Strange startup waveform on 7805 vReg and spikes on 5V output. - Page 1
Forum section: electronics / general

author: Psi
author_rank: Super Contributor
timestamp: August 10, 2022, 01:52:57 am
content: I'm investigating some damaged MCUs and have located this really odd looking 7805 startup.The Yellow trace is 12V input (5V/div)The purple trace is the 5V output of the Vreg.Obviously the 6.7V spikes on the 5V rail are the issue.Just wondering if anyone has seen something similar and knows what the issue is.Note: Those 5V spikes were very dim in original pic.I edited the image so you could see them better. They are burst of around 50mhz and last around 250ns

author: Psi
author_rank: Super Contributor
timestamp: August 10, 2022, 02:11:17 am
content: Neverminded, the scope trolled me.Tried probing with the ground spring on the scope and the spikes are gone.

author: Psi
author_rank: Super Contributor
timestamp: August 10, 2022, 04:20:06 am
content: The 5V rail is now fine.However I'm not liking the 12V going into the 7805.Below scope image was measured right on 7805 INPUT and GND pins (with scope spring)The power to the 7805 is being connected with a toggle switch.7805 has 8uF of input capacitance approx 6mm from the pins.Some of those dips are possibly just switch bounce.But I'm not liking those spikes above 12V.Is that caused by the sharp edge of the switch combined with some inductance on the wires to the 7805?I think maybe I just need a high ESR cap on the input.The ceramics are probably adding too much low ESR capacitance?

author: Psi
author_rank: Super Contributor
timestamp: August 10, 2022, 04:56:50 am
content: yeah, just adding a 10uF ceramic with 1 ohm in series seems to remove the spikes quite well.

author: SeanB
author_rank: Super Contributor
timestamp: August 10, 2022, 07:47:09 am
content: Yes, that is why the older datasheets and application notes say to use a regular electrolytic on input and output, to damp down that ringing, or use solid tantalum. Series resistor with the capacitor, and likely also a series resistor in the power line, value and wattage so that the highest power draw and lowest input voltage means regulator input is 8V, and nearest lower value in the appropriate power rating, will also help, and also adds to needing a smaller heatsink, as all power is dropped in the resistor.

author: tom66
author_rank: Super Contributor
timestamp: August 10, 2022, 04:49:32 pm
content: I usually add a small TVS to inputs of systems like this where the input is very low impedance, to provide additional attenuation of this effect. Typically sized to break down before the regulator chip does!

author: David Hess
author_rank: Super Contributor
timestamp: August 11, 2022, 12:09:12 am
content: Quote from: Psi on August 10, 2022, 04:20:06 amYep.QuoteYep, yep.If a small input capacitance is used, then the inductance back to the source can cause spikes like that. The solution is to add some bulk capacitance, like a 47 or 100 microfarad aluminum electrolytic, close to the regulator's input terminal.I think Bob Pease discussed this or a very similar problem.

author: Psi
author_rank: Super Contributor
timestamp: August 11, 2022, 04:36:44 am
content: Anyone know what causes the step response?

author: David Hess
author_rank: Super Contributor
timestamp: August 11, 2022, 08:14:40 am
content: Quote from: Psi on August 11, 2022, 04:36:44 amPoorly implemented current limiting might do that.

author: Psi
author_rank: Super Contributor
timestamp: August 11, 2022, 01:45:49 pm
content: Quote from: David Hess on August 11, 2022, 08:14:40 amHmm, i was powering it from a GWinstek psu, will try a battery.