Help me get rid of these transients! - Page 1
Forum section: electronics / projects

author: MattHollands
author_rank: Frequent Contributor
timestamp: August 19, 2016, 08:38:07 am
content: I am working on a circuit with 5 op amps and 7 LM3914s. I partially breadboarded it up with 1 op amp and 1 LM3914 and it worked just fine with my 12V power supply (http://www.digikey.com/product-search/en/power-supplies-external-internal-off-board/ac-dc-desktop-wall-adapters/589898?k=&pkeyword=&pv1376=4&pv48=2&pv1120=402&FV=fff40009%2Cfff8004a&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25).So I went ahead and produced some partially assembled PCBs and suddenly when I plug in my power supply I am getting 30V transients. I measured the DCR of my circuit (~6k4), and also guessed the capacitance (around 33uF due to 10uF bypass on power supply, 2 Op amp bypass + a bit extra) and simulated this on a breadboard, and sure enough suddenly the power supply transients are back.So I have two questions, how can I reduce the overshoot - should i just reduce the capacitance of my board? Is this LIKELY to damage an NE5532 or LM3914? It is above their rated voltages but only for ~40us.Thanks

author: Floyo
author_rank: Regular Contributor
timestamp: August 19, 2016, 08:55:21 am
content: Have a read through Linear's app note 88 (www.linear.com/docs/24956), It sounds like you might be suffering from the problem described. The basic gist of it is that ceramic caps plus wiring inductance resonate upon a step in voltage (plugging in the adapter), and that gives rise to the high voltage transients. Adding some ESR somewhere should dampen this resonance down.

author: Zero999
author_rank: Super Contributor
timestamp: August 19, 2016, 08:59:30 am
content: It's within the maximum voltage rating of the NE5532 and even the LM3914 will probably be fine with such a brief overvoltage.I'd be more worried about the leakage across the Y capacitor which will cause the whole circuit to float at half the mains potential and could damage sensitive components, if the wrong part of the circuit is earthed.You could add a transient suppressor diode to absorb the spikes: something with a break over voltage between 16V and 24V will do.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 19, 2016, 02:03:17 pm
content: Use electrolytic capacitors.Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: August 19, 2016, 05:13:02 pm
content: Literally solderanyelectrolytic cap (of course with at least 16V voltage rating) you have lying around in parallel with the input caps and see what happens.

author: MattHollands
author_rank: Frequent Contributor
timestamp: August 20, 2016, 02:22:50 am
content: @FloyoThat looks exactly like what I am experiencing - nice ap note. Thanks!@Hero999You're right about the 5532 voltage rating I ignored the fact that they're dual rail op amps.@T3sl4co1lV2 will use electrolytics@SiwastajaI don't have any electrolytics to hand - I am doing an internship abroad and don't have much of my kit with me.@EveryoneFor version two I am going to drop the bypass caps on the op amps down to the recommended 0.1uF ceramic caps. I am also going to use an RC filter (10ohm and 100uF electrolytic) to create a filtered supply for the op amps (they're doing audio filtering so I want a clean supply) and then I'll put the raw noisy supply for the LEDs on the LM3914 maybe with a 10uF electroytic capacitor. The filter shouldn't waste too much power because the op amps are driving a maximum of a few milliamps each. I will test the filter on a breadboard. Does this sound like a reasonable solution? (also trying to reduce noise on my power supply).