Capacitors Don't Always Block DC! Here's Why...
Added 2024-07-02 15:10:49 +0000 UTCMore Capacitor Knowledge - Use this information to protect your test equipment input/output from harmful DC spikes.
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Comments
A good intro that introduces the problem but then stops short. I was hoping for some recommendations for improving test equip input protection and guidelines for improving circuits.
Mark
2024-11-22 01:47:50 +0000 UTCGreat video Paul. This reminds me I have a origional peavey CS800 that I did an electrical restoration on some years ago. These old beasts have a very crude crowbar speaker protection circuit which does not stop the classic, On/off thump from the DC. I have been meaning to build a relay based speaker protection circuit to stop this as the thump from that amplifier is brutal. It has a very high energy power supply. Fortunatly the bose 800 speakers its driving has no crossover or tweeters to get damaged. Very robust speakers. Quite a crude design on peaveys behalf. Once an output transistor shorts in that amplifier, the damage is quite brutal from that crowbar circuit. Great video on some of the simpler things that most including myself have overlooked for years!
Ben Harper
2024-11-03 12:18:38 +0000 UTCGood refresher. I am working on several Carver M400 Amps right now. They are known for the initial thump. I think the thump is associated with other variables along with the the initial spike explained in Mr. C's video.
Kent Murphy
2024-10-19 21:20:53 +0000 UTCThanks, Paul. It's very easy to forget about or overlook the little things.
Robert Calk Jr.
2024-09-14 21:08:58 +0000 UTCTHANK YOU for a useful and interesting video!
Jeffry Blackmon
2024-08-13 08:05:11 +0000 UTCThis is the sort of video that convinces me that none of us fully grasp what we're doing with electronics, but in doing our best we certainly produce some incredible things.
Jules L
2024-08-04 22:08:11 +0000 UTCFrom this, I'd love to know more about protection circuits.
Keith-DS
2024-07-31 14:26:42 +0000 UTCProbably the exact reason I blew the front end of my Siglent signal generator. twice now.
Patrick
2024-07-12 03:08:55 +0000 UTC"...a semi conductor will just lose the semi part and just become a conductor. They become three legged jumpers." I'm going to use this from now on.
Arpi Batai
2024-07-10 15:02:54 +0000 UTCDoes this mean I have to trust the DC block to protect my front end....?
Paul Stallibrass
2024-07-09 10:21:15 +0000 UTCHi Paul. This video does not display an actual RF/DC block, it just displays an example of whats going on, "or whats happening" when DC is applied to a capacitor that's connected to the front end of a piece of test equipment.
Mr Carlson's Lab
2024-07-09 06:37:40 +0000 UTCMany thanks Paul. I have recently bought a spectrum analyzer and a DC block to protect the front end. I am going to build your one with LED's to give the front end a better chance of surviving when connecting other equipment....you just never know...!
Paul Stallibrass
2024-07-09 00:42:13 +0000 UTCYes, compiling a list that could be considered for future videos might be a good idea.
Simon York
2024-07-08 22:02:03 +0000 UTCDepends on the frequency you're operating at - generally, you want the cap to have a very low impedance at the frequencies of interest. For a 1 ohm impedance or less, this formula for the capacitance is useful: C >= 1/(6.28*f) , where 'f' is the minimum frequency you're interested in. As an example, your 10pF cap would present about 16kΩ to a test signal in the middle of the AM broadcast band. That's not gonna matter too much if the input impedance of your test device is a megohm or more at that frequency, but if it's 50Ω, you're massively attenuating your signal before it even gets into the 'scope/counter.
Don Tidrow
2024-07-06 19:17:52 +0000 UTCTLDR: Transients aren't DC. :-) People who don't design electronic systems for a living often don't realize how risky power-on and power-off transients can be, and need to be accounted for in your design.
