connectors or not. My fuzzy memory recalls that the 3.5" power cables were on the same wires as the 5.25", just with a physically smaller connector.

 

I'm confused... The 8250. 16450, 16550 UARTs are +5VDC Vdd? Op Amps and PNPs use negative biasing and the PC PS supplies -12VDC but I honestly don't know where or how it is used?

We used to use the complementary pair of RS232 chips, DS1488 driver and the DS1489 receiver in early industrial applications, in conjunction with a PC for program loading..
Max.

 

such as the DS1488

I figured there was an OPAMP in there somewhere but the DS1488 does use ±12VDC biasing.

 

I figured there was an OPAMP in there somewhere but the DS1488 does use ±12VDC biasing.

It's powered by V+ and V- (typically +/-12 V) not biased with them. There's a separate ground connection that the inputs are referred to. Internal diodes place the transition point at two diode drops above ground, largely independent of the supply voltages.

 

The actual UART chips interface to a line driver chip (such as the DS1488) which translates the logic levels to RS-232 levels. It's the driver chips that actually needed the +12 V and -12 V. I believe that, typically, this was the only thing that the -12 V was used for originally. There was also a -5 V supply that was provided to the I/O bus intended for use on stuff like sound cards.

-5V was required for the DRAMs too!

 

If I connect the -12v and +12v outputs together will it make 24v?

Together makes magic smoke. If you measure from -12 to +12, you will get 24V BUT the -12 and +12 are relative to ground/common.
So, if you had a 24 VDC relay that you wanted to activate from something on the 5V supply, you would have to build a level shifter or optoisolate the relay. You would be limited tot he max current of the smallest supply which is likely the -12V one.