How to not die when dealing with electronic components

[DeusM]

So, here are some more of my dumb (EDIT: noob) questions. Although this is a serious one that could prevent many from injuries 
So I know that cap can store some electricity after disconnected I'm always nervous when opening some electronic device like an amp or a power supply because I don't know what things inside it can electrify me. Off course I make sure everything is disconnected but I want to know exactly what things can do that and how to prevent it. So my questions are:

1) How much time can a cap store electricity.

2) How can I know if one is charged.

3) How can I discharge it.

4) What about transformers?

5) I read somewhere that you can damage some components like transistors, I think, if you touch them because static electricity you can have in your body. Is that true?

6) Any other recommendation?



So like always hope you guys can help me 

[Transmogrifox]

#1) I assume you mean how much "time".  An ideal capacitor can store the energy forever if it is just charged up and left open with nothing to bleed into.  Real circuits have all kinds of leakage current paths and capacitors have their own leakage.

Typical it can be dangerous for several minutes but I would treat it as live any time you open the box, even if it has been hours (unless you know the circuit).  For example certain power supplies I handle at work will have a dangerous amount of energy on them up to 2 minutes.

#2  A volt meter can tell you if it's charged, or make an LED probe where you have a resistor in series with an LED, and all wrapped in good thick insulating material so you don't contact anything conductive when you probe.

#3  We usually discharge them with a resistor.  The LED + resistor thing is nice because you can see it is safe when the light goes out.  More complex, but nice, is I wired up a 500V depletion mode MOSFET that self-biases off a resistor & LED to ground so the LED stays brightly illuminated until the voltage is below a certain threshold. 

Some people short the capacitor with a screwdriver.  There are 2 possible problems with this:
a)  If the capacitor bank is really large (a lot of energy storage) the arc flash can be explosive and potentially dangerous.
b) It's really hard on capacitors to rapidly charge and discharge.  You can damage the amp by doing this every time you work in it.

Resistor discharge is the best, and either use a DMM (meter) or LED indicator to tell when it is sufficiently discharged.

#4  Transformers don't keep their energy long enough to even open the box.  These can be dangerous if there is a capacitor charged up and you do something to discharge the cap into the transformer.  If you get energy stored in the transformer it can kick back to a high enough voltage to make sure you get all of the energy from a low voltage cap bank (even 5V cap bank could be dangerous).  Because transformers kick back when there is energy stored they find a place to discharge the energy -- sometimes this makes smoke or sparks.

#5  Yes you can damage semiconductor devices with static electricity.  This is true and this is a big deal at electronics manufacturing facilities -- they have taped off areas where you can only go with heel grounders, special anti-static smock and gloves.  Going into one of those places is in some ways like going to a pagan temple with all the rituals you need to perform in order to ensure you aren't storing static electricity.

I have never got the wrist-strap and ESD work pad and never damaged anything except JFETs when I'm being careless.  The key is to make sure you discharge static electricity to something other than your circuit before you touch the circuit or reach into your parts bin.

#6  Even low voltage circuits can be dangerous if you have inductors and capacitors together or step-up transformers.  It's the stored energy that is potentially dangerous.  High voltage is what ensures the energy is delivered into your body and an inductor can make extremely high voltages out of really low voltages  I have a tube stompbox that converts 12V to 320V with a little inductor -- the high voltage capacitor is every bit as dangerous as in a real amp even though the circuit takes a seemingly harmless input voltage.  Don't let things like 9V wall adapters make you think everything is safe unless you really understand the circuit you're dealing with.  If you see tubes, don't assume it must be a starved-plate mode circuit just because it has 12V input.  Make sure you take a meter and find out for certain there is nothing getting pumped up above 12V.

Another way low voltages can be dangerous is from arc flash (burns).  Typically not so much with stompboxes, but if you have a source like a car battery, there is a lot of energy there and a car battery could make a dangerous arc under the right conditions.

[Phoenix]

1) How much EDIT:time? can a cap store electricity.

2) How can I know if one is charged.

3) How can I discharge it.

4) What about transformers?

5) I read somewhere that you can damage some components like transistors, I think, if you touch them because static electricity you can have in your body. Is that true?

6) Any other recommendation?

1 Depends on the size of the cap (in farads, not it's physical size), the voltage it has been charged to, it's leakage current, and any external impedance it's connected to. Which is to say, a perfect capacitor would hold its charge FOREVER. Real world caps aren't perfect, but they can still hold charge for decades. Best practice is to have a discharge path for any capacitor that could hold a dangerous voltage (so called bleeder resistor). This is not always implemented though, particularly in older equipment.

