Identifying transients

Thread Starter

k1ng 1337

Joined Sep 11, 2020
539
Hi,

While browsing a datasheet I came across this excerpt which compounds some difficulty I've been having understanding transients. Allow me a moment to attempt to explain my conundrum:

LM317 National Semiconductor: "When external capacitors are used with any IC regulator it is sometimes necessary to add protection diodes to prevent the capacitors from discharging through low current points into the regulator. Most 10 µF capacitors have low enough internal series resistance to deliver 20A spikes when shorted. Although the surge is short, there is enough energy to damage parts of the IC. When an output capacitor is connected to a regulator and the input is shorted, the output capacitor will discharge into the output of the regulator. The discharge current depends on the value of the capacitor, the output voltage of the regulator, and the rate of decrease of VIN. In the LM117, this discharge path is through a large junction that is able to sustain 15A surge with no problem. This is not true of other types of positive regulators. For output capacitors of 25 µF or less, there is no need to use diodes. The bypass capacitor on the adjustment terminal can discharge through a low current junction. Discharge occurs when either the input, or the output, is shorted. Internal to the LM117 is a 50Ω resistor which limits the peak discharge current. No protection is needed for output voltages of 25V or less and 10 µF capacitance. Figure 3 shows an LM117 with protection diodes included for use with outputs greater than 25V and high values of output capacitance".

The above is fairly straightforward but it got me thinking: At which point do I implement measures to prevent large current spikes to a device and how do I identify and calculate the existence / possibility of transients with basic test equipment and datasheets?

For example, I have a Raspberry Pi 4, Pi Pico (ADC) and several low power modules. Currently the situation is the modules are powered by the 3.3V pin of the Pico which is powered by the USB port of the Pi 4. Is it wise to provide each device (or groups of devices) with 1) Protection diodes 2) Own power source (LM317?) 3) Own power supply as there are capacitors all over the place?

Any thoughts on the matter are appreciated.

Regards,
Mark
 

ericgibbs

Joined Jan 29, 2010
15,526
hi Mark,
When external capacitors are used with any IC regulator it is sometimes necessary to add protection diodes to prevent the capacitors from discharging through low current points into the regulator. Most 10 µF capacitors have low enough internal series resistance to deliver 20A spikes, back into the LM317, when shorted.

The reverse diode across the LM317 is to protect the LM317 from a reverse voltage/current.

Consider the LM317 was set for 15V and had capacitors on the output for smoothing.
If for some reason the input voltage to the LM317 was removed, ie: switch Off , shorted, out, etc.
The LM317 internal circuitry would be reverse biassed at 15V, due to the charge on the output capacitors and a current spike could flow back into the LM317.
If in doubt, just add a 1N4003 in reverse across the LM317.

E
Check Note 'E' in this image.
EG 1544.gif
 
Last edited:

Thread Starter

k1ng 1337

Joined Sep 11, 2020
539
hi Mark,
When external capacitors are used with any IC regulator it is sometimes necessary to add protection diodes to prevent the capacitors from discharging through low current points into the regulator. Most 10 µF capacitors have low enough internal series resistance to deliver 20A spikes, back into the LM317, when shorted.

The reverse diode across the LM317 is to protect the LM317 from a reverse voltage/current.

Consider the LM317 was set for 15V and had capacitors on the output for smoothing.
If for some reason the input voltage to the LM317 was removed, ie: switch Off , shorted, out, etc.
The LM317 internal circuitry would be reverse biassed at 15V, due to the charge on the output capacitors and a current spike could flow back into the LM317.
If in doubt, just add a 1N4003 in reverse across the LM317.

E
Check Note 'E' in this image.
View attachment 264325
Thanks for the reply though you just repeated what was already made clear in the datasheet. I want to know what parameters to look for when deciding if a device should have protection diodes or it's own power source hence my example. I believe it boils down to input and output impedances though I'm not sure how to calculate them when datasheets only provide block diagrams etc.. Looks like this is going to be another problem I'll have to hash out myself as the algebra presents itself. In the meantime I'll throw diodes all over the place and suffer their side effects to my circuits.
 
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