Measure 2MΩ, should be 100Ω - 50KΩ , what do I do wrong?

[Marco420]

Hi EEVBloggers,

I'm new here, please help me out with a noob question.
I'm measuring a rectifier from a Honda marine engine. In the service manual they described the resistance values between the wiring.
My measurements are all in Mega ohms, and should be in Kilo ohms.
For example:

What I measure:
2.450 M ohm
2.331 M ohm

From service manual:
100  -  50 K
Unit: Ohm

Note:
Use a tester that is equivalent to or higher than the performance specified; Internal resistance: 20 kOhm/VDC, 9 kOhm/VAC.

I'm measuring with a
ZOYI ZT702S Digital Multimeter/Scope
Pretty nice device, pretty accurate for beginners..

I know the rectifier is working fine, it's from a reference motor that is working good.

What I'm I doing wrong here?

Thanx for helping,
Marco

[DavidAlfa]

A DMM will rarely measure ohms for diodes, the voltage in that mode is too low, perhabs older DMMS with more strong output could.
Try it in diode mode, you should see 0.6V in one direction and nothing in the other.
 

[Marco420]

Okay, thanx, I'll try tomorrow...

So that's why they noted:
"Use a tester that is equivalent to or higher than the performance specified; Internal resistance: 20 kOhm/VDC, 9 kOhm/VAC."
Correct?

I'm not sure what 20kΩ/VDC means actually...

[Kim Christensen]

Ohms per volt is a specification used for old analog VOMs...
A VOM with a "20kΩ/VDC" spec would load the circuit under test with 20kΩ when switched to the 1V scale. On the 10V scale, it would present a 200kΩ load. 100V scale: 2MΩ.

Most digital meters have a fixed input impedance of 10MΩ no matter what voltage range they are set to.

[TimFox]

20 kohm/V is a spec for an analog passive VOM, such as a Simpson 260.
It means that the voltmeter function is a 50 uA meter in series with a "multiplier" resistor to obtain the voltage full scale.
Such meters had lower resistance for AC, therefore the 9 kohm/VAC spec.

[Marco420]

Thanx all for explaining !

@David, i measured in diode mode, and got

0.534v on some points where the service manual mentioned to measure 100Ω - 50kΩ
(what you mentioned)

2.235v on some points where the value should be 1kΩ - 200kΩ

Is there a way to convert diode measurements in voltage to resistant values in ohms mentioned in the manual?

Thanx a mil for helping out !

[Ian.M]

Not at all easily.   Also your DMM diode test range may be incapable of reaching some test current vs voltage points that the original 20K/V analog multimeter used by Honda when writing the manual could reach on its ohms ranges.

You *could* simply buy the cheapest 20K/V analog multimeter you can find and hope its similar enough to what Honda expect you to use.  e.g. https://www.amazon.nl/Ashley-GAO-Multimeter-Elektrische-Multitester/dp/B093DFSLNK

Its got a lower 4.5 K/V on its AC ranges, but that's unlikely to be important unless you need to measure AC voltages with the engine running, which wont be needed unless you are checking the output of an alternator winding before the diode plate or an (optional) AC lighting coil of an outboard's flywheel magneto.  Edit: Even so, I cant think of any AC voltage from an engine from the analog multimeter era which would be sensitive to 4.5K/V loading vs 9K/V.

[strawberry]

usually multimeter diode mode give ~1mA

rectifier should be on secondary coil charging circuit. usually loosely coupled to give 'constant/limited' current (no worries when shorted output for short time)
and electronic circuit that limits maximum charge voltage

[TimFox]

Strictly speaking, a VOM in resistance mode connected to a non-linear diode is not really a "measurement", but an "indication".
The total circuit is a battery in series with an adjustable resistor and microammeter, connected in series with the diode.
For a real diode, the indication is a function of the details of that circuit.
For a typical DMM, the circuit is simpler to understand:  some use a constant-current source in series with the diode, so the display is a measure of the voltage across the diode at that fixed current level.
However, some digital meters measure the ratio of the device voltage to the voltage across an internal resistor, which is not a fixed current through the series circuit.
The original instructions, from when VOMs were still common, merely distinguish a shorted diode from one in reasonable condition, since diodes typically fail short-circuited.