Switching power sources, battery charging

[kellyseye]

If the device is primarily mains-powered there is no real concern for power consumption and a simple RELAY would allow switching of practically any amperage at the cost of a few 10's of mA.

Sometimes the simplest way is best.

[Demon]

There's a few reasons why I wanted to stay away from a relay:

- they must be slower than transistors.
- mechanical parts mean wear.
- and I really was looking for an excuse to finally use mosfets.


I don't know why the design I posted at the top uses 2 mosfets back-to-back. According to this site (and many others) P-channel mosfets are connected "backwards" and you only need one to act as a switch:
http://www.electronics-tutorials.ws/...or/tran_7.html
(scroll down to P-channel MOSFET Switch)

Robert


EDIT: Updated design

[kellyseye]

There's a few reasons why I wanted to stay away from a relay:

- they must be slower than transistors.
- mechanical parts mean wear.
- and I really was looking for an excuse to finally use mosfets.

I sympathise with the 'excuse' but the first two resons don't parse..... a simple capacitor will prevent the volt drop for the millisecond(s) the relay switches and relay contact reliability is usually measured in the 'millions' (or 10's of millions) of operations....... Indeed, in this particular application you'll find most manufacturers use relays too - simple and cheap.

[Charlie]

First Robert, let me say sorry for starting to help and then falling off the internet. I had to be away for a couple days without access.

The original problem... Based on what you've posted, just hook the float charger to the battery and power your circuit from that. Your siren won't care about voltage from about 10 V to about 15 V.

A sealed lead acid battery can remain on a float charge forever without damage. Large commercial systems do it all the time - for example the telephone system works this way. Hook up your trickle charger to the battery and measure the voltage - if it's between 13.2 V and 14.4 V (2.2 - 2.4 volts per cell) you are good to go. Cars are usually 13.8 V.

The current required for multiple LEDs is an old problem, and the reason "scanning" LEDs was invented. You basically switch them on and off fast enough that your eyes don't pick up the blinking (anything over about 100 Hz). You use about a 10% duty cycle and increase the current a little bit if you wish. You can experiment to adjust perceived brightness. You will end up with less current than the LCDs.

I'll see if I can sketch you a VFET circuit when I have a second - still catching up on email.

[Demon]

I'm concerned with this wallwart made-in-china trickle charger. I wanted it for testing and maintaining my new 12V battery, but I wouldn't trust it in permanent service in a metal enclosure. I've read reports of them overheating.

I'm googling LM317 app notes and such looking for a reliable 12V 7Ah automatic charger.

The DSC1000 runs without a battery connected (I ran it over 1 year when the battery died). That's why I wanted to run off mains first, and battery during blackouts.

This charger seems interesting:
http://www.sentex.ca/~mec1995/circ/lader.htm

Robert

[Charlie]

You can certainly roll your own - they are pretty simple. As I mentioned, you want a voltage between 13.2 V and 14.4 V. Limit current for thermal reasons because the battery can handle a lot more current than a simple charger can provide when it is really flat. If you want to get fancy, remove the charge every once in awhile and measure the voltage as this will give you the state of charge, or set it up so that you provide under 2A when the battery voltage is about 12.6V, then reduces as it rises further.

The amp hour rating of your battery is not relevant to the design. and overheating is only a potential issue if you are charging a totally pancaked battery. In normal use you really don't need to worry.

The 317 is a bit light, but conceptually you could use it as a reference with a couple power transistors that turn on as required.

The circuit in the link seems fine.

[Demon]

... overheating is only a potential issue if you are charging a totally pancaked battery. In normal use you really don't need to worry.

What if the battery dies, sulfates, whatever?

The 317 is a bit light, but conceptually you could use it as a reference with a couple power transistors that turn on as required.

The circuit in the link seems fine.

I'm open to using higher rated components, I'm not merchandising. I try to follow his math on how to select resistors but I get lost.

Robert