Low voltage cut off

[goodtimes]

OK, here is the deal. I am working on a little project, and since I am better with a tube bender than I am a soldering iron.....I need a bit of help.

I am building a relay center for my BMW. This is going to control aftermarket accessories (lights, etc). Basically it will be a glorified set of board mounted relays and fuses (speaking of fuses, does anyone have a source for board mount ATO fuse holders?). Now, to protect the battery (dual battery set ups are to heavy), I want this relay center to shut all the aftermarket accy's off if the voltage starts dropping off (important when the bike only produces an excess of ~70 watts).

My original intent was to build the relay center, buy a cheap *whatever* that had a low voltage cut off circuit (I recall seeing a cig. lighter adapter that had one a while back), gut the device in order to steal the low voltage cut off circuit, then intigrate that circuit into my relay center. Of course, I can't find such a device anymore....so I am resorting to what is proving to be a non-electronics guy's worse nightmare (well, not really, but it is a pain).

I need a source for either a low voltage cut off device (12 volt system, cut off point ~11 volts), or I need some help designing the circuit. I have looked around the 'net, and to be honest, most of what I saw just confused the hell out of me. In addition, it seems that most devices are designed for circuits of 5v or less, &/or cannot handle much of a load. I need the circuit to support a load of about 450mW (to open a NC relay).

So, do we have any electronic guru's running around that want to get in on the fun?

:costumed-smiley-007

 

[Martyn]

Battery Brain sells a low voltage cut off that you install on the + battery terminal https://www.asseenontvnetwork.com/vcc/cartergroup/batterybrain/247655/

If it's too pricey we are working with an other manufacturer and should have something less expensive in a month or so.

 

[bfoxg8r]

Here is one on ebay:

http://cgi.ebay.com/ebaymotors/PSPC...ryZ33574QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

 

[DaveInDenver]

If what you want is a switch that turns off below a certain voltage, it would be pretty simple to build a little circuit that controls a relay. I can sketch up something for you. We're talking about a 8 pin DIP, a handful of resistors, maybe a transistor depending on how big of a relay you want to switch. How crunched for time are you, I'm kinda swamped at the moment, probably be this afternoon.

 

[nvprospector]

It works VERY NICELY and also acts as an On/Off switch.


The "First Schematic" shows a Mosfet Driving a "Load" (or Possibly a Relay).

A Relay would be Required if your "Load is a Grounded device".
A Relay than Isolates the loads Ground.

If your Load is NOT Directly Grounded, Than the load can be driven directly by the Mosfet.
In this Situation, the Mosfet SINKS the Negative of the load to Ground.

Alternately, The "Second Schematic" show a Simpler way to use a Relay with this circuit.



NOTE: Either the 33K or the 150K resistor, Either side of the potentiometer may need
to be adjusted in value to give you the appropriate range of control you require.
** This is due to Variation in these IC's.

The 5 Volt Regulator on the Circuit Board is to Stablize the trip point of the IC.
View attachment 11853


Here is the PCB to make your own circuit board if you or know someone that can make them
View attachment 11854

 

[DaveInDenver]

I was thinking along the similar lines. Adjust the 5K pot until you get the drop out voltage you want. As the voltage drops below the voltage set by it, the relay (N.O., BTW) would open. Might have to play with the 150K resistor to get the right hysteresis. That resistor sets the point where the circuit would close the relay. I would have to mess with it some more, this is just a quick first cut.

View attachment 11855

BTW, Motorola used to make the ideal chip, a MC3425. I think it's obsolete now, but it looks like the MC34161 is the replacement for it. This is an OV/LV detector, so you could even make a window voltage detector to protect from both over and under voltage conditions.

http://www.onsemi.com/pub/Collateral/MC34161-D.PDF

 

[goodtimes]

I appears that I forgot to add one critical piece of information....size. I don't have much room to work with.

The Battery Brain was what I originally thought of, after seeing the install on Chucks Tacoma at the ExTrophy, but it is to big. I have an area about 4 inches wide, 6 inches long, and 2 inches tall to fit the entire relay center....BMW just didn't leave much space for this sort of thing.

NVprospector, that is basically what I am looking for (along with a call out of the components and a functional description---because electronics are not my forte), but I need to shrink the package considerably.

bfoxg8r--that is about what I have seen in the past, but was unable to find. I can find one locally to take measurements off of....

DaveinDenver, no hurry. It will probably be the end of December before I have time to actually build this.

Thanks for all the quick responses!

 

[ntsqd]

A link that might come in handy: http://max8888.orcon.net.nz/pcbs.htm
(Firefox stopped some sort of pop-up though, no idea what it was.)

 

[nvprospector]

NVprospector, that is basically what I am looking for (along with a call out of the components and a functional description---because electronics are not my forte), but I need to shrink the package considerably.

You can shrik it down even more if you use surface mount components. Since you said that you don't make circuits I would not go that route. The image of the PCB is the actual size. You can shrink it down by using a thicker clad and running smaller traces.

 

[suntinez]

A link that might come in handy: http://max8888.orcon.net.nz/pcbs.htm
(Firefox stopped some sort of pop-up though, no idea what it was.)

Damn, that is cool! I'm an EE and we used to send our schematics out for prototypes and wait weeks to get them back. You could probably even do multi-layers this way. Amazing.

 

[TheRoadie]

The company I work for got one of these a few years ago when we wanted to make 1-2 prototypes faster than the email 2-day shops could deliver. Not for home use, although there ARE web sites showing how to make one out of a Dremel and a few stepper motors.

http://www.sunequipco.com/PCB/pcb-532002.htm

 

[goodtimes]

A link that might come in handy: http://max8888.orcon.net.nz/pcbs.htm
(Firefox stopped some sort of pop-up though, no idea what it was.)

I have seen that process before, and is what I am planning on using if I go this route. I may try it just to say I have done it....I like playing with stuff like that. I need to figure out how to accurately drill the board though. I wish I still had a user friendly CAD program....I could get the dimensions off of that, then set the board up in the NC mill at work........

So, just how much power can one shove through a trace on a PCB before letting the smoke out?

Yup...another relatively simple project is spinning out of control....

[insert picture of crashing plane here]

 

[TheRoadie]

So, just how much power can one shove through a trace on a PCB before letting the smoke out?

http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/

 

[DaveInDenver]

So, just how much power can one shove through a trace on a PCB before letting the smoke out?

No limit (within reason), just how much copper you want to lay down. Reference IPC-2221. You are limited by the physical size of the traces and so really you are looking at a few tens of amps for very thick planes.

 

[goodtimes]

http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/

Interesting. You got anything in english? (did I mention electronics isn't really my specialty???)

Lets say I'm looking at a total of 100 watts (which is about 30% higher than I can reasonably expect to see, since I only have ~70 watts available). I want to keep the overall package size as small as I can, but realistically, I can only go so small, as I need to fit 4 relays, 4 fuses, and whatever components I use for the low voltage cut off circuit onto the board.

Is there a huge cost increase as you get thicker material on the boards? Are there any downsides to using thicker material other than cost? Obviously, I don't want to spend a money where it isn't needed....but I need to keep the package as small as possible. Having 1/3" wide traces probably isn't the best way to accomplish that.....