Input to transistor turning on when connected to unpowered source. - Page 1
Forum section: electronics / beginners

author: peckett
author_rank: Newbie
timestamp: August 25, 2022, 04:56:50 am
content: HelloI have to electronic circuit boards, on acts as a controller and the other as a display panel.I want to disable the buffer IC on the display panel until the controller is connected and powered.I added a simple transistor switch to the enable pin of the buffer so it is pulled high unless the 3V3 from the controller is connected through a cable in which case the transistor will turn on and enable the buffer.When I power the display panel and connect the data cable from the display panel to the controller I get about 1.5V on the input of the transistor even though the controller is unpowered and I cannot work out where it is coming from.I cut the 3V3 track going from my switching regulator to the associated pin on the connector of the controller and the input signal of the transistor went low as expected.Please can someone help me understand where this mysterious 1.5V signal is coming from? I even tried connecting the transistor input signal to another logic IC and got the same result.I also tried adding pulldown resistor to input of transistor but still it is getting pulled to 1.5V from somewhere.

author: capt bullshot
author_rank: Super Contributor
timestamp: August 25, 2022, 08:02:55 am
content: Guessing:You've got some lines called xxx_Ret / MISO from the display to the controller. If these are outputs at the display side and inputs at the controller side, and by some chance are at "high" level when the display is powered but not the controller, you get a reverse power flow through these lines.These lines at "high" level can inject power into the controller circuitry through the usual protection diodes within the ICs used at the controller side. This brings up the VCC of the controller to any value from zero to about nominal VCC - 0.5V. Often the controller doesn't operate correctly or not at all in this state, but puts out undefined voltages at other pins (like your enable pin).To solve this choose one or more of these hints- take care all outputs from display to controller are at "low" level if the controller isn't powered. This is always recommended, because such leakage current from "high" outputs to unpowered circuitry can cause all kinds of weird effects- put a low enough value pull down resistor at the enable input. Might work as a simple workaround.- use a resistor from base to emitter of this transistor to create a voltage divider that turns on the transistor e.g. above 2V. This is always good practice, otherwise the transistor could be turned on by very small leakage currents or stray voltages.

author: peckett
author_rank: Newbie
timestamp: August 26, 2022, 12:34:40 am
content: That is a good guess, I did not know that was possible. Good to know!So I have added 100R pull down resistors from the outputs of the display panels on the MISO pins and that fixes that problem.Now when the controller is powered and then if I pull the power out the pull down resistors fail to do their job and the circuit acts as before generating the 1.5V.I have now also added the base emitter resistor so that the base voltage is about 1V when powered by 3V3 when the controller is powered so when the 1.5V signal is present the base voltage is only at ~0.4V and the transistor does not turn on.Not sure what you meant by the pull down resistor on the enable pin? The display is enabled active low so needs to be pulled high.Thanks for your help

author: capt bullshot
author_rank: Super Contributor
timestamp: August 26, 2022, 07:32:03 am
content: Quote from: peckett on August 26, 2022, 12:34:40 amI meant connecting this resistor to pin 8 of your connector, not realizing it was just connected to 3.3V at the controller side. So basically the same as you did.