Stupid relay question....

[rascalseven]

Okay, it's late, and I'm not getting my mind around this at the moment.  I just finished a tube mic and am doing pattern switching with a relay, and built a little 12vdc supply for this relay in the power supply.  Using an Alcatel 12v relay (360 ohm resistance on the coil) my control voltage drops to just over 1 volt when the relay is engaged.  It 'clicks' (twice, like a flam on a drum), clearly not staying engaged (with only 1 volt steady on it).  I'm regulating this voltage with a 7812 regulator, and it looks good until I engage the relay.

I'm sure this is a simple thing to fix, but in other things I've built I've not had this problem.  What obvious thing am I overlooking in the middle of the night??? 

Would be mucho grateful for any light shedding.... thanks.

JC

 

[Kingston]

Just how small is your PSU transformer? Sounds like it runs out of current with the relay. It eats 35mA from that 12volt line.

It's probably not thermal shutdown since it happens so quickly and there only 0.5watt of activity there with the relay. But test this too. Measure both ends of the regulator, should be at least 15V on the input even with thermal shutdown.

 

[rascalseven]

Wow.... I see a big mistake of mine:  I added the 12v circuit off the secondary of the power transformer without taking into account the rest of the psu circuit attached to that same secondary ( ).  I'm getting 71vdc at the regulator input, and the filter cap for the 12v circuit is getting really hot!!

I need to revisit the whole psu design and do some better math to source that voltage for this 12v regulator and then continue from there.

JC

 

[rascalseven]

Fixed... I altered the psu design to provide a proper 12vdc voltage, and all works well now (and no caps on fire!!).

;D