Modified car adaptor

I've found these car adaptors useful:-


and I've modified one with a voltmeter on one side and an ammeter on the other.



I used a couple of small voltmeter modules from Ebay. https://www.ebay.co.uk/itm/Mini-Vol...-0-28-DC-0-100V/263849308282?var=563212566097

To measure the current, I used one these:- https://uk.farnell.com/ohmite/lvk25r005fer/current-sense-res-0r005-1-2-w/dp/1462326 to sense the current and one of these https://uk.farnell.com/texas-instruments/ina199a3dckr/current-sense-amplifier-14khz/dp/3118145 to amplify the voltage. The output of the amplifier connects directly to the voltage input of the current meter.

Here is the circuit:-

Update to this.

When I built it, I had used the wrong shunt resistor, a 50 mΩ one. https://uk.farnell.com/ohmite/lvk24r050der/current-sense-res-0r05-0-5-1-w/dp/1462321 and I had improvised an additional shunt to get close to 5 mΩ.

I then got the polarity wrong and blew the IC. The displays contain diodes already.

The 5 mΩ resistor arrived today and fitted it. It turns out that the 5 mΩ resistor is a 1224 resistor, while the 50 mΩ resistor is 2412, so one has the current flow along the long axis, and in the other the current flow along the short axis. As they are both four-terminal resistors, it wasn't as obvious to me as it should have been, and after I fitted it, I found it didn't work. I had also added a diode in to protect the INA199A3DCKR.

The new layout of the 5 mΩ resistor means that I'll have to make a new board. I used veroboard.

I'll post again when it works.

Well that's finished and working.

The circuit has been modified slightly. There's a diode feeding the INA199A3DCKR and a 10 nF capacitor between it's positive and negative supplies. The amplifier and the two displays are fed from the input, so it's only the output current that is measured.

The green current display is far dimmer than the red voltage display.

The meters are auto-ranging, and will read up to 100 V, and will move the decimal point so they can read 9.99 and then 10.0 but I think that the A2D converter in them is only around 10 bits. Its resolution is around 0.1 V, making the last digit of current a bit useless. It's certainly not possible to get it to read 0.01 so there is no way that it has a resolution of 10 mA.

That's not such a problem with the voltage display, as the whole thing won't work below about 5 V, and it's nearly always going to be over 10 V so the display resolution will only ever be 0.1 V