Homebrew luanch controller question

[Lugnut]

I'm going to build a launch controller at some point soon (still tweaking the schematics). Its going to look like the St Josephs Industries ELC-50 (same enclosure, same voltage meter in fact), but have it banana clipped into an external 12v source in a tool box.

http://www.saintjosephindustries.com/elc50.html

Quick question: If I want to do a true measure of battery strength with the meter, I imagine the reading must be taken under load, rather than straight from the battery, true? If so, whats the best way to do so? 600k resistor? I'm not an EE so be kind.

Should that volt reading be done as a parallel circuit, meaning that it can be turned off prior to closing the launch circuit (ala continuty check switch)? I had originally envisioned the meter coming on line as the primary arming switch is activated and stay active through launch sequence.

Main concern is that I don't want to impart too much amperage draw on the ignitor to possibly trip it. I had wanted to possibly incorporate LEDs for the arming switch and pad select (3 pad switchable unit), a piezo continuity buzzer, plus the meter, but I'm afraid I'm trying to put 2 gallons of water in a 1 gallon bucket. I gather under 20mA primary circuit load is considered 'safe' for Estes style ignitors (?). Perhaps an "A-off-B" toggle switch between the battery check circuit and the continuity check circuit to prevent to much amperage draw at any one time is the safe way to go.

Any thoughts?

 

[DJ Delorie]

Use an incandescent lamp as a load, like a car light bulb. Wire it and it's test switch right to the battery lines, independent of anything else. That way, your meter will be able to tell you both loaded and unloaded voltages.

You can put as much load on the battery as you want, as long as it doesn't flow through the igniter. Only the continuity indicator's current need go through the igniter.

 

[Mad Rocketeer]

It's probably obvious, but you want to be able to turn the lamp off, and you don't want to run it while launching. It's OK to leave the volt meter wired across the battery all the time.


The Reasons:
The volt meter (if it's any good) will have a very high input impedance, probably well over a megaohm. That translates into a very small current drain, which should not adversely affect anything.

The lamp (if it's sized to be a true test of the battery under load) will draw significant current. You don't want it on and draining the battery (or heating up) for any longer than necessary to get your reading, and you don't want it competing for current while you're firing the igniters. Remember that you will be essentially shorting the battery when ingniting, so a lamp that gives you a realistic view of the battery under those conditions will be of relatively low impedance (resistance), so it will matter more than a mere continuity test lamp might. The lamp will be connected to the battery by shorter (and possibly heavier) wires than the igniters will, making the effect just a little more so.

Confused yet?

 

[Lugnut]

Actually that makes good sense. Thanks for the input guys.

Mark

 

[Mad Rocketeer]

I think I speak for all when I say, glad to help.

Steve