An Engineer in Wonderland – DVMs from Santa’s workshop

I like to make at least one present for someone at Christmas.

littledvmpic.jpgI 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.

little-dvm.jpgI 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.


Should you feel the need, respond to with ‘Little DVM’ in the title.

Please don’t respond below as our spam blocking system doesn’t work and the inbox is overwhelmed by all kinds of generous offers from all kinds of rather annoying people.

No email addresses are collected for marketing (or any other) purposes from responses to this blog.



  1. Hi David
    Type ‘ mini digital volt meter ‘

  2. Hi Alice,
    Like the little DVM project. Do you have info. on the supplier of the self-powered modules? I have a couple of ideas…..

Leave a Reply

Your email address will not be published. Required fields are marked *