I use large bypass/decoupling caps(10-470uf) across power lines and ICs and I never place a current limiting resistor in series with them. I was wondering if over time, that would damage the device because of the huge current demands from charging (especially if the power is being turned on and off several times an hour)?

Also, are bleed resistors recommended in designs where several large (10-470uf) caps are used and where power is being turned on and off several times an hour?


2 Answers 2


It's a design tradeoff like any other, and the fact that you do it one way does not mean that's the only way it's done.

As a decoupling/bypass cap, a resistor in line affects the performance of the capacitor - changing it from a C with some parasitic and inbuilt L&R (generally something one seeks to minimize) to one with considerably more external R as well, and it acts "more like a filter" or "like a filter at a different frequency."

Complexity and expense no object, it's possible to have a turn-on circuit that charges slowly and then switches to a low-resistance path. But it's complexity and expense and parts count and more things to go wrong, too. NTC thermistors are sometimes used as a crude (IMHO) approach to this ideal.

Bleed resistors MOSTLY have to do with systems that have voltages large enough to harm people who might work on them, though systems with lots of capacitance at low voltage can make alarming sparks even if they lack sufficient voltage to be an electrocution hazard. Other than a small amount (if appropriately chosen) of power wasteage, there's not much against them (but they are one more part.) Again, you could add more complexity and parts and switch them in when shutdown (only.) That's pretty rare in practice. On the other hand, if there's a "power-on" LED (and resistor) there's no need for ANOTHER bleed resistor.


Indeed, managing capacitor inrush current is a common challenge in electronic systems. Excessive inrush current can collapse supplies having high output impedance, trip fuses, cause output overshoot in regulators, and damage sensitive in-line components (usually semiconductors).

Personally, I've not heard of in-rush currents reducing the reliability of the capacitors themselves. Capacitors tend to be sensitive to over-voltage stresses, and suffer reduced reliability when exposed to chronic voltage stress. Current stress in a capacitor, on the other hand, is usually only a problem inasmuch as it generates heat. As long as the transient internal temperature rise during your hot-plug event is not enough to melt part of the capacitor, you are unlikely to observe an appreciable reduction in lifetime when turning on and off several times a day. You can find models for capacitor reliability that show the effect of chronic high temperatures (see here for example), estimate the temperature rise associated with your inrush event, and adjust for the duty cycle of your application (on the order of 2*1ms/day) to get a quantitative feel.

One application where in-rush control is important is server backplanes, where line cards are hot-plugged in to planes that are powering other, active loads. Not collapsing the source is key in this situation. There are IC products called "hot-swap controllers" that exist primarily to manage in-rush in this situation. You may glean some more insight by reading about hot-plug controllers.

As for bleed off resistors, they are only used in certain designs, especially where intrinsic safety is a concern or where the pins of the large capacitor may be exposed to other electronic systems (i.e. plug-and-play type situations).


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