[sdiy] DMM problems [OT]

[Seb Francis]

Well I've been getting _somewhere_ with this DMM.  Still don't know 
where the fault lies, but at least I'm somewhat closer to understanding 
how the Ohm-meter is working.

The heart of this DMM is a MAX134 which has a very helpful datasheet 
apart from the fact that it doesn't state the pin functions for the MQFP 
package (the one I have).  And I don't think the pinouts for the PLCC 
package are the same (is there a common convention for pin relations 
between these 2 different 44 pin SM packages??)  Anyway, I've mailed 
Maxim so maybe I'll get some info out of them.

 From the datasheet and what I can make out tracing the PCB the MAX134 
switches a 1.25V source through a series combination of a reference 
resistor (depending on the range), the 'input protection network' and 
the unknown resistor.  The 'input protection network' seems just to be a 
PTC resistor which measures 600R (dunno if this is right - it's marked C 
884, but it seems reasonable).  The MAX134 then measures the voltage 
across the unknown resistor (and I know the voltage measuring is working 
fine in other modes).

As far as I can see there are no MUX chips or protection diodes used in 
the ohm-meter circuit, so I can only assume the fault is either in the 
MAX134 or the 1.25V source (which unfortunately I can't identify as I 
don't know the pinouts of the MAX134).

So the search goes on!  Thanks everyone for all the help so far, and 
apologies for the OT use of email bandwidth.

Seb



Harry Bissell Jr wrote:
> OK Alex...  (Trebeck - Jeopardy TV show :^)
>
> The Final Jeopardy catagory is 
> "My DMM ohms scale doesn't work"
>
> And my question is...
>
> What is "an input protection diode on the ohms
> scale is damaged maybe due to some circuit voltage
> applied... and that diode now has a reasonably
> constant offset voltage... which the meter is reading
> on each scale as being a ratio of the full scale
> reading, plus or minus some 'who knows' factor"
>
> I would trace that out... and look for some defective
> part.
>
> Maybe possible that a MUX chip that switches the modes
> may be damaged ???
>
> H^) harry
>
> <blonde mode on>
>
> uhhh...  "what is.... don't use the ohms scale ???"
>
> </bmo>
>
> --- Seb Francis <seb at burnit.co.uk> wrote:
>
>   
>> Unfortunately not this simple .. I tried different
>> leads (and even just 
>> shorted the terminals with a banana lead)
>>
>> The interesting thing is that it gives very
>> different readings in 
>> different ranges:
>>
>> Continuity tester mode: 31.8R + bleep
>> Lowest: 32.0R
>> Low: 424R
>> Mid: 2.30K
>> High: 20.8K
>> Higher: 208K
>> Highest: 0.21M
>>
>> All other modes: diode tester, cap meter, voltmeter,
>> etc. work perfectly.
>>
>> Thanks anyway,
>> Seb
>>
>>
>> anthony wrote:
>>     
>>> This happens to me when the wire is about ready to
>>>       
>> break. The lead 
>>     
>>> wire is maybe only one filament or two of actual
>>>       
>> copper wire - ergo it 
>>     
>>> has a higher resistance.
>>>
>>> ----- Original Message ----- From: "Seb Francis"
>>>       
>> <seb at burnit.co.uk>
>>     
>>> To: "Synth DIY" <synth-diy at dropmix.xs4all.nl>
>>> Sent: Wednesday, March 29, 2006 4:17 PM
>>> Subject: Re: [sdiy] DMM problems [OT]
>>>
>>>
>>>       
>>>> Seb Francis wrote:
>>>>         
>>>>> It reads about 500R in autoranging mode and
>>>>>           
>> different readings in 
>>     
>>>>> each range when manually ranged (from about 30R
>>>>>           
>> up to 20M depending 
>>     
>>>>> on range).
>>>>>
>>>>>           
>>>> Sorry, should have said "with the inputs
>>>>         
>> shorted".  When measuring a 
>>     
>>>> resistor the reading is similarly way off (much
>>>>         
>> higher than it should 
>>     
>>>> be)
>>>>
>>>> Seb
>>>>
>>>>
>>>>
>>>>
>>>>         
>>>
>>>
>>>       
>>     
>
>
>
>