Passing analog video without power

[vivitern]

Okay, the title sounds odd, I'll admit. I need my project to work without power. That is, a complete power failure may happen, and no additional power (such as a battery or supercap) is available mainly due to size and cost restrictions. The ground will always be connected during a power failure. The power failure may also include, for example, a short circuit placing the 3.3V buck regulator into a protection mode, causing a loss of power.

Here is the scenario. I'm developing a video processor which has multiple video inputs (two in most cases.) It uses an analog switch TS3A24157 datasheet:http://www.kynix.com/uploadfiles/pdf9675/TS3A24157DGSR.pdf, depending on version) to select the desired video feed, all software controlled. With power, it selects either one. However, when no power is present, I don't get a good video signal, because the mux goes into some kind of null state, with both inputs kind of conducting but very weakly.

The result is without power, a video picture which looks like this:

[Attachment 43744 - Click to enlarge]

The output amplitude is large enough to allow a TV card to sync onto it, but not large enough to provide a clear picture.

So, I need some kind of circuit which activates when there is no power and passes through either video input (one or the other, but not both) as a sort of fail safe for when power is lost or a bad hardware problem occurs.

I was thinking of a PMOS, as a negative gate potential will turn that device on, but wasn't sure how to make that work properly.

As this is video, we are talking about low on resistance; preferably less than 10 ohms. The analog mux probably has quite a high resistance when powered down, so as long as the circuit which provides the backup has considerably lower resistance, it should be possible to override it.

Also, it should be small, and low cost to implement. The current board is just 50mm x 39mm.

[pandy]

So, I need some kind of circuit which activates when there is no power and passes through either video input (one or the other, but not both) as a sort of fail safe for when power is lost or a bad hardware problem occurs.

I would go for miniature relay e.g. reed or small signal relay in DIL/SIL/other package - this is easiest and most reliable way to perform this functionality. Semiconductors without power are bad...
Alternatively as required current seem to be low - consider to use some lithium coin battery or alternatively sufficiently large capacitor (super cap?) as battery substitute (power current seem to be around 1.2uA)

Not sure if link will work for you https://eu.mouser.com/Electromechanical/Relays/_/N-5g31?P=1z0x216Z1yzxc13Z1z0x3udZ1yhq...t&Ns=Pricing|0

https://eu.mouser.com/Electromechanical/Relays/Reed-Relays/_/N-5g37?P=1yvsbamZ1yvsbatZ...d&Ns=Pricing|0

[Cornucopia]

This is what UPSes are for.

Scott

[JVRaines]

Well, that's what the AC outlet is for, too. "Battery not available" is a given in this problem.

I vote for pandy's solid-state relay.

[johnmeyer]

Yes, a semiconductor switch without power doesn't get the minute current flow into the base of the transistor or the gate of the SCR/Triac to keep the switch "open." Your only option is a mechanical switch that is not spring loaded (i.e., won't spring back when the current to the coil is removed).