2 You need to use a voltmeter/DMM/digital multimeter to check for the presence of voltage. Make sure to be careful when doing this - get probes for the meter with crocodile/alligator clips on the end, keep one hand in your pocket, attach one lead with a croc clip, then you can use a regular sharp probe to take the measurement. That way you only need one hand anywhere near anything dangerous, and are not risking a discharge path through your heart.

3 Use a discharge resistor. Some DMM's have a so-called "low impedance mode" which puts about 10k resitance across whatever's being measured - and also gives you the benefit of being able to simultaneously watch the voltage discharge. This feature is usually only seen on "electricians" DMM's though, not the type most suited for electronics work.
If your DMM does not have a low impedance mode, then you can make a discharge resistor probe. Get a power resistor (say 5W or larger) with a value somewhere between 10k-100k (exact value is not critical), a crocodile lead, and some adhesive heatshrink in a couple of diameters. Clip the croc lead in the middle, strip the cut ends, slip some heatshrink over the leads, and slip some heatshrink big enough to fit over the power resistor over one of the leads too if it's too small to fit over the croc jaws, then solder the two cut croc leads to the power resistor, slip the small heatshrink over the joints, shrink it, then slip the larger heatshrink over the power resistor and joints too (make sure it's long enough to cover the whole power resistor and the two solder joints at both ends, even though they're already covered by their own heatshrink - there'll still be a gap where that butts up against the resistor), and shrink it as well.
Now you apply the discharge resistor in the same manner as I described above for measuring voltage on capacitors, one hand in pocket, use the croc clips to attach.
Make sure to leave the resistor on while you're working on anything - capacitors exhibit a phenomenon called dielectric absorption, where after they've been discharged, if the load is removed (your DMM in low impedance mode or your discharge resistor), then it will "recover" it's voltage somewhat. If you have a DMM with low impedance mode, you can watch this happen by using low impedance to discharge, then switch to regular voltage mode (which has a high 10Mega ohm resistance), and watch the voltage creep back up. Different types of capacitors exhibit this phenomenon to greater and lesser degrees, old electrolytic caps show a great degree of this.

4 Transformers do not store energy. However they can be extremely dangerous when powered, especially in tube equipment where high voltages are present. Never do a job "live" that you can do with the power turned off (actually - never just turn the power off - make sure to actually unplug. If a powerpoint has been miswired or you live in a country without polarised sockets, then power can still be present and dangerous within equipment). There are certain tasks that you cannot perform without power applied, but ideally you should do this as little as possible, make sure never to do this when tired, frustrated, if you've consumed any alcohol, medicines that can affect your alertness, motor skills etc, or any illicit drugs. One mistake can be enough to kill you. Many people (myself included) have experienced electric shocks and survived, but there is no guarantee of that.
Oh, also make sure to always where rubber soled shoes, do not work in a wet or damp environment (particularly floors or work surfaces), and also do not work if you're excessively sweaty, as this increases your electrical conductivity, placing you at more risk of shock, injury or death.

5 Yes, static electricity can damage electronic components. All types of transistors may be effected, although mosfets are by far the most susceptable. As our resident guru RG Keen is a known to say: "The gate of a mosfet is a piece of glass about 20V thick." That means that any voltage difference between gate and source larger than that 20V limit will punch a hole straight through it, and it will not be a mosfet anymore, but a mosded. Many people are very cavalier about ESD (electrostatic discharge) precautions, stating that "I've never had any problem". The flaw in that argument is the false belief that ESD damage is alway immediately obvious and completely destructive. In reality, ESD can simply degrade performance or reliability, so you may have "unexplained" failures years down the road due to inadequate ESD procedures at assembly or maintenance. Given how simple ESD management is, there is no reason not to purchase the most basic of equipment. All you need is an ESD matt, and a wrist strap. The wrist strap connects to the ESD matt, usually by a 10mm snap and 4mm banana jack adapter, and that's all there is to it. The wrist strap has a 1MEG resistor in it which provides a controlled discharge path for you (and also limits current if you reference your matt to safety ground so that you're not directly referenced to ground incase of electric shock - but you won't need to reference your matt to ground, you only need to do that under special circumstances).
The matt itself has a so-called "static dissipative" top layer, and a bottom layer which is conductive. This means that if there's a voltage difference between the matt and something that you're about to place on it, it doesn't immediately zap out like a static shock, but discharges slowly until it equalises, preventing damage. When the piece of equipment is on the matt, and you have your wrist strap on, you all equalize to the same potential, which is the fancy way of saying voltage.
A small matt and wrist strap can be had for around $20 - cheaper if you buy on eBay or the like. You only need to avoid destroying a dozen germanium transistors or a couple of dozen mosfets for it to have paid for itself already.

