| Electronics > Repair |
| Why do I keep blowing scope channels, when measuring ripple?! |
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| Geoff-AU:
--- Quote from: Wallace Gasiewicz on June 30, 2024, 10:33:46 pm ---Never, Never, Never use 1X probe. throw it away. Use 10X. --- End quote --- Mantra like this isn't helpful without giving the "why". rwgast has watched some Dave videos suggesting to use 1x but you say to use 10x. Confusion results. 10x probes burden the circuit less, and they have lower capacitance which allows higher frequency operation. 1x probes do not have the full penalty of the oscilloscope's input amplifier noise to contend with, so they are better for low signal levels. Don't forget that probes have a voltage vs frequency derating as well. --- Quote from: rwgast_lowlevellogicdesin on June 30, 2024, 06:35:09 pm ---I looked away for a second and then looked back and my trace was gone.... --- End quote --- --- Quote from: rwgast_lowlevellogicdesin on July 01, 2024, 03:16:02 am ---I was just pulling the load resistor out by hand, guess thats a bad idea. --- End quote --- Ahh.. while looking away did you happen to pull the load resistor? Now we are getting closer to the real sequence of events :) 10uH is a fairly small inductance, but 1500uH is a fair bit of energy storage. Inductors can generate quite impressive voltages when you open-circuit them. In this case, you had 700mA flowing through the inductor and then disconnected the load. The inductor will generate a substantial voltage across itself in order to keep 700mA flowing, and once it reaches a high voltage the only place left to dump that energy is into your probe. So your scope probably copped >1kV and popped the front end. You could avoid the voltage spike by putting a flyback diode across the inductor (very common practice for suppressing relay coil transients). Generally for power supply filtering it's safer to use capacitors to do the bulk of the filtering work and use smaller inductor values to avoid voltage spikes like this. Have a capacitor as the last filter element, as well, and it will improve voltage regulation of the output. |
| rwgast_lowlevellogicdesin:
What is a safe value for inductors? About how big of a cap should i parallel after the inductor, to make sure and surpress transients? I usually just model a filter in elsie and go with it, can programs like lt spice and microcap model the transients when the inductors magnetic field collapses? Lastly would this have happened in 10x mode? |
| Wallace Gasiewicz:
I think they are correct about the inductor, If this is the case and you used a 10X probe I believe that you would blow the probe and it is possible the scope would not be damaged. Passive Probes are quite inexpensive these days and usually fixable. A Gas Discharge tube or a MOV inside the PS might prevent this but maybe they would not be fast enough, lightening arrestors do 'fire" but the equipment is not always saved. Any addition to the probe changes capacitance but I just measures a 150 V gas discharge and it measured ONE pF at one MHz. MOV have higher capacitance. HP used gas discharge on some of their different inputs, like the 3456. But I do not know of any of these devices used on scopes. I do not think fuses are fast enough. Perhaps someone else knows if these devices are fast enough under these circumstances. |
| IanB:
Surely a good piece of advice is do not work on a device under test while the scope is connected to it? Detach the scope, make the desired alterations, and then reattach the scope probes. For that matter, do not try to modify a device while it is powered up. There is always the risk of a slip of the fingers and an inadvertent short circuit that can damage things. |
| tggzzz:
--- Quote from: rwgast_lowlevellogicdesin on July 01, 2024, 11:10:07 am ---What is a safe value for inductors? About how big of a cap should i parallel after the inductor, to make sure and surpress transients? I usually just model a filter in elsie and go with it, can programs like lt spice and microcap model the transients when the inductors magnetic field collapses? Lastly would this have happened in 10x mode? --- End quote --- Vinduced = Ldi/dt. So work out the current change in the time interval. Spice will correctly calculate the transients in model circuit you specify. That should give you a general feel for what's happening. The issue is that you have to correctly specify all components in the model; that is non-trivial. Knowing what is necessary and sufficient requires skill, judgement and experience. You do realise that L+C=>resonant circuit - don't you? |
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