Hello,

You will have some troubles when you will have to work on a circuit with capacitors that carry 2500 Volts and the bleeder resistors are broke.
It was the powersupply of a tube based (2 X 4CX250) RF amplifier.

Bertus

 

Hello,

You will have some troubles when you will have to work on a circuit with capacitors that carry 2500 Volts and the bleeder resistors are broke.
It was the powersupply of a tube based (2 X 4CX250) RF amplifier.

Bertus

You need this:

Obviously, you need a smaller one. But it was a normal thing to have around the shop in the hollow state days.

 

The current is greatly limited anyway by the decaying charge. The instantaneous current at the moment of contact is high - but still not as bad as shorting it with a screwdriver.

The idea of using a lightbulb is that it has a PTC (positive temperature coefficient) and although the initial in rush current will be high, the resistance will rapidly rise limiting maximum current, there by lowering stress on the capacitor.

As the charge on the capacitor decays, the current through the bulb's filament also decays allowing it to cool, thus decreasing it's resistance; the discharge curve instead of a gradual decrease like it would be across a resistor, is much closer to a straight line.

And no screwdrivers were harmed during this discharge

 

The current is greatly limited anyway by the decaying charge. The instantaneous current at the moment of contact is high - but still not as bad as shorting it with a screwdriver.

Just be sure that if you have a test lead clipped to the screwdriver that you have the other end securely clipped to ground and it is not dangling free where it can touch your elbow.

Don't ask me how I happen to know this.

OK it wasn't an actual cap but the high voltage anode on a CRT television and the power was still on.

 

Wanna trade stories of stupidity? When I was a kid and just at the very beginnings of learning about electricity I had a 12 volt transformer, one of those large finned rectifiers and a car radio. I wanted to play the car radio in the house so I hooked it up. Got a TERRIBLE hum. Someone told me I needed a filter capacitor, so I found one. Something like a 470µF at 16 volts DC (electrolytic) I put it across the secondary (output) of the transformer but that did nothing to clean up the hum. So I put it across the primary.

Interesting story: The guts of that capacitor disappeared in an instant. Oh, there were some remnants of foil and paper, but largely there was nothing left after the energetic bang.

Depending on the type of cap, some hold charges longer, some shorter. Electrolytics are typically leaky in that they don't hold their full charge. I've tried that trick, charging up a cap and leaving it lay around where it would get touched. Much to my dissatisfaction the cap did not hold the charge for long. So assuming you have a high quality capacitor charged at one time up to 120 volts, it can hold that charge for quite some time. And without a bleeder resistor to drain off the charge it will remain potentially shocking. I once accidentally touched the leads of a photo flash capacitor. Talk about swearing like a sailor!

 

So, if a capacitor had 50 kF and was charged up to 120V, it would behave like a continuous voltage source of 120V DC?
I guess it would take way more time to charge a 50 kF at 120V than a 50 uF at 120V, right?

 

So, if a capacitor had 50 kF and was charged up to 120V, it would behave like a continuous voltage source of 120V DC?
I guess it would take way more time to charge a 50 kF at 120V than a 50 uF at 120V, right?

"50kF"?111 - sometimes I wonder how much capacitance it would take to blur the distinction between capacitor and battery......

Someone published a blog years back describing how they replaced their car battery with a "beer crate" bank of supercapacitors.

 

Wanna trade stories of stupidity? When I was a kid and just at the very beginnings of learning about electricity I had a 12 volt transformer, one of those large finned rectifiers and a car radio. I wanted to play the car radio in the house so I hooked it up. Got a TERRIBLE hum. Someone told me I needed a filter capacitor, so I found one. Something like a 470µF at 16 volts DC (electrolytic) I put it across the secondary (output) of the transformer but that did nothing to clean up the hum. So I put it across the primary.

Interesting story: The guts of that capacitor disappeared in an instant. Oh, there were some remnants of foil and paper, but largely there was nothing left after the energetic bang.

Depending on the type of cap, some hold charges longer, some shorter. Electrolytics are typically leaky in that they don't hold their full charge. I've tried that trick, charging up a cap and leaving it lay around where it would get touched. Much to my dissatisfaction the cap did not hold the charge for long. So assuming you have a high quality capacitor charged at one time up to 120 volts, it can hold that charge for quite some time. And without a bleeder resistor to drain off the charge it will remain potentially shocking. I once accidentally touched the leads of a photo flash capacitor. Talk about swearing like a sailor!

I remember an electronics instructor giving a demo of the dangers of connecting an electrolytic to 120VAC

 

When I was a tech at HP, I had a big power supply blowing up caps like firecrackers. There were two color coded stud rectifier diodes that had the opposite polarity and the assemblers had a tendency to swap them. I showed the production engineer and he made me carry the supply back to the assembly supervisor to show her what happens when her people screw up.

The electrolyte that leaked on the floor was removing the wax. The guts of most of the smaller caps had been blown out; the big ones had pressure relief plugs.

They did a better job after that.

 

"50kF"? - sometimes I wonder how much capacitance it would take to blur the distinction between capacitor and battery......

