# Trying to repair some clamp meters

Discussion in 'Repairing Electronics' started by BGAmodz, Sep 25, 2013.

1. ### Nigel GoodwinSuper ModeratorMost Helpful Member

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Connect it as a simple potentiometer, and feed it from a 5V AC supply.

However, if you don't understand such simple basics as that (simple ohms law) how do you expect to repair things?.

2. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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You can use your 6.3 V xformer and apply it to the ends of a potentiometer and then take the output from the wiper and the CCW end.

You can't exceed the wattage of the POT (P=V^2)/R

and the output Z is variable, so it would influence what's it's connected to.

e.g. A 1 ohm POT will likely smoke
A 10 M pot might have linearity issues if paralleled with a 10 M input Z meter.

==

Getting 5 V out is a little harder. You still have to find a good value for Rpot that doesn't exceed the power dissipation of the POT @ 5V.

Find the current through the POT at 5 V drop.

Use tat current to drop (6.3-5) V and do it with a series resistor. Again check resistor wattage.

==

You could just do 3 or 4 fixed voltage dividers with two resistors.

++++

Other more complicated options can come into play such as an OP amp buffer.

Last edited: Oct 14, 2013
3. ### BGAmodzMember

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Hey thanks bro for keeping up with me here , well am still ambitious about this , and am kinda enjoying it actually .

So i have an idea about the AD736 chip ; I want to make a test to see if output gets proportinal with the input , but its noted on the datasheet that the chip provides 200 mV full scale input range .
Does that mean i should not apply an ac voltage above 200 mV at input of the IC ??

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5. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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Yep, that's what it means. The datasheet also states that in some cases you can apply more.

The datasheet also states that the gizmo can be configured to respond to DC. So, 0-200 mV DC would be 0-200 mV DC out.

I circuit, you haven't even got close. You did appear to figure out the inputs are differential.

Here's a resistor divider calculator: http://www.electronicproducts.com/Voltage_Divider_Resistance_Calculator.aspx

6. ### BGAmodzMember

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But the input should be always ac i guess , or am wrong ? , because this is an RMS to DC converter .

7. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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In your meter, the answer is yes, the input needs to be AC.

If you breadboard the chip outside the meter, TRMS(AC+DC ) is an acceptable test.

We have to go back to theory and practice,

12 VDC is also 12 V RMS when you delve into the mathematics.

12 VDC will supply 12 Watts to a 1 ohm load and so will 12 VAC. This really satisfies the definition.

12V DC with a 1 V AC on top of it is 11 V RMS.

Now practically, I mostly care about the AC and DC components and not the True RMS of the waveform/

Some meters will allow TRMS: AC, DC, AC+DC

Now, lets go wierd. AC Coupling: TRMS; so we get the RMS value of the AC component
DC coupling TRMS; We get the True RMS of the sum of the AC and DC components

Useful yes:

TRMS AC; gives you an idea of the ripple on a power supply
TRMS: DC compared to TRMS AC+DC gives you a really good comparison of ripple.

Stuff gets complicated. SO, it stays at your understanding or a need to know basis. That happens all the time.

8. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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From page 1 of the datasheet:

9. ### BGAmodzMember

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Allright then , i have a breadboard by the way , so i will make the tests with both ac and dc inputs .

Edit :

according to the datasheet the TRMS input range is from 0 mV to 200 mV , that means that the output also has to be in that scale as i understand , but why on my previous tests i had voltages above that scale on IN and OUTPUT ???

Last edited: Oct 16, 2013
10. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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1. auto range broken forces the input higher.
2. We never really measured the input diferentially.
3. Some measurements were taken with a bad resistor.
4. The datasheet says that > 200 mV is OK under certan configurations.
5. knowing the supply voltage can help. An output is never Exactly at the supply voltage unless shorted.

Which is why I didn;t think it was important to test the IC out of the circuit yet. Differentially, you have to get less than 200 mV using manual range. If you can't, then I would expect the problem to be BEFORE the chip.

The BAD RESISTOR threw a curve ball. So, I'm more interested in getting the differential voltage below 200 mV before going anywhere else. Attenuation HAS to be before the TRMS converter. Autorange just messes everything up.

Stuff like Relative, could be before the TRMS IC.

DC volts (As in when using the DC input) could also be present there, but the chip would not process it as TRMS. Without a schematic, there is no way to know.

Last edited: Oct 16, 2013