Now, that's not the end to ESD, you also need to protect the devices themselves (that is any active components - transistors, IC's etc, passives like resistors and capacitors are immune, although some LED's are also susceptable - but most aren't) when storing them.
Anywhere reputable that you purchase active devices from will ship them in ESD safe packaging - you want pink foam (static dissipative) at minimum, grey or black foam (conductive) is better, pink bags are static dissipative, but best is the metallized foil bags which are static shielded. The pink stuff will prevent things from building up a charge from rubbing up against each other during shipping, but if someone comes along that has been dragging their feet along the carpet and touches it, that static charge will zap straight through the bag and zap your bits. The grey/black foam will keep all device leads at the same potential, so offers better protection against this than the pink stuff, but the static charge can still potentially punch through the device encapsulation and destroy it.
Only the static shielded bags (and tubes that IC's and other bulk parts come in) will completely protect devices.

Once you get stuff home, try to keep it in the original packaging. When you want to use it, pull it out, put the package on your matt, put on your wrist strap, then you can take out the parts you need or pour the parts out onto the matt. If you'd like a box/container, you can buy static safe storage from electronics suppliers, but this can get expensive. Options vary widely depending on where you live. You can also get static shielding sprays that will put a coating on regular plastic boxes and trays, but this is usually an even more expensive option.

6 Sorry, I think I've typed enough already. Probably information overload time! Anyway, hope you find some of that useful.

[DeusM]

Wow! Thanks guys for the answers! Sorry about the typo. Din't know about the ESD Matt. I'll try to buy one when I start working In my pedal.
So, what If the condenser is too small or the circuit too "crowded" for putting the crocodiles in its legs? What about putting one prove in the circuit path and another in the top of the cap? And what about those screwdrivers with an LED on top that electricians use for checking if there's power coming out of the socket?

Sorry if my English is too bad 

[Phoenix]

So, what If the condenser is too small or the circuit too "crowded" for putting the crocodiles in its legs? What about putting one prove in the circuit path and another in the top of the cap?

I think you're asking if you can discharge a capacitor through it's outer casing? No, that won't work, all modern capacitors have the outside insulated from the internals. If something really is too small to get croc clips on, then it's probably not very dangerous anyway (but you should still be cautious), you can use sharp multimeter type probes instead. Be careful not to slip!

And what about those screwdrivers with an LED on top that electricians use for checking if there's power coming out of the socket?

Those voltage detection screwdrivers (which no real electrician would use anyway, they're dangerous in their own right) don't contain an LED, they have a neon bulb. Neon bulbs are no good for discharging capacitors as they need a so-called "stike" voltage to initiate the glow discharge in the noble gas mixture, usually around 60-90V, then need a maintaining voltage to continue conducting, usually about 30-40% lower than the stike voltage, and then will stop conducting. So they will not discharge anything that is below their strike voltage at all (which could still be a dangerously high voltage), and will only discharge something that is above their strike voltage to their dropout voltage (which is still a dangerously high voltage). Also, they are simply not always designed for this - they generally do not have a lead attached, only the one probe which is placed in contact with something you are testing for the presence of voltage, and then have an exposed metal ped on the end of the "screwdriver" where you place a finger, which allows your body to act as a capacitor with the surrounding environment to complete the circuit. Basically, they connect your body straight to the wall voltage less the neon bulb voltage and (hopefully) the fitted current limiting resistor (not always fitted). This is why they are so dangerous and no electrician in their right mind would use one.

Sorry if my English is too bad 

I can tell you right now that your English is better than my anything! Us native English speakers have it easy, with most of the rest of the world catering to us, I'm always impressed by bi/multilingual people. I tried learning a few languages in school, I think I remember more Latin than anything else (gee, that's helpful!).

