555 properties when Vcc open - Page 1
Forum section: electronics / projects

author: psteichen
author_rank: Contributor
timestamp: September 04, 2017, 03:24:28 pm
content: Hi folks. I've been having difficulty with a design. Power keeps leaking out of caps by unknown means. I've eliminated all other sources, so it has to be the 555s. But I can't find technical data anywhere that describes the pin properties of the 555 when Vcc is open (which is necessary at times in my design). Does anyone have knowledge or sources they could share? The schematic doesn't help. I can't visualize how all those internal transistors interact. I already did a basic check with a multimeter. No pins seem to have any direct continuity to any other pins with Vcc open, but do they for a few milliseconds? Do they drain to a certain voltage? Do they drain in response to voltage on another pin? Nobody seems to have documented these properties, and I really don't want to have to reinvent the wheel if I don't have to....I'm mostly interested in the Out, Discharge, Threshold, and Trigger pins in that order of importance.

author: Zero999
author_rank: Super Contributor
timestamp: September 04, 2017, 03:48:52 pm
content: It sounds like you're doing something you shouldn't.The 555's output isn't specified, when nothing is connected to VCC, because it's not designed to have a voltage connected to it, without VCC.If it's a CMOS 555, it's quite likely there are ESD protection diodes going from all pins, including the output to both power pins. Cathode to +V and anode to 0V of course, as they're not supposed to conduct, in normal operation.It's hardly surprising your meter doesn't measure any continuity, because the test voltage is usually under one diode drop: a few hundred mV is quite common for many meters. You could try the diode test function instead.Please post a schematic, showing what you're trying to do.

author: Ian.M
author_rank: Super Contributor
timestamp: September 04, 2017, 04:03:24 pm
content: Assuming a classic bipolar NE555 or clone. just about the only pin that you can be certain wont draw current with Vcc disconnected would be Discharge.Control is connected to a potential divider resistor ladder. Threshold is two diode drops + a resistor away from Gnd. Reset is one reverse B-E junction zenering + 2 diode drops away from Gnd. Trigger would need two reverse B-E junctions to zener then would be two diode drops and a resistor away from Gnd. The Output would be a resistor and one diode drop away from Vcc, then there are various paths to Gnd. Reset and Trigger will probably be damaged by applied voltages large enough for them to draw current with Vcc missing. They'd probably degrade gradually, drawing more input current in normal operation.The 555 is NOT suitable for applications where it will have voltages on its inputs with no supply - you will need to redesign to avoid that.

author: psteichen
author_rank: Contributor
timestamp: September 04, 2017, 06:30:10 pm
content: The recommendation to use the diode test function was a good one. It turns out that Out, Threshold, and Control all drain to ground limited by a Vf of around 2V.Vcc itself tests similarly, although a few mVs continue to flow even down to the test voltage of the multimeter.This result brings up another question. Is the 555 internally protected within its normal operating voltage range to prevent capacitor charges from frying the chip through the Threshold or Control pins when power is removed.Would the Out and Discharge pins also be internally protected to prevent exceeding the ~200 mA sink & source limits? I've been using resistors to limit current but maybe it's unnecessary?Sent from my SM-G900P using Tapatalk

author: psteichen
author_rank: Contributor
timestamp: September 04, 2017, 07:59:23 pm
content: To answer my own question, it appears that Threshold and Control are internally protected by resistors. But Out and Discharge are not. But I suck at schematics, so could be wrong...Sent from my SM-G900P using Tapatalk