i need to find what the current is for

* a line level audio signal (eg the red/white Out Left/Right plugs
from a VCR or CD player)

* a composite video signal (eg the yellow Video Out plug from a VCR)

because I have a switch box with 1/2 amp relays, which would be used
to send/switch audio/video from different sources to a single place,
and want to know if the relays will be able to handle these signals
without being fried.

thanks in advance

Mad Scientist wrote:

i need to find what the current is for

* a line level audio signal (eg the red/white Out Left/Right plugs
from a VCR or CD player)

* a composite video signal (eg the yellow Video Out plug from a VCR)

About zero, since it's a voltage driven system.

For a 1V input (not far off either example) into a 10k load, you're
looking at something like 100 microamps.



because I have a switch box with 1/2 amp relays, which would be used
to send/switch audio/video from different sources to a single place,
and want to know if the relays will be able to handle these signals
without being fried.

No problem.

Mad Scientist wrote:

* a line level audio signal (eg the red/white Out Left/Right plugs
from a VCR or CD player)

Effectively zero. Input impedance is something like 1K ohms or higher
on most inputs, so the amount of current flowing is minimal.

* a composite video signal (eg the yellow Video Out plug from a VCR)

Here the input impedance is 75 ohms, so with a 12V video line there can
be measurable current flowing. Far less than the half-amp you care about.
But remember, this stuff has a 6 MHz bandwidth; it needs to go into a
constant-impedance switch matrix. If you just use random relays off the
shelf, you will get mismatches and lots of video ghosting.

because I have a switch box with 1/2 amp relays, which would be used
to send/switch audio/video from different sources to a single place,
and want to know if the relays will be able to handle these signals
without being fried.

Current is the least of your worries.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

thank you both for your posts... so I don't have to be worrying about
it frying my board. but just connectiong the video from point A to
point B won't work well? isn't a relay just a switch? is it a question
of signal loss? would one of those video amplifiers you see at radio
shack work?

But remember, this stuff has a 6 MHz bandwidth; it needs to go into a
constant-impedance switch matrix. If you just use random relays off the
shelf, you will get mismatches and lots of video ghosting.
Current is the least of your worries.

Mad Scientist wrote:
thank you both for your posts... so I don't have to be worrying about
it frying my board. but just connectiong the video from point A to
point B won't work well? isn't a relay just a switch? is it a question
of signal loss? would one of those video amplifiers you see at radio
shack work?

A relay is just a switch. But try it with a standard household switch;
you will find that you get trouble because there are substantial reflections.
This is stuff up at radio frequencies you're talking about.

Amplifying the signal will just amplify the reflections too.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

What do consumer A/B video boxes use to get around this?
Are there special shielded relays that would work?
Can I modify the circuit in some other way to fix the problem?

A relay is just a switch. But try it with a standard household switch;
you will find that you get trouble because there are substantial reflections.
This is stuff up at radio frequencies you're talking about.
Amplifying the signal will just amplify the reflections too.

Mad Scientist wrote:
What do consumer A/B video boxes use to get around this?

Open one up and see... most of them use active switching electronics, but
it is amazing to see how the PC boards are designed.... very short lead
lengths on everything, and the spacing of the traces on the board are designed
to keep impedances constant.

Take the cover off on one of the better VGA switches and just see how
the board is designed as a transmission line.

Are there special shielded relays that would work?

You can buy constant impedance coaxial switches. They are very, very
expensive, but they are commonly used in RF switching applications.

Can I modify the circuit in some other way to fix the problem?

You might be able to get by using the existing relays and running micro
coax right up to them, cutting the board traces off. You'd still get a
bump, but it might not be too bad of one depending on how much smearing you
are willing to tolerate. It would definitely be a lot more work than it's
worth, though.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."