preventing "phantom power" from powering my circuit

I looked for help in another website but couldn't find an answer, so I will post here.

After making failed circuits in the past, I learned that the cause is that the parallel port was producing "phantom power" (aka powering the circuit). This is because I plugged my circuit into the parallel port before adding the external 5VDC to it.

Based on other ideas, My circuit idea is this:

On power-up, the 555 timer makes the output disable a 3-state buffer for a short period of time so that all outputs are high-impedance. (I forgot pull-up resistors on the buffer outputs). Shortly after, the left-most buffers are always enabled and then data from the port can communicate bidirectionally with the micro-controller. (data returns as parallel port statuses).

The micro-controller is on a separate board and the thick lines represent ribbon cable. VCC is 5VDC and both VCC and GND are connected to the separate board through the same ribbon cable as well.

My question is, will this circuit work well for two lines of bi-directional data between the parallel port and the micro-controller without having any phantom power? (I'll be plugging the circuit into the parallel port first before VCC is connected to any power).


So then after I thought of replacing all those buffers with opto-couplers (2 of 4N25's) with cathode of the input internal diode and emitter of internal NPN in the opto to ground. and anode to data lines from the port. Here's my new circuit:


Am I on the right track with the optocoupler idea or is there something else I need to add? The MSB lines are the DI, DI, and ICLK lines and each one is connected to a 47K pull-up resistor.

The opto isolators should work.

The issue with phantom power is that many ICs won't let the input voltages go much above the supply rails. Then a high input will pull the power rails up.

Some ICs have different input protection, and the input voltage can be above the supply voltage, and then the input circuit can't power the IC. If you look at the maximum input voltage range, that will tell you which applies.

For instance, a 74AHCT125 from Diodes Inc. has these figures:-
VCC Supply Voltage Range -0.5 to +7.0 V
VI Input Voltage Range -0.5 to +7.0 V
(from https://www.diodes.com/assets/Datasheets/74AHCT125.pdf)

so that IC won't let the inputs power the IC.

On the other hand, a 74HCT125 from Nexperia says:-
VCC supply voltage -0.5 +7 V
IIK input clamping current VI < -0.5 V or VI > VCC + 0.5 V 20 mA
(from https://assets.nexperia.com/documents/data-sheet/74HC_HCT125.pdf)

Now on that IC, the input can't be more than 0.5 V greater than the supply, so a large input voltage will pull up the supply.

The danger there is that the circuit behaviour can depend on which make of IC is used, even if they are the same number.

so I could just rip out my 74HC125 and use a 74AHCT125 instead? is there another catch?

and why doesn't the AHCT datasheet have a VI > x condition like the HC one does for clamping current?

And I just noticed the local store I normally visit does not carry this chip in the 74AC/74ACT versions. Only the 74HC/74HCT versions. I'll check another local store and if that fails, I'll go the opto-coupler route. Would 4n25 be perfect or am I better to use 4n35? The lines marked MSB-1 MSB-2 MSB-3 and MSB are bi-directional and the values are either always ground or high impedance.

I'll just leave this file right here, courtesy of Mouser Electronics and Diodes Inc.
Warning though: these tiny little parts cost a whole 10 and a half cents (quantity of 100).

These things worked like a champ on my latest board design. Bidirectional, handles two Vcc/Vdd supplies, works from 1.65 up to 5.5 volts, 24mA drive, and probably faster than any opto isolator I've run across.

Well, I'm making this circuit for data testing purposes at low speeds (around 100bps) so speed to me isn't an issue. I just would rather use my computer and my circuit to test my microcontroller circuit than to do millions of manual key presses. (it drives me nuts when I have to become a manual serial port in a circuit)

I recommend taking a look. At about 25...30 cents in small quantities, it doesn't require any external components. OK, you can put a 0.1μF on the supply if you want. All you need is a high signal or +V from the connectors on each side. (It needs to see each +V supply so it can tell if it is on or not.)

Thanks but I'm going for parts that are locally available to me at this time

mik3ca said:

so I could just rip out my 74HC125 and use a 74AHCT125 instead? is there another catch?

and why doesn't the AHCT datasheet have a VI > x condition like the HC one does for clamping current?

Click to expand...

The AHCT device has input clamping that uses a zener diode. The HC device clamps using normal diodes to the +ve supply. So the AHCT device can have an input voltage at 5 V with the supply at 0 V, but that's not possible with the HC device. The specification for the AHCT has the recommended maximum input voltage as 5 V, while the HC has the recommended maximum input voltage as Vcc.

You can just change devices as long as you check that datasheet for the brand of device you use has an input range that doesn't quote Vcc, but has a number of volts.