Electronics > Repair
HP 34401a DMM with leaking segments
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qu1ck:
@dreamcat4


--- Quote ---for example wanted to know if by installing the oled it might affects negatively the precision of measurements. but maybe there was already some testing with that in mind?
--- End quote ---

I don't have any precision standards to verify this, my 34401a is literally the most precise thing on my bench. But I did measure a AD587 based reference before and after the mod and the difference was well within temp drift of the source. Take that for what it's worth.


--- Quote ---it seems qu1ck gets the power in from a +12v J5 marked there as DISPLAYPWR ?

not sure where that is supposed to hook up to. but yes that was my main concern with the oled mod.
--- End quote ---

On the display that I used there is a boost converter to 12v. I provisioned DISPLAYPWR hookup to use it as power source there and desolder/cut off the onboard converter on the display. I ended up not doing that because I couldn't measure any noise coupling into meters power lines. But the option is there.

@floobydust

--- Quote ---3.12" OLED need 12-15VDC at 25-50mA typ, depending on how much is lit up. About 2/3W.
If this is generated by an on-board boost-converter, it will draw a lot from a 3.3V rail, up to 250mA.
--- End quote ---

Correct, 250mA is a very high figure though. In this application the display is only showing some text and small icons, at most 10-20% is lit up. My measurements showed <100mA load on the 3v3 line. That's including the stm32 microcontroller.
dreamcat4:
those details are a great help and i appreciate the reply qu1ck. it would seem i am in a pretty similar situation over here. as it happens got some voltage refs from digikey recently. a few different ones but nothing ultra precision. and also a variety of common jellybean regulators. after watching dave's recent video on this topic, i stocked up

your solution looks pretty reasonable so please let me say thank you for your hard work there
floobydust:
I took the 250mA value from example Blue 3.2" 256x64 OLED datasheet it's 3.3V boosted to 12V and 100% lit for 250mA. You can't pull that much off the +17.4V rail so I wasn't' clear where power is coming from, on the in-guard side.
RikV:
Thanks for this great job! This will certainly be useful one day when I find a 34401A for a reasonable price....
cellularmitosis:

--- Quote from: imo on August 14, 2019, 07:27:53 am ---Yes it can do via rs232 with the 31 address. It sends out strings (scientific floating point, like "+9.99995790E+00") with results.
For example you may read the rs232 strings with an external MCU and display them on any display you wish then..

--- End quote ---

Like this  ;D  https://www.eevblog.com/forum/testgear/external-display-for-agilent-34401a-(or-any-dmm-with-rs232-output-stream)/msg4344103/#msg4344103
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