Long term effects of -0.6V on Electrolytic cap? - Page 1
Forum section: electronics / beginners

author: pelud
author_rank: Regular Contributor
timestamp: November 13, 2014, 05:56:39 pm
content: Hi all,I am working on a simple programmable power supply, and I noticed that when shutting down the output I sometimes get a negative voltage on one of the electrolytic caps. I've added a diode to protect it, but will having -0.6V from the drop of the diode still damage or lower the lifetime of the cap?Thanks,JohnEdit: clarified some grammar

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 13, 2014, 06:14:36 pm
content: What is the voltage rating of the cap?What is the circuit?What function is the cap?How much negative voltage was it experiencing?For how long? (was it only a turn on/off transient, or continuous)?

author: pelud
author_rank: Regular Contributor
timestamp: November 13, 2014, 06:24:42 pm
content: The cap is on the output of the power supply, and is 22uF / 50V. In one variation of the circuit it can be continuous.You can see the circuit here:https://www.eevblog.com/forum/projects/lm317-based-cvcc-lab-supply/msg534276/#msg534276The cap in question is "C_5" on the PSOC Creator schematic.I have an updated version that I can post later today, but the basic circuit is the same.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 13, 2014, 06:44:44 pm
content: Quote from: Richard Crowley on November 13, 2014, 06:14:36 pm

author: pelud
author_rank: Regular Contributor
timestamp: November 13, 2014, 06:54:17 pm
content: -0.6V, continuous

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 13, 2014, 08:10:17 pm
content: How much "reverse DC" were you measuring BEFORE the diode you added?How long did it last? Was it only brief during turn-on, or turn-off?It seems quite unlikely that your capacitor(s) need protective diodes.Most circuits see brief excursions of voltages during turn-on or turn-off that are harmless to the components.IMHO, you are trying to "prevent" something that is probably normal and was accounted for in the original design.

author: AG6QR
author_rank: Frequent Contributor
timestamp: November 13, 2014, 08:42:47 pm
content: You say the negative voltage happens when shutting down, but you also say the voltage is present continuously. Does that mean it's present continuously while the device is powered down? Or is it present continuously when powered up?If the problem is when the device is shut down, then putting a high value resistor across the cap will allow any charge to bleed off before it does harm. There's probably already enough leakage in components you already have, and/or 0.6V may not be enough to be concerned about, but if you're worried, add a resistor.Also, check the datasheet of the capacitor you're considering. The datasheets for aluminum electrolytics I've looked at generally say that a reverse voltage of less than 1V is nothing to worry about.

author: pelud
author_rank: Regular Contributor
timestamp: November 13, 2014, 09:26:53 pm
content: OK, there are actually two ways of reverse voltage on the cap. The long story is that in order to allow the output of the 317 to go to 0V, I added several diodes and a cap as a (poor) ~1.8V shunt regulator (D_3-D_5, C_4, see the link above for the schematic) which acts as a virtual ground for the return from the output. This way I can basically set the output voltage that the 317 sees to 1.8V to make the real output voltage 0. It additionally provides the benefit that it shifts the voltages into the common mode range of the op-amps.This also creates the first opportunity to create the negative voltage, in that if the ADJ pin is brought to ground, the output will go to about -0.5V via R_6. This can be continuous if something is set wrong in the software on the micro, such as one of the calibration constants.The second time is when the power is removed, C_4 still has 1.8V on it, which in turn discharges into C_5, which can temporarily put -1.8V on it. This is what I added the diode for, is to protect from the 1.8V. This will be transient, as C_5 will get discharged by the 100k voltage divider.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 13, 2014, 10:39:03 pm
content: 1) The voltage is a very small fraction of the rated capacitor voltage (whether positive or negative).2) It is only a transient (turn on/off) event, not a static operating condition.For those reasons, I wouldn't give it a second thought. Protection diodes seems way over the top.I would seriously consider generating a negative rail by a more straightforward method.We are operating in the dark here because we don't know the source of the raw supply voltage.If it is rectified from an AC source, you could probably make a low-current negative supply operated from the same raw source, etc. Just saying.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 13, 2014, 11:47:31 pm
content: Electrolytics are generally rated for something like 3V or 5%, whichever is the smaller (magnitude) reverse condition.Tim

author: pelud
author_rank: Regular Contributor
timestamp: November 14, 2014, 04:22:52 pm
content: Thanks for the replies. I didn't realize this would be answered by the capacitor manufacturer application notes.I found this in the nichicon application note:http://www.nichicon.co.jp/english/products/pdf/aluminum.pdf"Normally a cathode foil has a withstand voltage of about1V because of the natural oxide layer so it can withstand areverse voltage as much as a diode's withstand reversevoltage."I found a few other application notes say more or less the same thing. From what I gather from the other manufacturers, it appears that if it's less than 1V then it's ok, so long as it's transient.