Ideas -Ramblings

[Hattrick]

About to start making my first test box. I've looked at the forums and certainly found them most helpful for some of the schematics. Couple of questions.

1. Regular pitfalls or gotchas that I need to watch for.

2. anyone see an issue with using 18v Black and Decker rechargeable batteries, other than weight? (I have about 8 of them with chargers, My son has eaten a lot of drills with his welding in the past few years. finally got him a Dewalt)

[brtech]

There are two major variables in test box design:
1. One meter or three (and occasionally four)
2. Internal ohmmeter circuit or plugs to a purchased multimeter

What were you thinking about? We'll be better able to help you.

Higher voltage is not necessarily good. The basic equation is ohms law,
I=E/R. I= current in amps, E = Electro-motive force in volts, R=Resistance in ohms.

You want to measure, let's say, 5 ohms center scale. If you use 1.5v, this is:
I=1.5/5 = .3A = 300ma. That's a fairly large current, but not too bad.

Let's try it with 18V
I=18/5 = 3.6 A. That's huge. You don't want to do that.
The only way to use an 18v battery is to drop the voltage much lower. You could do that (a voltage divider for example).

The question above enters into this. If you want a 3 meter test box, you need three independent power sources. You could have 3 batteries, or you could do a DC to DC converter with 3 secondaries. The latter would fix the voltage problem too, because you could have an 18v primary and a 1V secondary. Well, 1V secondary regulation is hard, but you could do something reasonable. How much work do you want to get into, and how much do you know about designing DC to DC converters?

Of course the other question above enters into it too. If you use an external, purchased multimeter (with a 1X resistance scale), the battery is in the multimeter and is usually one or two AA or AAA cells.

[Hattrick]

ohhh... forgot to add, I ended up with an ME/EE from college. As for the DC to DC I have a few stepdowns planned already to accommodate 3 Old 9V Alog Meters. Heat is going to more of an issue, otherwise I'll add battery packs to make it easier.

1. 3 meters
2. External (edited for spaz purposes)

[brtech]

A volt meter is usable, but that means you need a current source. That's a pain. If you are comfortable with DC to DC, then sure, an 18V rechargeable will be fine.

The 9V has the problem above. Do the math. I=E/R = 9/5 = 1.8A center scale, and if the cord is, let's say, 1/2 ohm, then it's 9/.5 = 18A! Not a good design.

You want a current meter, and something reading 300ma direct, or a voltage divider to get to, say, 10ma.

[Hattrick]

Doh..... Bad morning arrrrrrrggh... was thinking of this all through another boring meeting. I'm making this way more complicated than it needs to be, for the sake of a few dollars. Actually it would be costing me more to run it down to the right levels on a system if I run out of the batteries in a few years will have to be redesigned . thanks for the wake up on this, more time for fewer results, I'm certainly not working smarter, but harder.

Thanks again!