Digital meter feedback?

Your vendor is deserving of negative feedback if he told you a

7107-based Digital Panel Meter (DPM) can handle what's called "high-side measurements". He didn't know what he was talking about.

Actually, it's only because of your caution and using a current-limited supply that your DPM is even working.

Now, there are two ways to do this. First, you can use an isolated DC-to_DC converter which will work over the automotive voltage range (11V to 16V, with load dump protection) for each DPM (you can double up if both shunts have the battery voltage as a common, but you may have to disable the negative segment of the first LED -- they'll both read negative amps).

The second involves some cobbling. You can use an op amp configured as a diff amp, along with some precision resistors (and probably a tweaker pot to offset null) to do the level translation. Also, with a single supply op amp, you may have a bit of trouble getting dowm to below

100uV (one count of your DPM is 100uV). From your prior post, this sounds like it might be a little too much, but it's your call. If you're interested, and are willing to cobble together an op amp circuit to help, post back.

Of course, you could still go retro, and use your 1 milliohm shunts with a couple of 0-50 amp analog panel meters (All Electronics P/N CAT# PMD-50A, $12 each in single quantities). Then you don't have to worry about power supplies.

Good luck Chris

to be used as a amp meter thus using the local supply the inputs to the meter must not be common to the supply. in other words, you need like a differential or separate common input that is not part of the common of the supply. maybe you should check a little closer to see if the has a low, common or - input along with the + input .. normally you simply connect the path through these two inputs with the shunt across the inputs. if you don't think the meter has - and + input then you could construct a simple op-amp using the - and + inputs in the same fashion and the output to drive the meter as a voltage level meter instead. but in my opinion, if the meter is already designed to be an amp meter then it should have an isolated common and

• input or a - and + input.. you don't use the common to the supply to connect any current path..

--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

How is this meter supposed to be used?

I am measuring current in zip cord, and I do have available the negative wire, one side of which runs to system ground (a lead-acid battery negative terminal).

However, when I tried the meter on the black wire in this way, it showed "-00.4" amps with no current imposed on the shunt and supply current to the meter limited to 0.040 amps. So just using it on the ground wire, and having one side of the shunt and the 5V negative commoned did not work properly.

Ken C

"Ken C"

** With a floating DC supply, apparently.

Most DPMs that use LED displays work with "common negative".

But a few need the DC supply to be isolated from ground.

Can you check if the minus (-) input is linked to minus ( or 0) supply ?

........ Phil

Hi, Ken. Most of these 7107-based meters are supposed to be hooked up like this (view in fixed font or M$ Notepad):

| | .-----------. V | | | | | +5V | |In+ | | | o-----o o-------------o | | | | | | Current | | | | | | | | .-. | | | | 1 milli- | | | | | DPM | ohm | | | | | | shunt '-' | | | | | V | | | | | 0V | |In- | | o-----o o-------------o | | | | | | | | | '-----------' V | (created by AACircuit v1.28.6 beta 04/19/05

There's separate connections for power and the measurement. The 5V supply on the left is supposed to be isolated from the voltage measurement across the shunt on the right.

The way I was reading your post, I thought you might be doing this:

| | +12V TOTALLY WRONG -- DO NOT DO THIS! | o-------------------------------. | | | .-----------. | | | | | | +5V | |In+ | | | o-----o o-------------o | | | | | | Current | | | | | | | | .-. | | | | 1 milli- | | | | | DPM | ohm | | | | | | shunt '-' | | | | | V | | | | | 0V | |In- | | o--o--o o-------------o | | | | | | === | | | | GND '-----------' | | .-. | Load | | | | | | '-' | | | === | GND | (created by AACircuit v1.28.6 beta 04/19/05

Typically, you're not supposed to connect the - of the voltage measurement with the - of the power supply, and you're definitely not supposed to have an input voltage greater than the power supply.

Look at the instructions again, or get in touch with the vendor. I'm pretty sure you've got separate connections for the power supply and the + and - inputs. If you do this right (e.g. get an isolated power supply for the DPM, and keep it separate from the shunt resistor input), this shouldn't be hard (as long as the DPM hasn't been smoked).