[DeusM]

Don't think yours is worse than mine, too much text correction going on here  I think its beacuse we are pretty much used to it (English) since most movies and internet pages are in that language. I went to an English institute anyway, when I was younger because I liked it very much. Most of the kids there went because they parent told them to go, so I was always the smart kid there. Not so much for the rest of my life  So many stories to tell...


I meant using the "screwdriver" to see if the cap was charged. I might said that in the wrong context. But yeah. I remember now you have to put your finger and that seems dangerous 

My multimeter has sharp type probes but, I don't think it has that low impedance mode you talked about. Here's a picture of it:

Don¿t mind the child-ish VDC - AC thing. It's  from an other era   
Is the low impedance mode only needed for discharging the cap? After re-reading your post I now understand now it's for that purpose right? I don't know why I had the idea it was for protecting the multimeter.

Maybe it has. I don't know  I'm so bad at this thing called electronics. I need to keep reading since I started from zero. I've been reading for over a year now, not so constantly due to studies and other stuff but so far I get to understand some of its principles but since not all sources have all the stuff I need, I find myself looking for the answers in many different places. Like here for example  After I'm done with finals I'm going to keep reading since I ordered the components for my first "real" pedal 

[Phoenix]

Don't think yours is worse than mine, too much text correction going on here 

I meant I don't speak any language other than English, but thanks for the compliment 

BTW, please add a width to your image tag so that it will fit on peoples screens. If you don't know how to do that, click on "quote" above my post and you can see the code.

That multimeter is really only good for low voltage things, I wouldn't use it on anything that does not have an external double-insulated power supply or runs on batteries. It's only fused on the milliamp range, not the amp range and could potentially EXPLODE if connected incorrectly to anything with a moderate voltage. I also wouldn't use it for measuring voltage on any capacitors that're in something that plugs into the wall. Pedals ONLY. It's not even moderately safe. The exact same meter is widely known for being available from eBay from as little as $1! with free shipping, and for being given away for free at Harbour Freight (a big-box hardware store chain in the US).

And no, it does not have a low impedance mode.

Low impedance mode is only found on electrician type meters (electricians and electronics require very different feature sets in multimeters). It is not primarily used for discharging capacitors. Rather, it is used to prevent false readings caused by so-called "ghost voltages".

I'm so bad at this thing called electronics. I need to keep reading since I started from zero. I've been reading for over a year now, not so constantly due to studies and other stuff but so far I get to understand some of its principles but since not all sources have all the stuff I need, I find myself looking for the answers in many different places. Like here for example  After I'm done with finals I'm going to keep reading since I ordered the components for my first "real" pedal 

Don't worry, everything takes time. I've been at this a very long time. You'll get there if you keep at it!

[DeusM]

I meant I don't speak any language other than English, but thanks for the compliment 

Yeah, reading it now I understand what you meant

BTW, please add a width to your image tag so that it will fit on peoples screens. If you don't know how to do that, click

Yup, sorry, I noticed it was to big and didn't know how to do it. The more you know! 

That multimeter is really only good for low voltage things, I wouldn't use it on anything that does not have an external double-insulated power supply or runs on batteries. It's only fused on the milliamp range, not the amp range and could potentially EXPLODE if connected incorrectly to anything with a moderate voltage. I also wouldn't use it for measuring voltage on any capacitors that're in something that plugs into the wall. Pedals ONLY. It's not even moderately safe. The exact same meter is widely known for being available from eBay from as little as $1! with free shipping, and for being given away for free at Harbour Freight (a big-box hardware store chain in the US).


And no, it does not have a low impedance mode.


Low impedance mode is only found on electrician type meters (electricians and electronics require very different feature sets in multimeters). It is not primarily used for discharging capacitors. Rather, it is used to prevent false readings caused by so-called "ghost voltages".

LOL. What a piece of crap. 1 dollar? Holy S. It costed me like a lot more. They even give it for free? The wonders of living in a 1st world country.

Gonna have to upgrade soon...

Don't worry, everything takes time. I've been at this a very long time. You'll get there if you keep at it!

I know I will!! And I'll be able to give something back to this great community 

[digi2t]

So, here are some more of my dumb questions.

Assuming that a question is "dumb" is the usually the first step towards catastrophe.

[DeusM]

Assuming that a question is "dumb" is the usually the first step towards catastrophe.

That is so true. I don't believe there are dumb questions, but I din't found other words to express my self 
I though about "noob question" but I din't wanted to sound too amateur 

[DeusM]

Also nice avatar. Another Bruce Lee fan here! 

