... and if I built a "wild" voltmeter?

• ... and if I built a "wild" voltmeter?
  Posted by DiTBho on 14 Apr, 2021 16:09
• I own a semi-professional DMM Rigol DM3058E, but I wound't let it out the laboratory since it's a bit expensive (I paid it 520 euro). So for a crazy project  I am considering the purchase of "low cost" DMM, but I need it with a serial RS232 line
  

  • Mastech MS8218 (battery powered)
  • Mastech M9803R (230VAC powered)
  • Brymen BM857 (battery powered)

  look interesting, but they are a bit expensive, ~ 180 .. 250 euro (the Brymen BM857 require a RS232 adapter, so I have already included the 60 euro of cost in the 250 euro)
  
  In my opinion they are not professional but decent multimeters and able to measure more things than what I need: I only need a digital voltmeter with a RS232 line, and I need to measure voltage in the { -24V .. 24V } DC, in three working scales
  
  

  • 0..100mV (here I need +/-0.25mV of precision)
  • 0..10V (here I need +/-10mV of precision)
  • 0..100V (here I need +/-100mV of precision)

  
  I am thinking about it, and ... in theory I can use a fast SPI ADC 12bit coupled with some OA, SIL rele' to switch circuits, a MPU to manage measures and digital signal conditioning (interactive-mean? min/max values in a time window?), would it be reasonable? and If yes, what about the protection and analog circuit to interface the probe?
  
  Is there a project to look at as reference, or - better - is there a kit?
  
  Just to consider it makes sense or if I'd better buy something "second hand" on eBay.
• #1 Reply
  Posted by Fungus on 14 Apr, 2021 16:38
• Quote from: DiTBho on 14 Apr, 2021 16:09

  I own a semi-professional DMM Rigol DM3058E, but I wound't let it out the laboratory since it's a bit expensive (I paid it 520 euro). So for a crazy project  I am considering the purchase of "low cost" DMM, but I need it with a serial RS232 line
  

  • Mastech MS8218 (battery powered)
  • Mastech M9803R (230VAC powered)
  • Brymen BM857 (battery powered)

  look interesting, but they are a bit expensive, ~ 180 .. 250 euro (the Brymen BM857 require a RS232 adapter, so I have already included the 60 euro of cost in the 250 euro)
  

  
  The Brymen BM857 is definitely one of the most awesome meters available, for reasons already given in other threads.
  
  Your pricing seems off though, a BM857 is 150 Euros and the RS232 adapter is only 40 Euros, taxes included.
  
  https://brymen.eu/shop/bm857s/
  
  If you buy one from Welectron you can probably get free shipping, too.
  
  The only spanner in the works might be Brexit.
  
• #2 Reply
  Posted by DiTBho on 14 Apr, 2021 16:41
• Quote from: Fungus on 14 Apr, 2021 16:38

  Your pricing seems off though, a BM857 is 150 Euros and the RS232 adapter is only 40 Euros, taxes included.
  

  
  

  Quote

  Hallo,
  Tatsächlich können wir es sogar noch günstiger anbieten - für 60 EUR. Zusammen werden es 250 EUR sein. Multimeter + Zubehör-Kit BRUA-85XA wie auf dem Bild, das dieser Nachricht beigefügt ist. Lassen Sie mich einfach wissen, wenn Sie an einem solchen Geschäft interessiert sind.
  

  
  Translated:
  
  

  Quote

  Hello,
  In fact, we can even offer it even cheaper - for 60 EUR. Together it will be 250 EUR. Multimeter + accessory kit brua-85xa as shown in the image attached to this message. Just let me know if you are interested in such a business.
  

  
  This is what a local seller replied me.
• #3 Reply
  Posted by DiTBho on 14 Apr, 2021 16:51
• So it's 190 euro including the optical RS232 cable if bought directly from the Brymen's shop!
  Thanks a lot for that great news 
• #4 Reply
  Posted by msquared on 14 Apr, 2021 17:24
• Have you considered the Owon XDM1041?
  
  It's not officially released yet but it has RS232, AC powered, better specs than the Mastech and can currently be had for ~130EU on fleabay.
  
  It's being discussed here:
  https://www.eevblog.com/forum/testgear/owon-xdm1041-the-unknown-multimeter/
  
  
• #5 Reply
  Posted by Fungus on 14 Apr, 2021 17:41
• Quote from: DiTBho on 14 Apr, 2021 16:51

  So it's 190 euro including the optical RS232 cable if bought directly from the Brymen's shop!
  Thanks a lot for that great news 
  

  
  The welectron "Plus Pack" is a bargain, too. You get a case and gold leads for a bit more than the cost of postage on the Brymen site:
  
  https://www.welectron.com/Brymen-BM857s-Multimeter_1
  
• #6 Reply
  Posted by Kleinstein on 14 Apr, 2021 20:36
• Some of the meters have quite a good price / performance. One may be able to build one cheaper, but this usually comes with some limitiations, like less protection, no / poor AC performance and no nice case. With a nice case and so on, a DIY solution is likely not  that good in price / performance. The professional DMMs usually come with more features, even if one does not really need them.  A DIY solution may be interresting if one has special needs, like relatively high speed (the cheap meters tend to be relatively slow), very high impedance (the cheap meters are usually 10 MOhms) or a special range like sub µV resolution (here a used HP3478 may be an option if GPIB is OK).
  
  If one does not need lowest noise, the input protection can be mainly via a higher value resistor and some clamps.
• #7 Reply
  Posted by DiTBho on 14 Apr, 2021 21:11
• Quote from: Kleinstein on 14 Apr, 2021 20:36

  If one does not need lowest noise, the input protection can be mainly via a higher value resistor and some clamps.
  

  
  Great idea! I saw a similar trick in a kit sold by MikroE. A kind of "low cost" ADC evaluation board with a minimal input extra protection.