Here's one thing you can try. I'm sure your shunt resistor is OK. Use your bench power supply to power up the DPM, with the input + and - shorted. It should read zero, +/- one digit. Now, connect up a 1.5V battery, a 10K resistor, and a 1K pot to the DPM like this:

| 1.5V | | | .-. | | |10K | -----------. | | | | | '-' | +5V | |In+ | | o-----o o-------------o | | | | | | | | | | | .-. | | | .-->| |1K | | DPM | | | | | | | | '-' | | | | | | | | | | | 0V | |In- | | | o--o--o o--------o----o | | | | | | | '-----------' | (created by AACircuit v1.28.6 beta 04/19/05

You should read from zero to about 1400 counts as the pot is turned. If this doesn't happen, the meter is probably toast.

By the way, you might want to download Andy's ASCII Circuits so you can better describe what you're doing, if you still need help.

Good luck Chris

"Ken C" schreef in bericht news: snipped-for-privacy@4ax.com...

A possibility is an incorrect zero adjustment. You can check by shorting the input connections. Adjust if necessary.

From your description I got the idea that one testconnection has been connected to power ground. The drawing suggest it to be the plus of the testconnections but it may be the minus as the drawing is wrong anyway. You may try to check with an Ohmmeter.

If the minus is grounded, you may try to correct the read-out using the adjustment. So adjust to 00.0 with no load. The idea behind is that the (input)amplifier has some bias voltage that was overcompensated for.

petrus bitbyter

How can I tell (without loss of magic smoke) if my meter power supply is isolated from my equipment power supply?

if i use the meter at home, the main equipment will be running off a switching power supply. How can I be sure that the 13.8 V switching supply will be isolated from the 9V wall wart I will use for powering the meter, when they will both be plugged into the same 110V convenience outlet?

How about in the field, where the 9V supply will be a DC-to-DC unit connected to the equipment supply?

Ken C

Hi, Ken. The 9V wall wart has transformer isolation from the line, as should the home 13.8V switcher. In the absence of a significant voltage between the input and GND, you shouldn't have problems. It's only when the voltage being measured is, say, at line potential that this is a problem.

In a DC-to-DC converter, you should be looking for one that's rated as isolated. The spec sheet shows the isolation between input and output power. Even the minimal 500V isolation rating provided by the least expensive isolated DC-to-DC converter is more than enough.

Just for reference, one choice might be the Mean Well SCW03A-05, which converts 9 to 18VDC into an isolated 5V@600mA. Kind of overkill, available at Jameco as their P/N 213268PS for $15.95. You might want to look at the product to give you an idea:

Actually, I guess I should also mention that the temperature and vibration extremes in a car make an analog meter less than ideal. If your meter does need an isolated power supply, you actually might be stuck with the DPM.

Also, since the maximum input to the DC-to-DC converter is 18V, you should protect against load dump by using a low ohm series resistor, followed by a big cap and a transzorb at the input of your power supply, like this (view in fixed font or M$ Notepad):

| | V | 2.2 ohm 1 watt .------------. .-----. | |B+ ___ | | | | | |o----|___|-o----o----o 9-18V +5Vo---o o---o | +| | | | | | | | 1000uF --- /-/ | | | | .-. | 25WV --- ^ | DC-to-DC | | | | | | | P6KE16A | Converter | | DPM | | | | | | | | | | '-' | === === | | | | | | GND GND | | | | | | | | | o---o | .-o GND COMo---o | | | | | | | | | | | '------------' '-----' | | | V | === | GND | | (created by AACircuit v1.28.6 beta 04/19/05

Good luck Chris

Thanks, Chris.

At least I can solve my provlem for $15.95 plus delivery.

I tested the DC adapter part of my Sima SUP-2 "universal power adapter" and found that the input ground and output ground are shorted

-- suggesting that it uses DC regulators. No good.

I have a noname DC-DC converter used to power DVD players on airplanes. The input ground and ouput ground measure as completely open. How can I test it to see if there is enough isolation?

Ken C

Hi, Ken. Plug it in and see if it doesn't say. Use a 10K resistor to find out:

| | .------------------------------------. | | DC-to-DC | | | Converter | | | + In - + Out - | | '---o------o---------------o-----o---' | | o o o o | | | ___ | | | '------o----|___|------o-----' | 10K (created by AACircuit v1.28.6 beta 04/19/05

If you've got good isolation, you should be able to put a meter across the 10K resistor, and then see if there's a voltage across it as you connect the ends of the resistor to the primary and secondary. Be sure to check primary - to secondary + and pri. + to sec. -, as well as pri.

- to sec - and pri + to sec +.

Good luck Chris

Sorry -- use a 100K resistor with your DVM.

Chris