Digital Ammeter 

This is a re-purpose of the box which housed an accessory shunt which was made obsolete by a re-modeling of the device it was an accessory of.

This device fills a need for an ammeter with galvanic isolation and wide application. By virtue of the excellently low offset voltage of the Op-Amp OP-07 it has in insertion drop of only 50 millivolts.

An effort was made to minimize battery drain, especially when off. The LCD display therefore has no backlight and is the minimum size to display the current being measured. 

  To simplify the design, a micro-controller was selected which has a built-in socket for a lithium rechargeable battery. It has separate regulators for the USB input and this battery, and selects the source with the higher voltage. It also has a charger for the lithium battery. Unfortunately it does not have a 5V output and the display needs at least 4.8V. Since the display current is only 6 MA I boosted the 3.3V output of the micro-controller with a 1.5V alkaline cell. The lithium battery uses 18 MA.

To simplify operation, power is controlled by grounding the micro-controller’s enable line at one position of the switch. This drops the lithium battery drain to 75 micro-amperes. Unfortunately, the display current only drops to 1 milliamp. This required inserting the “Auto-Shutoff” circuit.

One position shows the voltages of both batteries. The remaining 10 select current ranges in a 5;15 sequence from 50 micro-amps to 5 amperes.

The display is externally mounted and may be rotated to provide optimum contrast.