Source
r/AskElectronics•
itzkold

Resistors in RC filter keep dying

Troubleshooting

I built this RC filter off of a design from a thread on the diyaudio forums. I had been using it in a different circuit for several months without problem - albeit in a slightly different configuration, with an extra set of capacitors and resistors, and with 2200uF caps in lieu of the 4700uF (didn't have any 2200uFs in =>35v).

I had some extra boards so I decided to pop the filter into my "ghetto bench supply" and after a about a month my first 470mOhm 2W resistor died. It failed open, no visible damage.

It took me quite a while to figure out where the fault was, and I didn't think much of it. So I replaced the dead resistor and the replacement died almost immediately after. I don't think I even had a load connected. I was using these Chinese foil resistors (2512 package 2W) and figured they were bad.

I went back to the 1W 2512 Ralec resistors that I've used for months previously without issue, and after 2-3 weeks or so, one just combusted in front of me at ~500mA (China ammeter probably not very reliable) powering one of my shitty circuits. There is a 2A polyfuse after the RC filter that wasn't even warm as I held it while the resistor was smoldering, so I didn't think there was a short at the load.

The first 2 Walter resistors that died were on the low side, and the Ralec that died last was on the high side.

What am I (indubitably) doing wrong here?

Posted Oct 12, 2018, 11:14 PM UTC

135
piezeppelin•

You might want to measure what your inrush currents are when you apply power to the board.

3
itzkoldOP•

good idea

1
itzkoldOP•

highest i've been able to catch it is around 1.2A

no resistors died in the exercise

discharging caps gets annoying pretty quickly

1
InductorMan•

With a multimeter though? These inrushes would be measured in tens of milliseconds. Your meter just couldn't see it.

2
itzkoldOP•

How else?

1
Pocok5•

Two channel oscilloscope.

2
InductorMan•

You'd need an oscilloscope. Or else a very nice meter, but I don't think even my nice meter can sample that fast. Alternatively if you thought the peak was above 1A you could attach a Schottky diode in series with a small (1uF) capacitor to the resistor, and attach your meter across the capacitor. This will "catch" the peak voltage (minus the diode voltage, which will be 0.2-0.6V depending on the speed of the pulse and the consequent current that flows into the cap).

2
coneross•

Measure the voltage across your resistor with your meter in DC volts.

Measure again with your meter in AC volts.

Report back.

2
itzkoldOP•

.007vdc high side resistor only

all other readings ac/dc low/high are 0

dmm is "true rms"

1
coneross•

Looks like you are OK on steady state. Look at inrush like u/piezeppelin suggested.

3
itzkoldOP•

https://www.reddit.com/r/AskElectronics/comments/9np56j/resistors_in_rc_filter_keep_dying/e7oeh8b/

1
sideways_blow_bang•

The .47 ohm resistors need to be large cement type power resistors(or equivalent) if any amount of current is to be drawn.

2
ltonto•

What's your resistor mounting setup look like? SMD resistors required the PCB pads act as heatsinks, and their rated power would only be achieved with each SMD pad being an infinitely large copper plane on the PCB.

If resistors are basically free-floating in the air, reckon on 0.25W or so for 2512 package, if that.

Through-hole components will meet their rated power handling in free air, though.

2
itzkoldOP•

it's a rather small board - no bigger than it needs to be - but there's a decent amount of 2oz copper and the traces are as large as they could be, think ~2mm

1
ltonto•

What's your load? A DC average of 500mA night week be an RMS value of much higher. A 2.0A pulse at 25% duty is 0.5A average, but 1.0A RMS.

0.5A RMS is only 0.125W through a 0.5R resistor, so even if heatsinking was inadequate, that's just not much power. There's always the finger test: can you hold your finger on the resistors for an appreciable amount of time?

2
itzkoldOP•

well i've been using it as a bench supply for my shitty projects so it varies. i can't recall what the load was like when the first 2 resistors failed for certain, but i'm pretty sure the second one just died with no load. i was thinking it could be the capacitors charging or discharging or something - i have no idea.

normally the resistors are cool. in my other use case for the filter (with 2 sets of resistors and 3 2200uF caps) i have it hooked up to a 150mA constant current load and they do not even get warm, ergo i thought i could use it in the "ghetto bench supply"

1
Enlightenment777•

  1. Are the resistors popping when you first turn on the power? If yes, then the initial inrush current surge might be the cause.

  2. Possibly change R1 & R2 to a lower value, such as 0.2 then see what happens, if pop again then try 0.1

  3. Possibly add a low-resistance series resistor in front of C2 to lower it's charge up current demands at power-up.

2
itzkoldOP•

what do you mean by "popping"? they seem to have acted completely normally even in failure except the one time one of them caught fire

edit oh you mean failing, only second time i believe. definitely not the 3rd and not sure about the first time.

i apologize for the shitty schematic - i accidentally overwrote the design files for the revision that went to fab and just quickly redid it - but there's 2 C2s in the schematic in the OP

you mean like this?

1
Enlightenment777•

yes, that R3 is what I mean, but also it might not be enough, because maybe the load on right side is the cause of the R1/R2 resistor destruction, in which case you might want to lower the resistance of R1/R2.

I'm not sure if any of this will fix your problem, just throwing out some ideas that I would consider.

2
formervoater2•

Even if your resistor is rated for the power, you still need to get the heat out of it or it just builds up and blows open. Either use heatsinking or use through hole resistors with a bigger package that have the surface area to dissipate the heat.

1