[digi2t]

"Noob" is OK. I've been doing this pedal building schtick for several years now, but I still consider myself a noob. Probably always will. And remember....

Empty your mind, be formless. Shapeless, like water. If you put water into a cup, it becomes the cup. You put water into a bottle and it becomes the bottle.

Just be sure that water's not anywhere near you when your playing with caps though. Roger, out.

[Kipper4]

Just out of interest what sort of equipment are you dealing with?
High voltage valve amps, fridges and freezers or just 9v pedals?

[antonis]

Just an add to calm down some anxiety disorders... 

Every self-respected PS should have some kind of bleeding circuit - usually just a humble resistor across + & - nodes of rectifier/filter capacitor(s)..
(sometimes that resistor is set in series with an LED to form an working indicator for PS..)

I'm not aware if there are strict regulations about the size of bleeding resistor (for creating some typical RC time constant for discharge..) but it's freequently said that the number of mounting screws/bolts of an instument is set so that the reqired time for dismantling it overcomes the discharging time of it;s power supply.. 

[EBK]

Assuming that a question is "dumb" is the usually the first step towards catastrophe.

That is so true. I don't believe there are dumb questions, but I din't found other words to express my self 
I though about "noob question" but I din't wanted to sound too amateur 

You could just simply call them "questions," without any self-deprecation. 

[Phoenix]

Just an add to calm down some anxiety disorders... 

Every self-respected PS should have some kind of bleeding circuit - usually just a humble resistor across + & - nodes of rectifier/filter capacitor(s)..
(sometimes that resistor is set in series with an LED to form an working indicator for PS..)

I'm not aware if there are strict regulations about the size of bleeding resistor (for creating some typical RC time constant for discharge..) but it's freequently said that the number of mounting screws/bolts of an instument is set so that the reqired time for dismantling it overcomes the discharging time of it;s power supply.. 

LOTS of vintage equipment, especially from the tube era, but even into 80's solid-state amps, do NOT have bleeder resistors on the bulk filter caps. The tube stuff would rely on the tubes warm heaters to discharge them, but if they've been out for service, have blown filaments, or the amp wasn't on long enough for the heaters to warm up but the filter caps have already been charged, then those caps will be at full B+ voltage.
The solid-state stuff would rely on the transistors to self-discharge, but if they're blown, same story.
And when are we most likely to be poking around inside stuff? When something's not working right...

I'm also not aware of any regulations requiring or specifying bleeder resistors (except as it applies to non-isolated power supplies like DC-DC converters and capacitive droppers) on linear power supplies, but I do recall something to the effect of a 30 second time constant being suggested as best practice, although for the life of me I don't remember where. I know this rule-of-thumb is not followed at all in current production tube amps (what I have most experience with), but most of the RC combinations I remember off the top of my head are under 60 seconds.

So yes, every self respecting power supply SHOULD have a bleeder resistor - but NEVER count on it.

[digi2t]

Just an add to calm down some anxiety disorders... 

Every self-respected PS should have some kind of bleeding circuit - usually just a humble resistor across + & - nodes of rectifier/filter capacitor(s)..
(sometimes that resistor is set in series with an LED to form an working indicator for PS..)

I'm not aware if there are strict regulations about the size of bleeding resistor (for creating some typical RC time constant for discharge..) but it's freequently said that the number of mounting screws/bolts of an instument is set so that the reqired time for dismantling it overcomes the discharging time of it;s power supply.. 

Antonis has a point there. I see this often in tube amps.

[antonis]

I'm also not aware of any regulations requiring or specifying bleeder resistors

Maybe R.G. or some other guy involved in PS design/build could enlighten us... 

Together with the "dilemma" of compromise between fast discharge vs high inrush current in case of short On-Off switching time... 

[anotherjim]

That meter is, I think, absolutely fine for low voltage work, which includes pedals, synths and other low voltage/power stuff.
I use the same one for everyday work, but that doesn't include valve amps or switch mode power supplies.

Don't be tempted to measure current with it. Put a blob of paint in the triangle pointer on the selector switch so it's easier to see what the range is. Don't ever plug into the 10A socket.

The supplied probe leads are very bad and probably the least safe thing to them.

[antonis]

Don't be tempted to measure current with it.

Not even on mA socket...!!

It's extremely easy to make damage by turning the selector while test leads on circuit...

IMHO, meters without completely separate  test lead sockets for current measurement shouldn't be able to placed in market...!!!