Display Battery Backup

[dssteven]

Seamless DC power switching

Hey everybody!

So my original post here had tons of info and I was getting no replies. I'll sum it up a bit quicker this time so maybe you guys can understand what I'm actually looking for. I need something that will allow me to seamlessly switch from my primary power supply to my backup powersupply whenever my primary supply is disconnected (or too low).

The equipment is powered by 12VDC normally and draws a decent amount of current although I'm not sure exactly how much but will update when I know. Please let me know if you have any ideas on how to seamlessly switch power supplies. I had a relay idea but I believe the 5ms-9ms delay may be too long. I'll give it a shot when I get a chance but I highly doubt it will work. The attached picture shows a bit more information on what I'm looking for.

Thanks!

Dan

 

[dssteven]

Revised the original post for clarification since there were no replies. Didn't want to make a new post and clutter the forums. Writing this so it isn't sitting at the bottom of the list now

Also thought I'd make note of this:
http://www.intersil.com/content/dam/Intersil/documents/fn31/fn3183.pdf

This is perfect...except it is for micro controllers and has a maximum peak input current of 38mA for primary and 30mA for secondary supply. Does this mean that the maximum output current depends on what power supply is active in the switch? It would appear that way but I just want to clarify. The equipment I'm using will draw as much current as it needs (which I'm still unsure as to how much that is exactly... but I'd guess over 100mA)

 

[weird_dave]

If your display doesn't mind a small voltage drop, you could just use 2 diodes.
Join the 2 cathodes together and connect that to the +ve of the display, then each anode goes to a supply +ve, job done

That intersil part isn't suitable on its own due to the current limitation, but scroll down to figure 11 or 13.

 

[dssteven]

Join the 2 cathodes together and connect that to the +ve of the display, then each anode goes to a supply +ve, job done

Wow I never would have come up with an idea as simple as this. Why is it that only one diode is activated at a time?

That intersil part isn't suitable on its own due to the current limitation, but scroll down to figure 11 or 13.

Yeah I noticed this. Thanks for your suggestion it may save me tons of trouble designing a PCB for the intersil part and the transistors and resistors needed as described in Figure 11 of that datasheet...

Thanks!

 

[dssteven]

Nevermind I understand how it works now. Thanks for showing me the way weird dave!

 

[weird_dave]

You're welcome

 

[dssteven]

Back to this! Would this circuit do the following?
1. Allow the higher charged battery to be the only source of current for the load
2. Recharge the secondary battery when the primary battery was on and
3. not overcharge the secondary battery.

Let me know if you have any suggestions to fix the following potential issues. I haven't tried the circuit yet in action. Thought I'd check with you guys first

The switch there is to simulate the primary power supply being disconnected

Edit: I should also mention that the primary supply is 12V and the secondary is 11V. The primary is kept fully charged by the engine via a Voltage regulator

 

[CDRIVE]

This is an automobile?

 

[weird_dave]

The resistor between the 2 batteries is a problem. If the primary voltage drops, it will be charged by the backup battery. You might get away with putting a diode in series with it to prevent that.
Really need to know a bit more about what you're trying to achieve at this point, I can't see an engine in the diagram!
It sounds like your 12V primary is a car/leisure battery being charged by an engine, where exactly is the voltage regulation in here?
I've assumed the 100ohm to the right is simulating the load of your display

 

[CDRIVE]

A small incandescent lamp, in place of the resistor, is usually better choice for this kind of application. As W.Dave said though, you'll need a Diode in series too. A Schottky Diode will minimize voltage drop.

 

[dssteven]

Basically this is a large piece of construction equipment (specifically, a large cement screed). It has a display where it shows heaps of data as well as allows for calibration of different settings. It runs on an 80hp combustion engine with a 12V primary battery being charged via an alternator.

I use this tool here to simulate: falstad.com/circuit
the tool would not let current flow through the circuit without resistance somewhere in that loop so I added it just to please the applet. I will certainly look into adding a Schottky diode to make sure the primary battery isn't being charged by the secondary, thank you for that suggestion! Otherwise though, should that circuit do the trick with a simple diode in place of that 100 ohm resistor between batteries? and can I use Schottky diodes in place of the two diodes already there?

On a different topic, although I enjoy the ease of simply using a java applet for circuit simulation I should probably upgrade. Suggested free/cheap program? LTspice or something is one right? I'd google it but I'm already writing a reply so I might as well ask also...

Thanks for the help CDrive and Weird Dave!

 

[weird_dave]

I'll assume the display is currently working fine being powered by the alternator/battery (there can be all sorts of problems with this if the equipment isn't designed for it)

You can't replace the resistor with a diode since there wouldn't be anything to limit the current, the diode would let out all its smoke.

Does the display state the power consumption on it? We need this to work out the diode rating.

Yup, make them all Schottky

 

[dssteven]

I'll assume the display is currently working fine being powered by the alternator/battery (there can be all sorts of problems with this if the equipment isn't designed for it)

haha good assumption. Yeah, this has been in production for a long time and the general setup works fine. I'm just looking to add this as a feature (or redundancy).

You can't replace the resistor with a diode since there wouldn't be anything to limit the current, the diode would let out all its smoke.
Does the display state the power consumption on it? We need this to work out the diode rating.
Yup, make them all Schottky

Well, I was reading 600-700 mA at 12VDC so... 7-8 W... I'll have to take it apart tomorrow and see if it says on the back of it (it's in a nice little housing I can't take apart right now) I would imagine it isn't more than 8W but I'll check tomorrow for a concrete answer.

Thanks again!

 

[CDRIVE]

<snip>Well, I was reading 600-700 mA at 12VDC so... 7-8 W... I'll have to take it apart tomorrow and see if it says on the back of it (it's in a nice little housing I can't take apart right now) I would imagine it isn't more than 8W but I'll check tomorrow for a concrete answer.

Thanks again!

This is why I said a small automotive lamp will probably work fine.