Someone published a blog years back describing how they replaced their car battery with a "beer crate" bank of supercapacitors.

Yes, I said 50kF to put a huge capacitance example. I'm trying to comprehend a little more what capacitance means.

 

Lets say you have a capacitor that has 50kF, and it is charged to some voltage say 12V. If you were to draw 1A from it for 50000 seconds, the voltage of the cap would drop by 1V to 11V. I would consider this in the same ballpark as a 60Ah car battery.

 

Big caps like with 10 kV ratings will charge themselves without a "keeper" which is a short across the terminals. Meter movements not in circuit would have them too.

So, very nasty,

@Yaakov Used something similar when working on a 0-100 kV 0.1 A supply, 1000 W RF transmitters (2-3Kv inside) and a 15 kV at 1.5 A supply.

Put 120 AC into a piece of electronics that wants say 12 V DC and there are lots of firecrackers.

A 400 VDC rated oil cap really made some noise when discharging with a screwdriver.

We rigged lockers in high school to discharge a cap when the latch was lifted resulting a "big bang".

Somebody at work made "contact explosive" material and put it on door latches. Another guy made some Thermite https://en.wikipedia.org/wiki/Thermite and we went outside to watch it melt a hunk of metal.

 

Capacitance means "Capacity". In this case the capacity to hold a charge of a specific magnitude and potential. The magnitude and potential being the total charge and ability to deliver it back.

Caps are sort of like batteries in that they can be charged. Batteries are typically chemical reactions whereas capacitors are on an atomic basis, not to be confused with atomic power. Atoms are pushed from normal orbits into excited states and they desire to get back to normal. When a cap is charged it exists in an excited state. When the leads are shorted the electrons rush through the circuit in an effort to re-establish equilibrium, the normal state.

The difference between a battery and a capacitor is their ability to deliver their entire potential charge. Batteries can deliver specific amperage at a specific rate. A capacitor has (in theory) the ability to deliver its full charge in an instant (as it may be). This isn't a perfect description of a capacitors ability to deliver its charge but for the sake of ignoring ESR (Equivalent Series Resistance) and other imperfections, for all intents and purposes, a cap can deliver its entire charge in a single moment. Batteries can't.

 

Yes, I said 50kF to put a huge capacitance example. I'm trying to comprehend a little more what capacitance means.

Capacitance is not *TOTALLY* different to a static charge, but for most practical purposes the physics are a bit different.

In simple term: its 2 conductive plates separated by a dielectric (which could even be just air). whether stacked or rolled into a cylindrical form - its all about total surface area. In aluminium capacitors - the electrolyte is not actually the dielectric. The plates are etched till they're pretty much porous - which creates a huge surface area. The dielectric is actually aluminium oxide formed by a forming current during manufacture.

Batteries store energy in the form of a reversible chemical change.

 

An interesting thread: strange that no-one has mentioned super-capacitors yet ;-)

The WWII equivalent of a boost converter was the rotary converter, typically a 12 or 24 volt motor attached to an HV dynamo within a single unit. When I was new to electrics, I acquired one of these that was part of an R/T set. Yes, I did... I touched the output terminals before the unit had finished spinning down. The 500V arc punched a hole right through my thumbnail.

You learn some lessons the hard way, but maybe these are the ones you remember best.

 

An interesting thread: strange that no-one has mentioned super-capacitors yet ;-)
.

Super caps are low voltage, they won’t provide hugh current into a high resistance load.

Photo flash caps, on the other hand...

 

The WWII equivalent of a boost converter was the rotary converter

Or the vibrator supply for valve car radios - a sort of mechanical switch mode supply?

 

Or the vibrator supply for valve car radios - a sort of mechanical switch mode supply?

When I was about 7 years old I discovered that hooking a “transistor battery” through a switch to a 120-12V transformer secondary resulted in a very noticeable output on the primary when the switch was operated.

Massive “joy buzzer”.

 

Nothing like the rig I set up once. Took some very fine aluminum wires and laid them carefully on the toilet seat. Put a flash light battery spring under the front of the seat to hold it up slightly and made some contacts at the back. When you sat down you charged a capacitor. When you got up and the contacts broke, the breakdown of the magnetic field coupled with the charge of the capacitor gave you a helping kick to get up off the seat. It was powered by a single D cell battery. I think the transformer I may have used might have been a 10:1 step-down connected as a step up. It got quite a few comments by visitors to the water closet.

 

Nothing like the rig I set up once. Took some very fine aluminum wires and laid them carefully on the toilet seat. Put a flash light battery spring under the front of the seat to hold it up slightly and made some contacts at the back. When you sat down you charged a capacitor. When you got up and the contacts broke, the breakdown of the magnetic field coupled with the charge of the capacitor gave you a helping kick to get up off the seat. It was powered by a single D cell battery. I think the transformer I may have used might have been a 10:1 step-down connected as a step up. It got quite a few comments by visitors to the water closet.

I won’t say you are weird, Tony, but...

Today, you’d have lawsuits to contend with!