I have a small design issue I'm trying to figure out:
I have a Fluke 287 I will be using on the bench for the time being until I can save up for a bench meter, but that's awhile away... I already have it modded to run off a 12V wallwart witch goes to a LM7810, and a 9V battery, both diode'd for backflow. When the wallwart is off, the 9V battery provides power for clock backup. When the wallwart is on, it provides the power from the 10V LM7810.
The issue arises when the wallwart is off (please don't say just see if it is on or off) - the 9V battery still has enough to power the meter. When off, the meter draws about 200uA IIRC. It turns on at about 17mA, backlight low is 29mA, and high is 50mA. The idea was to design a simple circuit that limits current to ~10mA or less (therefore it can't turn on) when on battery. The additional problem with this is I don't want components that put an additional parasitic load on the battery, unless all of it can stay within the low uA range. I should get 2, maybe 3 months off a 500mA 9V Alkaline battery...
It seems something simple got very complex quickly and out of my realm of knowledge.
I've tried ChatGPT and Claude for suggestions, but they keep choosing components with incorrect values or things that won't work together (go figure).
Do any of you experts have recommendations or some light reading?
Can you please post a copy of your circuit, a picture of a hand drawn circuit in jpg, png?
Hand drawn and photographed is perfectly acceptable.
Please include ALL hardware, power supplies, component names and pin labels.
@LarryD - attached. @jim-p Hi Jim - I owe you some beer money for helping me with the power board a few months back. But no, normally that would work fine if I was using it portable. I use the meter sometimes for over an hour, and can't really afford to replace them frequently, this way works better for me.
The circuit is on a perfboard at the moment, here is a diagram for it from memory.
I know the 7810 getting 12V is on the edge for minimums, but it is holding stable, and no issues from the meter.
If you're interested, I’ve created an animated video that explains how the TL431 works and explores its practical applications. It could help make things clearer for you. You can check it out here:
I’m confused … you are powering your meter from a wall wart ?
The 9v battery inside will last for years and typically is not isolated from the measurement side - creating a hazard when measuring higher voltages , also easy to damage the meter ( if have a measurement connection common to the battery connection)
If you are doing that , do yourself a favour and buy a battery.
I'd likewise keep supplying batteries, our workshop has 5 Fluke DMMS, we probably change a battery for any of them about once every 2 months.
The battery's last for eons, 100 working hours, is not a problem.
I think you are over thinking the situation with a DMM worth more than a $1000, AUD.
