# Can I use a resistor before a bridge rectifier to lower inrush current?

I have been looking for ways to limit inrush current in a power supply circuit I am designing. In LTspice, with no current limiting, I see a spike of around 24A before my caps are charged.

Looking around, I see NTC inrush current limiters as a common solution as well as some ideas with inductors and resistors inline with the capacitor, each with its own advantages and disadvantages.

My idea though was to put a resistor before the bridge rectifier in my circuit, so the resistor would always provide some resistance before the capacitors got charged. Playing around again in LTspice, this drops my inrush current to about 4A (and changes with the resistor value obviously).

Is this ever done? Are there any major good or bad reasons to do this? I suspect no one does this due to decreased power efficiency, but is that the only reason?

• en.wikipedia.org/wiki/Inrush_current_limiter This article may be of some small help. Oct 27, 2015 at 1:41
• In common cases, the inrush current isn't severe enough to warrant mitigation. As long as you're not using oversized capacitors, your components will handle the inrush well enough. Oct 27, 2015 at 5:28
• How does your simulation look like, does it contain a realistic enough model of a transformer (I am assuming you recitify some transformer output)? If you just rectify after some AC voltage source, then you get unrealistic inrush values. In a lot of cases the DC resistance and the voltage drop due to excessive current is enough to reasonably limit the inrush into your smoothing caps. Oct 27, 2015 at 9:07
• What is the Issc (single cycle surge current) rating of your rectifier? Studying the datasheet may show it's already over 24A. If not, upgrading the rectifier may be the most cost effective (and efficient) solution.
– user16324
Oct 27, 2015 at 11:49
• An important point that I've not noticed being made is that an iron core transforme feeding a capacitor will produce very short large I spikes as Vmains rises above Vcap and this can cause bad EMI . Adding even quite a small series resistor spreads and rounds the peaks. Oct 27, 2015 at 13:56

There are good and complicated solutions, relay, FET, and simpler, thermistor, but it still may be possible to use the simplest, fixed resistor, without too much dissipation for your application.

If you get an inrush, does it matter? Well, only if it breaks something. So what could break? Supply fuse, transformer, or diode rectifiers.

Fuses have 'T' rated versions that take a long time to blow, for just this application. The transformer is a heavy lump of copper, that's not going to fail.

Read the specification of your diodes carefully. You may be surprised at how much the 'single cycle surge current' is. In a 1N40xx (cheapo workhorse mains diode), the continuous current is 1A, the surge is 30A. For 1N54xx series, the figures are 3A and 200A. This is specifically to allow them to survive inrush. You may find you already have enough stray resistance in your circuit to limit the current to the safe surge value. If not, you maybe won't need much more, and it still may give you acceptable efficiency. If it doesn't, then try smarter solutions.

• And can get very "smart" and very expensive: electronicdesign.com/power/… Oct 27, 2015 at 6:19
• Add to list of "what could break?": anything on the same AC phase that wants a sinusoidal input. Inrush current can be responsible for ugly harmonics. Oct 27, 2015 at 13:05

Using a resistive element to limit inrush current is done.

However, placing a permanent resistor in the AC (or DC ) lines is very inefficient as it is always in-circuit.

The use of a Negative temperature coefficient (NTC) thermistors is often done instead as it provides a level of resistance during the precharge event that is then reduced due to heating. This however does impose a "cooldown" period to facilitate the NTC returning to room temperature resistance in the event of a power loss or power cycle.

An established method is to use a form of soft-start

http://www.ti.com/lit/an/slva156/slva156.pdf

simulate this circuit – Schematic created using CircuitLab

There is equally monolithic chips that provide the same functionality.