I purchased 3 “Voltage Current Display, DROK DC 4.5-100V Digital Voltmeter Ammeter Multimeter Panel, 0-50A Volt Tester Meter Amp Detector, LED Voltage Amperage Monitor Gauge for Automotive Motor Battery” and I cannot get these meters to work. I have followed the instructions exactly, but the display seems to be unpowered even though I connected the V+ from my power supply to the middle lead. Please let me know how I can use this meter and turn the display on.