How to power a dvm with the source voltage it's reading?

Post by **ShowMaster** » Sat Mar 31, 2012 3:47 pm

I've been asked to come up with an inexpensive way to power some $4 DVM's to constantly read our solar station system. 20 auto batteries and a 40+amp solar charging array. 5000w gen backup as well that will soon auto power on if the 12v buss drops too low.
My dilemma is that most inexpensive dvm's can't measure the voltage powering them. The meters use a 9v battery. My thought is to use 2 HK uebc type modules in series to make 10 volts. I don't think there is a single higher voltage one? Has anyone used there devices that know if the inputs voltage is isolated from the output? I'm sure the negative leads are common and I don't know if the DVM will care about that? The more expensive $30+ commercial devices isolate everything -&-+.
I'm looking for any simple solution or voltage pump circuits. I'd rather use something already made I can piece together from hobby parts.
Other meters may have been a better choice but I came into this late after the meters were purchased.
Thanks
SM

Post by **Kilrah** » Sat Mar 31, 2012 3:58 pm

ShowMaster wrote:My thought is to use 2 HK uebc type modules in series to make 10 volts.

That certainly won't work. All hobby UBECs I've seen have a common ground between input and output. So putting 2 in series would just create a nice short circuit

What's the voltage the meters are supposed to measure? If it's higher than about 10V, AND the meters don't mind having a link between supply and input (that would have to be tested on one that would risk being sacrificed for the sake of science), you could simply use a 7809 voltage regulator.

Post by **cre8tiveleo** » Sat Mar 31, 2012 4:03 pm

12v ubec... if that helps

http://www.hobbyking.com/hobbyking/stor ... _23v_.html

gohsthb · Post by **gohsthb** » Sun Apr 01, 2012 12:34 am

MAybe you are looking for something like one of these:
http://www.jameco.com/webapp/wcs/stores ... CH%252BNAV
An isolated DC-DC converter.
-Gohst

Post by **ShowMaster** » Sun Apr 01, 2012 6:10 am

gohsthb wrote:MAybe you are looking for something like one of these:
http://www.jameco.com/webapp/wcs/stores ... CH%252BNAV
An isolated DC-DC converter.
-Gohst

Thanks for the link. Yes they look like my best option. I found out a little more about he meters. They are actually a red led type so I'm going to measure the current required to drive them. I did check and they can't measure the source that powers them.
SM

Post by **GrootWitbaas** » Sun Apr 01, 2012 6:47 am

Most cheap dvm needs very low supply and current. IMHO you don't need a ubec. You can get units (display only) ready made for this. I will have a look for some links on ebay when I had some sleep, just back from nightshift.

I made some circuits for the same purpose many years ago, it was self contained radio relay stations, solar powered with cutoff for both low and high, and also step by step shutdown, so lower priority rebroadcast systems was shut down 1st. I have to search, might still have one of them designs somewhere.

Groot

Sent from my iPhone using Tapatalk

Post by **ShowMaster** » Sun Apr 01, 2012 7:31 am

Thanks Groot for looking.
SM

Post by **Kilrah** » Sun Apr 01, 2012 9:00 am

Frankly I guess that buying new meters will be easier and cheaper than getting isolated converters or stuff like this...

Something like this kind of standalone meters for example:
http://www.ebay.com/itm/Digital-Voltmet ... 4164514191

Post by **GrootWitbaas** » Sun Apr 01, 2012 12:45 pm

Thats the modules I was referring to. About the circuit I used, I have to dig for it and see.

Groot

Sent from my iPhone using Tapatalk

SamoaFlyer · Post by **SamoaFlyer** » Sun Apr 01, 2012 12:59 pm

This one is pretty simple and cheap.

Post by **ShowMaster** » Sun Apr 01, 2012 3:50 pm

SamoaFlyer, I like the circuit and will breadboard it up. I'm hoping it will be isolated enough from the
Power source to not confuse the meter reading it? I wonder if I could drive a small transformer to fully isolate it with the 555? I'll measure he meters current draw with the red LEDs and see what it is. I think they're multiplexed so hopefully its low. If the circuit gets too pricy a new LCD led backlit meter that can read its source voltage may be required. Since it's club money I like to not buy anything new but this circuit I can donate if it works, thanks.
Any other circuits that are simple and inexpensive and make 9+ volts at as much current as a 3 place red Led display may require, TBD will be welcome. Ther's a big contest this weekend and the charging stations are in use so I can't open one up to take any meter current reading right now. The meters now use a push button and a 9v battery for power but it's a pain to change he battery when it goes dead so I'm trying to engineer a simple fix with all your help.
Thanks all
SM

SamoaFlyer · Post by **SamoaFlyer** » Sun Apr 01, 2012 6:21 pm

I doubt that this would work with LED meters - I assumed LCD. I used it with panel meters that used less than 1 mA. A small audio transformer could be used but you still need isolation between multiple meters.

Bill

Post by **ShowMaster** » Mon Apr 02, 2012 2:32 pm

I have new info because I started it all and so many have tried to help.
The meter is a led type from All Electronics. All is a surplus electronics store. The meter appears to run on 5 volts and someone that wired it all up used a 7805 to make 5 volts from a 9v battery and push button. The meter draws 60 ma@5v when on. Going by All's posted specs the grounds are common for power and the meter input a/d. All's specs say it can read it's own voltage and its input imp. Is >100mohms. I have no make or model # yetbut will peruse that this week.
The persons wiring this all up had limited knowledge of electronics but made it work several years ago. Now that I'm learning more about this it may be as simple as eleminating the 9v battery and just connecting the 7805 to the solar 12-14v rail it's reading. I may disconnect the meter from reading the source, power it from the source first, and then add a 1k or higher resistor in the input line first time to protect it. Thanks everyone for your help on this so far but as I learn more about it may be a simple fix with a soldering iron.
The eBay meter suggestion is also a one, thanks there. These meters cost $15 and if I blow one up I'll go to eBay for a replacement ha ha.
I'm hoping for the simple fix this week when I open up the panel and move some wires around.
SM

gohsthb · Post by **gohsthb** » Mon Apr 02, 2012 3:26 pm

Take your normal voltmeter and connect it between the +13V(Solar rail) and the + on the 9V battery. If you are only getting like 4 volts then it will probably work. Also check that the grounds are connected with your meter. Just hook one lead on the meter - and the other to the solar -. You should get 0V if they are connected.
-Gohst

Post by **ShowMaster** » Mon Apr 02, 2012 7:39 pm

gohsthb wrote:Take your normal voltmeter and connect it between the +13V(Solar rail) and the + on the 9V battery. If you are only getting like 4 volts then it will probably work. Also check that the grounds are connected with your meter. Just hook one lead on the meter - and the other to the solar -. You should get 0V if they are connected.
-Gohst

Will do, thanks.
SM

Post by **GrootWitbaas** » Tue Apr 03, 2012 9:40 am

Also Note, on any solar system you want any measurement and controlling to use absolute minimum power. Those LED meter do eat a fair chunk of power. The meters listed above on ebay can be bought very cheap, if you search a bit on ebay. have Fun

Post by **ShowMaster** » Fri Mar 01, 2013 4:36 pm

$2.50 12v DC to DC converter fix.
I found several for $2.50 ea locally. Isolated in and outs. Replaced the 9v battery with it and now when the meter read PB is pushed 12 volts is sent to a 78L05 that powers the meter. I'm able to power and also read the source voltage with the meter now. No more replacing the 9V battery every other month. It gets a lot of use.
One less field chore getting in the way of flying.
SM

Sent from my iPod touch using Tapatalk.

openrcforums