• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Mixing Voltages

Not open for further replies.


New Member
If I had a fire alarm system, or any thing for that matter, and I wanted to add battery back-up...
System normally powered by a 12V DC transformer, but if I added a 9V battery, would this cause problems?


Well-Known Member
Most Helpful Member
Two Schottky 1A rectifiers (1n5819), cathodes to the alarm, one anode to +12Vdc, other anode to the battery+, battery- to 0V (the other terminal of the 12Vdc supply).


Well-Known Member
I don't think that I've ever seen a fire alarm system that didn't already have a battery backup. And they have always been 12 Volt SLA batteries.

But, for other products...

It is very subjective. It all depends on what parts of the system need what voltages, and how much current they need. Some things may work, but probably not for very long.

For instance, if it were drawing 1/2 amp, then as soon as it switched over to the battery, the battery voltage will drop to about 6 Volts due to the internal resistance of the battery. Not many 12 Volt systems can run reliably at 6 Volts.

Note: 9 Volt battery discharge information from: http://www.powerstream.com/9V-Alkaline-tests.htm
Not open for further replies.

Latest threads

EE World Online Articles