Multimeter MOD

[AtomSoft]

Hey everyone i got another IDEA from being a cheap-o

I have 2 multimeters and i want to make one use a wall adapter. It uses a 9v Battery right now. Instead of buying batteries all the time because i seem to forget to shut it off i would like to plug it to the wall..

This would be easy if my ADAPTER which is Rated 9v @ 650mA was actually 9v..

The problem is when i use the meter to test the supply it shows 12-13v ... i hear there is this thing where there will be more voltage with no load.

Would it be safe to plug it in? Or should i add something to the line to ensure it doesnt blow my meter?

BTW my meter is a Greenlee 93-606:

**broken link removed**

 

[ronv]

Sounds like an unregulated supply. I doubt your meter draws much current so it may stay pretty high. Load it down with a couple hundred ohms and see what it reads. Better yet measure its current with the other meter and load the supply with the same current. You might want a regulated supply.

 

[AtomSoft]

I was thinking of using a LM1117 and a 12v supply to create a 9v supply... Would that be even better? I dont know how to do the calculations for the wattage it will dissipate.. and stuff but im sure i can figure it out and if not im sure i can ask here for help heh

 

[carmusic]

wattage of multimeter is very low, it is under 100ma, just take a 7809 or equivalent and it wont even need heatsink

 

[AtomSoft]

Wow i didnt even knew they made 9v regulators heh... cool! thanks ill get that ASAP... i was thinking since i would use a 12v supply i would have to dissipate 3v but then i noticed it would be fine since i just checked 3 datasheets for different 9v regulators and most have a dropout voltage of 2v - 2.5v

 

[Jon Chandler]

One thing to watch with some voltmeter chips is that the common terminal may be common with the negative battery. With a line power supply this could cause some grief or strange results! Try measuring from the positive meter probe to the positive battery terminal. If you read ~9 volts without the negative lead connected, you'll know that the grounds are common.

 

[carmusic]

since the supply is floating i dont know how it could make a problem

 

[crutschow]

Yes, since the adapter supply will be floating it will act the same as a battery and should cause no problems when supplying power to the multimeter.

 

[cr0sh]

Wow i didnt even knew they made 9v regulators heh... cool!

Yeah - there's a whole series in the 78xx area; if this is something you think you'll be needing, its worth looking into. 7805 (+5V), 7809 (+9V), and 7812 (+12V) are "typical"; less common are the 7803 (adj to 1.2V) and 7824 (+24V). There's also the "negative regulator" line - 79xx; they can provide a regulated negative voltage.

I think many of these aren't low-dropout (not sure, haven't investigated that much) - and nowadays there are plenty of other options, but none seem as "easy" to use as the 78/79xx line of regulators. Most now are switchmode regulators that require more "exotic" components many of us don't have "on hand", like small value coils; but they tend to be fairly low drop-out and much more efficient, so that's the tradeoff, I guess!

 

[KeepItSimpleStupid]

How about this: Plug-In 9-Volt Power Supply (Battery Eliminator) - 9V ADAPTER - Smarthome

Way back when I had a 9V battery eliminator in the shape of a 9V battery.

 

[AtomSoft]

Ill get me the 7809 and give it a try... that adapter is $15 heh... id rather buy the 7809 @ $0.50 ... hence the cheap part in the first post...

I already have so many 12v adapters heh...

 

[Shyamal]

Current measurement?

 

[KMoffett]

If you have another meter, why not use it and a bench supply to verify the first meter's current draw? And, instead of a large LM7809 go for a LM78L09 in a T0-92 package. It will handle 100mA. Or, a more likely available LM317L adjustable version.
And I almost forgot, with two resistors you can make a 5V regulator into a 9V one. Same arrangement as the adjustable ones, except a 5V reference rather than a 1.25V.

Ken

 

[AtomSoft]

All my adapters are 600mA to 1.5A

well im sure the meter has too many different arrangements for me to test the current for each one to see the max heh... I would love to use a LM317 or AMS1117 since its not too much current im sure... a LM7809 would have to be ordered and i already have a mouser order coming in today...

I guess ill give it a try with a adjustable regulator and see what happens

 

[cr0sh]

Because these regulators (LM78xx) aren't low-dropouts, try to keep your supply within a few volts of the regulator, and don't draw much current (not that the meter likely will), otherwise you'll need a heatsink on the regulator. Your 12V adaptors will probably be fine, but I would do a test-run with your meter as the load, and check the temperature every so often (if your meter has a temp sensor, tape that to regulator and measure as you test-run).

 

[AtomSoft]

yeah since i have 12v supplies and need 9v... hopefully with a 1.25-2v dropout i should be fine since it doesnt need much current anyway... i dont have a temp sensor... i think i have a lm35 somewhere but im sure my finger as the temp sensor will also work heh...

 

[crutschow]

well im sure the meter has too many different arrangements for me to test the current for each one to see the max

The meter mode that likely requires the most current is the lowest range ohms measurement setting since the meter generates a current through the resistance to make the measurement and measuring the lowest resistance requires the most current. Thus the worst-case would be the normal meter current plus the low-range resistance measurement current.

Note: Make sure the leads are connected to the ohms inputs and shorted together when you measure the circuit current.

 

[AtomSoft]

thanks for the TIP i will surely try that out...

 

[3lectrokid]

There is a nice explanation on why there is a higher voltage at sparkfun electronics LINK

 

[AtomSoft]

wow thanks!!! awesome link