Don Tidrow
2024-07-06 19:07:28 +0000 UTCAlso important for capacitors left in the draw. They may be charged up so when you take them out an put them into circuit, they could send a pulse where you didn't expect it. One always needs to remember to discharge them before putting them away - especially the big ones (high school jokers) but smaller ones too.
Rick Parsons
2024-07-06 10:19:19 +0000 UTCWould it therefore not be a better option to use a very low value capacitor (say 10pF) inline with a test probe, to limit the duration of the 'DC spike' into the front end of your test gear?
Vincent Stevens
2024-07-04 19:08:18 +0000 UTCI understand thanks.
Tim
2024-07-03 18:30:59 +0000 UTCHere is a video of Mr Carlson building the Spectrum Analyzer Protection circuit 8 years ago : https://www.youtube.com/watch?v=ETKyKC2Zj-M
Bjorn V
2024-07-03 08:01:15 +0000 UTCI find capacitors to be one of the simplest electronic components, but also one of the most intriguing. But I'm going to drop a little bomb here. Technically, a healthy capacitor does not pass DC, but also not AC (only a small amount of leakage current depending on the type of capacitor). Why am I saying this, let's go back to basics and see how a capacitor is made. A capacitor consists of 2 conductors separated by an INSULATOR (Dielectric). As we all know, an INSULATOR is an electrically non-conductive material. Let's do a test: We take 10V AC (60Hz) and connect a 1uF capacitor in parallel, a current of about 38mA now flows because the 1uF capacitor forms an impedance of about 265 Ohm. If we increase the 60Hz to 60Khz (60000Hz), the impedance decreases 1000 times to 0.265 Ohm, and the current increases to 3.8A If we repeat this experiment of 10V at 60Hz and 60Khz with a real resistor with a value of 265 Ohm (if this value existed) in parallel, the current at 60Hz and 60Khz (and higher) will remain the same. So how can 3.8A flow through an ISOLATOR? It's not possible !!! Where does this AC power come from? Well, this AC current is caused by the charging and discharging of the capacitor. In the experiment that Mr Carlson did with the DC voltage, you saw the LED light up briefly, this was due to the charging of the capacitor, the other LED lit up briefly due to the discharging of the capacitor. In both cases the DC voltage created a current, a charging and discharging current respectively. It is this charging and discharging current that AC does many times per second. The larger the capacitor, the higher the charge and discharge current becomes, and this increase is proportional to the frequency. As I said in the beginning, we are talking about a healthy capacitor, not a capacitor that has gone bad and has so much leakage that it starts to look more like a resistor. Grtz
Bjorn V
2024-07-03 07:42:35 +0000 UTCIn most cases no.
Mr Carlson's Lab
2024-07-03 06:41:15 +0000 UTCA "front end" is a generic name for the "sensitive" parts (the inputs) of a testing equipment such as: Oscilloscopes, Spectrum analyzers, Multimeters, capacity/inductance/impedance testers, or any other testers for that matter. Usually, a front end consists of a signal amplifier/conditioner of some sort and any excessive voltage applied to it can potentially destroy it (with a relative ease !) and is almost always very expensive to have it repaired. A good example is when you use a multimeter to measure capacitance. Say you've switched your multimeter to the capacity function and want to measure a capacitor value but that capacitor still holds some residual charge (is like a battery that has energy in it) so you could destroy the "front end" of your multimeter with that "residual voltage charge" . So to avoid it you'll need to have the capacitor discharged FIRST ! before commencing the test. So what breaks in a "front end" stage could be a number of components but basically any kind of semiconductors that the "front end" is made of, such as: a diode, a MOS-FET or a J-FET transistor, a specialized IC, an OP-AMP. or it could even be something as simple as a little fuse (well...) if you're lucky enough :) etc. I hope it helps.
Ovi4
2024-07-03 05:49:05 +0000 UTCTo put it another way: caps block DC, but when you first switch on DC, it is AC for a while, since the act of switching it is an "alternation". A sharp turn on of a "DC" signal can contain frequencies in the GHz (which will happily cause RF interference).
