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Ground loops and TTL supply from PSU questions

Hello,

I have two questions.

First, do I need to be concerned with ground loops for computer circuits? I am planning on taking 5V and Ground from a molex off of the PSU to power a small circuit that I am building inside of the computer case. The circuit will use that ground as a reference for a voltage (2V or so) coming off of the video card adaptor (AGP port).

Am I going to be okay referencing the voltage from the video card to the ground wire from the PSU, or should I reference it to something on the video card itself? If I do need to reference it to something on the card itself, would it be okay to tie the ground on the video card to the ground on the molex from the PSU?

My second question concerns using the 5V output from the PSU as the Vcc for a TTL chip. Should I use a voltage regulator to ensure that the supply is at

5V (and not too far over), or is it generally okay to just use the 5V as it comes off of the molex?

Thanks for any insights into these concerns,

Jon

Reply to
Jon Danniken
Loading thread data ...

On a sunny day (Thu, 23 Nov 2006 22:16:31 -0800) it happened "Jon Danniken" wrote in :

At the risk of being wrong (as always possible), I'd get the signal from that AGP differential. Then you can use normal ground from the connector, and any ground current will not interfere with the input.

No, use differential input, ground side diff input on AGP card.

Yes.

Reply to
Jan Panteltje

...

I disagree here. Jon should either use the +5 from the PSU, and just capacitate it smooth, or run a regulator off the +12. Any +5 regulator with only a +5 supply will put out less than +5V, because there is no headroom.

Thanks, Rich

Reply to
Rich Grise

On a sunny day (Fri, 24 Nov 2006 19:35:37 GMT) it happened Rich Grise wrote in :

Oh I agree, when using the 5V you should filter / decouple it locally. If the app is purely digital that PSU 5V will be OK, if it involves any analog processing you could use the 12V or a switcher on board from some other supply. To make it short, the 'molex' IS the connector from the PSU I think? Or did you mean one of thsoe power connectors as to the IDE drive?

Reply to
Jan Panteltje

I
a

of

video

itself,

Thanks, Jan The reason I want to tie the grounds together is to be able to use a transistor (NPN) to switch in the circuit. I would drive the transistor from a 4013 flip/flop, and connect the 2V coming from the AGP card to an appropriate resistor, and then to the Collector of the transistor. When switched on, the transistor would complete the circuit through the shared ground, and change the reference voltage from the AGP card (desired effect)

Otherwise, I'm thinking I would have to use a quad bilateral switch.

Here is a crude ascii drawing:

|--/\\/\\/---- From AGP voltage | | |/ From switching circuit --/\\/\\/---| |>

| | === GND

This is why I am hoping to tie the grounds in together, so that I can just use a transistor to switch in the circuit.

Thanks,

Jon

Reply to
Jon Danniken

some

Thanks Jan, and Rich.

Yes, the voltage will be coming off a molex connector from the PSU.. The chips involved will be a MAX232, a 4013 (flip flop), and possibly a 7404 (hex inverting buffer) if I need it. There will also be a few transistors switching things in and out..

I would definitely toss in a few caps across the supply as it enters the board, but will that be enough?

Thanks,

Jon

Reply to
Jon Danniken

On a sunny day (Fri, 24 Nov 2006 14:22:19 -0800) it happened "Jon Danniken" wrote in :

I would do it something like this (discrete), differential amp. _________________________________________ + | | [ ] [ ] | |--------------------------> signal d d -- g JFETs g--------- AGP s s | SIG |-------------| | | | AGP -------------------------- + GND | | | [ ] c | b----------------------| e NPN --- | \\ / temp comp diode | current source --- [ ] | | [ ] | | ------------------------------------ GND No need to tie any ground together,

Reply to
Jan Panteltje

On a sunny day (Fri, 24 Nov 2006 14:28:20 -0800) it happened "Jon Danniken" wrote in :

The PC PSU will have to work between about 4.75 and 5.25 I think, mine goes down to 4.81 according to bios.... That is within the TTL spec, CMOS HC[T] may have even bigger range, look it up. It should work!

Reply to
Jan Panteltje

Thanks, Jan, but if I am reading that correctly, you have the AGP actually turning the FETs on and off.

In the application I am doing, the switching is done by the remote circuit (by a flip flop), which connects the AGP voltage to ground via a resistor.

The reason for this circuit is to fool a voltage sensing chip on the AGP card into thinking that certain voltages on the AGP card are too low. This is done by connecting a parallel resistance to the existing resistor on the AGP card. By doing this, certain voltages on the AGP card will be raised, which is what I am trying to accomplish. (this actually works, I am just trying to automate it with a switching circuit so it isn't left on when it isn't needed)

I like the idea of keeping the AGP ground seperate, though, the safety of which is my main concern.

Thanks,

Jon

Reply to
Jon Danniken

transistors

goes down

it up.

Thanks, Jan, that will save me some needed space.

Jon

Reply to
Jon Danniken

On a sunny day (Fri, 24 Nov 2006 15:14:58 -0800) it happened "Jon Danniken" wrote in :

Ah now I get what you want. Well in that case it is the other way around, the diff amp goes on the AGP card and perhaps drives a power MOSFET or transistor. Your driver signal is the differential. But likely only a simple MOSFET will do, 5V on enough noise margin.

Reply to
Jan Panteltje

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