HF noise througth relay


2016-01-12 9:32 pm

I have an optical isolator supplied by lifepo4 battery.
I use a mechanical timer to switch it on/off but i get tired of the mechanical tic-tac.
I think to use a NE555 timer instead.
If i use e NE555 timer i would have to not supply it with my battery (it would pull it down), and to use a relay to trigger my battery on.
A relay trigger not supplied by my battery current but with external DC source at this time ruins the ultra low noise supply of my lifepo4 battery. HF noise goes througt the coil an i can hear the difference.
What can i use between the NE555 and the battery to switch the battery on with a perfect noise isolation ? would an optocoupler work ?

Thank you for your help



Paid Member
2003-06-12 7:04 pm
Maine USA
Yes, you can make this as complicated as you want.

A CMOS '555 sucks hardly any power, won't load a battery. Assuming it turns the audio on and off, its small switch-surge is negligible compared to the amplifier turn-on thump. A pair of transistors can switch the positive line with no fancy parts; switching the negative line may be simpler.


2016-01-12 9:32 pm
The lifepo4 battery feeds a optical/ethernet converter, 3,3V - 1A.
This battery is loader by a DC 5V supply + LM317, output 3,4V - 1A
I have the same setup (with 5V battery) on the Olimex etalon streamer that is after the optical/ethernet converter.
This isolation (and specially the network one) has such a huge effect on SQ that i would not going back to a more simple setup.

At this time, my mechanical timer switch the /battery output on/battery loader off / for 4 hours. The battery loader contact is cutted on both +/-, with non electrical contact at all to the loader when music is on.

Before, i had just a switch with a relay to connect/ disconnet the loader from battery. If relay coil was feed by loader circuit (LM317), It did ruin the lifepo4 noise reduction (i strongly heard it), same effect if i did cut the loader circuit on just one pole.

I want to upgrade the setup now, because of the mechanical tic tac noise, and because i get tired to startup my player each time i want to lesson music.

I want the battery to be always on load with the LM317 (so that i don't have to restart each time the converter ans player), and just pushing a button to isolate the battery from the loader for 4 hours when i lesson to music.

A Cmos 7555 would be ok if feed by the loader circuit + DPSP relay to disconnect it from the battery; but i am afraid to get noise back from LM317 throught the relay coil as it happens before on my first setup.
I am going to check if 7555 would pull to much current if i feed it with battery instead of loader circuit. I have to check too if there is not a logical problem to reconnect the loader if battery is emply .