My project uses the serial 7-segment display with 5v connected to VCC and using the serial interface. After some initial data display I want to shut down the S7S to save power, so I am planning to connect VCC through an appropriate NPN transistor and pull its base low at the appropriate time.
I have my whole project connected through a multimeter so I can monitor power consumption and that’s when I noticed something strange. Under normal operation the project pulls about 24mA (with all S7S segments turned off). Then when I (manually) disconnect VCC from the S7S it jumps to about 33mA. If I also disconnect the RX line then power drops to about 14mA. On the other hand, if I start over and disconnect RX first then power jumps a bit to around 27mA and when I disconnect VCC it then drops to ~14mA.
So it seems that when VCC is disconnected but RX is not, the S7S is somehow drawing even more power through the serial interface.
What’s going on, and how can I fix it? I’d really like to get the project down into that 14mA range. I suppose I could use 2 transistors, one to control power to VCC and one to control “power” to RX but that seems really odd.
There is a diode on the chip for ESD protection, connecting RX to Vcc (anode to RX). When RX is powered and Vcc is open, this diode will power up the chip from RX, and therefore the LEDs as well.
You could also try pulling reset low and see if holding the processor in reset will reduce your power draw.
Thanks for the explanation and the suggestion! I found that if I pull reset low, power draw remains around 24mA but if I subsequently disconnect VCC then power drops to around 14mA like I want. If I reverse the steps (disconnect VCC then pull reset low) draw remains around 24mA. Still seems a bit strange to me but I can work with this. Thanks again.
This phenomenon is known as phantom or ghost powering, and should be avoided at all costs as it can cause lots of problems with the chip that is off. You can put a 10K resistor in series with high impedance input lines to keep the unwanted current flow to a safe level.
connect VCC through an appropriate NPN transistor and pull its base low at the appropriate time.
That won’t work well, if at all. Most people use a high side switch based on a P-MOSFET or PNP bipolar transistor, for example (except R2 should be 10K or higher):
Thanks all. I think I have it, and the solution was simpler than I thought. I’m using an ATTiny84 as a controller, so I just connected the S7S to one of the pins in OUTPUT mode (S7S draws 15mA at most so that’s cool). No transistor needed. When it comes time to power down I set the RX and VCC pins to INPUT (in that order) which sets both lines to high impedance and that does the trick.
