Switching on power source dynamically

[gamozo]

One of the common issues I always run into are switching on power sources digitally. There are a few cases which I have issues, and I'm interested on insight on both (I know relays would work just fine, I'm looking for some solid state setup... and in particular something that would work with my parts bin (BJTs, diodes, and triacs)).

Current issue:

5v chips performing calculations which then need to be used later to switch on 12v line. This project runs off of a Molex connector on a computer PSU. The 12v and 5v rails are from the same source, and the ground is shared. Is there a way I can safely supply 12v given the state of the 5v chip? Basically it's as if a relay is used with a chip output to switch on a 12v line. Perhaps a single MOSFET would work here (sadly I only have BJTs... any fancy configuration work with them)?

Other common issue:

Same as above, but the 2 power sources differ. Like 5v input from computer, to switch on external 9v battery (once again, a relay would work, but I'm looking for something digital).

Right now everything is DC... I've got my triacs for my AC work of similar scenarios.

Sorry for the terrible descriptions... just about to head to bed.

-Brandon

[IanB]

Does the standby power rail and soft power switch capability of the PC power supply not help at all in your situation?

[gamozo]

I cannot sadly, it's a 'live' power supply that is in use on a running computer.

And it's switching on a 12v line that I want to be able to do, not switch on the power supply (not sure if I made that clear... I didn't explain well last night).

Basicially, I want to replace a physical switch on a 12v line, with a digital switch as I want to be able to switch it on and off using my computer (serial -> parallel), but the serial to parallel circuitry is all on 5v chips, thus I need to use the 5v to 'switch' on the 12v line.

A bit more information:

The load I need to supply 12v to, consumes about 100mA.

[robrenz]

This may do the trick.  just googled opticaly isolated analog switch.
http://www.clare.com/home/pdfs.nsf/www/CPC1009N.pdf/$file/CPC1009N.pdf

[gamozo]

Yeh... I guess I'll do that... I probably can just make my own with my parts bin too.

I've got some op amps that can take in a 15v supply. Would it be safe to use these as a switch? If so... what configuration?

Just tested it a bit:

Using an op amp, with Vs+ 12V and Vs- Gnd, with - input grounded, and + input ground or positive (depending on the parallel output), yields 12v exactly how I want. The only issue is I'm only getting 45mA out of the op amp, unlike the 130mA I need. But for now it's better than nothing. I can just use a few more op amps in parallel... I've got 86mA with 2 in parallel, and about 94mA with 4 op amps in parallel. The brightness is close enough until I get some optorelays)

[sonicj]

this setup is popular with HAMs for toggling push to talk.

[gamozo]

Hahahaha. I had some 4066's laying around. I just tested them out, 5v on Vcc to the chip, and I was able to witch 9 volts! Can't believe I didn't remember this, as I can vividly remember the internal schem image.

http://www.nxp.com/documents/data_sheet/74HC_HCT4066.pdf

[robrenz]

11 Volts is the max Vcc and Vcc is the max switchable voltage.  you said you need 12V

[gamozo]

My actual one is TI (15v it looks like):

http://www.ti.com/lit/ds/symlink/cd4066b.pdf

But I don't need exactly 12V, it's just for some LEDs. If anything, I can change the resistors myself and use 5v. But I'd prefer not to as they are SMD. 12V is optimal, 11V would be fine... it'd just be a bit dummer.

[robrenz]

Watch your on state resistance. It looks like it will be about 200 ohms. you may have to lower your current limiting resistors to the leds to get the brightness you want.

[gamozo]

Yeh. It's not my actual device, it's a device I'm working with (instrument cluster) so I've been trying to work around their parts. But it's getting to the point that I might just start using my own resistors and such.