Lily Finley
2024-07-03 04:13:32 +0000 UTCWould the front end not already be protected by a 1 Mega Ohm resistor to limit current?
Jonathan Hughes
2024-07-03 03:15:24 +0000 UTCWhat is the front end?
Tim
2024-07-03 01:23:00 +0000 UTCWhat has helped me to visualize the pulse, is to think of it like the kinetic balls that you can pull back one side and let go, which would be voltage in, and as it strikes the other balls, the energy is transferred to the other side causing an equal number of balls to swing away, giving you voltage out. The first ball (the electrons) stay on the same side, but the energy transfers to the other side pushing away the ball (electrons on the other plate) on the other side. With AC the ball comes back and transfers the energy back the other way thus passing AC through but blocking DC after the initial pulse. It's not a perfect metaphor, but it helps me understand what is going on inside capacitors.
Chip
2024-07-02 23:46:05 +0000 UTCnow that's a very cheap way to make 2 red leds with alternating cuurent, flash on a railway passing of a modeltrain, scale H0
Ronald Cobet
2024-07-02 23:02:28 +0000 UTCVery helpful Mr. C, thanks for sharing. Any plans for more of these sort of "taken for granted/not talked about kernels of wisdom" videos in the future would be excellent!
Chris
2024-07-02 22:19:59 +0000 UTCThis is something I didn't know. I've always heard the "Thump", especially noticeable when turning on my guitar amp. I always thought it was a normal thing and never really paid attention to it let alone think about the why. Thanks Mr. C.
James Roberts
2024-07-02 21:53:09 +0000 UTCVery helpful but short of building your circuit in Video 36 what other simple solutions are there?
Jim Gross
2024-07-02 20:53:26 +0000 UTCThank you, Mr. Carlson, for this valuable lesson. Ignorance of physics means the loss of $1k to $10k in lab equipment! There is no excuse for that: "I didn't know this law of nature!" That really adds up! A year ago, I fed +16 dBm instead of the maximum possible +10 dBm into an RF broadband amplifier during a restoration project of an ED80 military aircraft radio receiver to simulate an ED 10/81 multioscillator. Unfortunately 6 dB too much. Loss for replacement: 250 €! So please know exactly what you are doing in the lab. Please start small, ramp up your hobby with inexpensive measurement technology. Professional devices are designed precisely to their limit values in order to do what they are supposed to do. Inexpensive measurement technology forgives many mistakes. I have paid enough for this comment. By the way: I love my tube measurement technology because it forgives many a switching error. Often enough, I only dare to switch on modern measurement technology afterwards: "If you get in one circuit, you get one pulse!", very wise Mr. Carlson and I can confirm this from my experience in restoring modern measuring technology!
Martin Siebert
2024-07-02 16:50:55 +0000 UTCI'd love to add a simple protection circuit to my old suitcase phonograph if such a thing exists
Brian Ortiz
2024-07-02 16:21:58 +0000 UTCThank's for video 👍. Very well explained
gregthewolf
2024-07-02 16:10:01 +0000 UTCGreat description of the inrush current in a capacitor. I have also glossed over the fact that caps are not instantly charged. I won't forget it anymore. I use mostly solid state instruments - NanoVNA, TinySA Ultra, etc and don't want to destroy them. Thanks Paul.
Sheldon Butler
2024-07-02 16:01:03 +0000 UTCThank you Mr. Carlson, this greatly expands my understanding
Rob
2024-07-02 15:55:07 +0000 UTCYes, that would be a very good way of looking at it.
Mr Carlson's Lab
2024-07-02 15:51:04 +0000 UTCSo "capacitors block DC... Eventually" is a better way to think of it
Tom23rd
2024-07-02 15:38:33 +0000 UTCThank you Paul. Very helpful.
Robert Dreitlein
2024-07-02 15:32:47 +0000 UTCThank you!
Kenny Fidler
2024-07-02 15:31:50 +0000 UTC
