An Engineer in Wonderland – DVMs from Santa’s workshop
I like to make at least one present for someone at Christmas.
This time it was little pocket DVMs for those gathered around the Christmas tree that maintain their own cars and motorbikes.
A little time consuming, but the result was three happy relatives.
Inside each is a little self-powered DVM module from China.
They are pretty accurate from 3.3V to 30V.
Maximum forward voltage is 30V, and no reverse vollage is allowed.
30V max is OK for car use – although 60V would have been better – but the reverse bias restriction is obviously a no no, so part of the build was to add reverse protection.
I chose the classic mosfet-run-backwards circuit adding a 10V zener to protect the gate from too many volts, with the whole lot soldered straight to the module.
Current draw was a pretty steady – regardless of number displayed – at 10mA, rising to 20mA at 30V.
So the rather over-specified 55amp STMicroelectronics mosfets I happened to have lying around resulted in a reading error of a huge 20µV, if my calculations are correct.
33x35mm boxes came from Maplin, and I replaced the top with red Perspex to make a contrast-enhancing window.
The artistically angled display was unavoidable as I would have had to cut bits of the module to get them straight.
A thick layer of stick-back foam in the back of the box pushes the module against the window to hold everything steady, and means I have a lot of 2mm screws and some spare holes lying around.
Who’s a clever Alice
BTW, the whole experience made me think that a simple series diode could be used for protection, providing current draw was constant, and the module added the extra 0.6V (or whatever) voltage loss before it displayed a result.
As I remember, there is an extraordinarily clever LM10 circuit that could wrap around a microcontroller-based DVM to make its overall current consumption constant in this application, somewhere in this data sheet.
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