I am more an RF guy than a power supply guy, so I will need some help with this one. I can use a standard computer power supply for the main power supply 115-240 VAC in/12 VDC out @ 1 amp. I need to incorporate a circuit that can charge a 12V battery and switch over to it if the main power fails, then back to main power when the main power turns back on. The device I am running draws 150mA, and runs on 12VDC. Is it practical to base the circuit on using a JFET as a switch, so when the gate voltage goes to zero, it will conduct?

 

Hello,

Here are some link pages on SMPS:
http://www.educypedia.be/electronics/powerelectronicssmps.htm
http://www.educypedia.be/electronics/circuitssmps.htm

Greetings,
Bertus

 

I opted to use a DPDT relay. The primary power supply will energize the relay and provide power to the device while charging the battery pack. When the primary power is removed, the relay is supposed to de-energize and let the battery pack take over. The problem is, once the primary DC power is removed, the voltage from the battery pack is keeping the relay from de-energizing until it reaches 4VDC. How do I get the relay to de-energize when the primary power supply is removed?

 

Hello,

You can use a diode to isolate the supply from the battery.
Anode to powersupply, kathode to battery.

Greetings,
Bertus

 

I got it figured out. I used a 1N4002 and it worked. Thx