Transformers & Caps

[mykamatic]

Hey Everyone,

I had a few safety questions for SS amps. One: Do Transformers hold a charge for a long period of time (even unplugged)?

Two: Capacitors. I watched a tube repair video that showed splicing a 470ohm 1 watt resistor into a jumper clip, connecting
one side to the chassis (ground) and then touching the other side to the capacitor to drain it. Is there any reason this
information would not hold true to working on SS amps? I'm trying to get to the bottom of a problem & would like to make sure
safety comes first!

Thanks!

[JPHeisz]

Hi,
Transformers don`t hold a charge.
Caps can, depends on the circuit they are connected to and if it bleeds them down (most will). To be sure, you can do the resistor thing to drain them.
For most solid state gear, the caps are of relatively low voltage - 40V or so will not `shock`you but can take a good sizes chuck out of a screwdriver it you short them.

[mykamatic]

Rad. Thanks for the info!

[joecool85]

It's true about the caps being unlikely to shock you, although some can, it only takes 24v or so to penetrate the skin.  But it depends on humidity, skin type, moisture on skin etc.  So always be safe and discharge them.  Also, even if it doesn't shock you, it could fry components of the amp if a wire is crossed/sent to ground while working on it etc.  Always best practice to drain your caps.

Once you think they are drained, measure the voltage across the cap.  As long as you are below 1v you are effectively drained and won't cause any damage to your or your amp.

[DJPhil]

MJL21193 is spot on. I thought of a couple of things to add that may be useful.

Your question got me thinking, and off the top of my head I can't think of anything that would present a shock hazard in disconnected and unpowered equipment except capacitors. If anyone can think of something else, please let me know. Certainly there are hot components, sharp edges, and a few nasty chemicals if you like eating or breathing ground up parts, but nothing related to electrocution.

There's an excellent article on electrical safety on the ESP site (absolutely amazing site by the way, further exploration is highly recommended).

Capacitors store charge in an electric field. An ideal capacitor retains it's charge indefinitely, but real components have internal resistance and other imperfections that cause the charge to dissipate over time. There's also another odd phenomenon called 'dielectric soakage' where a completely discharged capacitor in a shorted state, after removing the short, charges back up to a percentage of it's original charge. You can see this effect most easily with larger electrolytics. Hook up a 10uF or so electrolytic to a voltmeter and charge it up to say 10V. Put a 500Ω or so resistor across the terminals and you can watch it discharge on the meter. Then remove the resistor and you can see the voltage bounce back up to 3V or more. It's a well known problem in some analog to digital circuits that use a capacitor for sample and hold, as it goons up the sampling on large input changes. All capacitors do this to some degree, but it can be reduced greatly by using certain materials. Just something to watch out for, especially on tube equipment where hundreds (or thousands!) of volts can bounce back up to potentially dangerous levels. Increasing the time the cap is grounded can help minimize soakage.

You can test a cap to see if it's holding a charge with a voltmeter. The important thing to remember is to be careful of the voltage rating of the meter! This is a big issue for CRT and microwave cap discharging (special HV probes are mandatory), an important issue for tube amps (check the ratings on the meter and the probes), and almost a non-issue for all but the highest power SS amps. Almost every meter out there can easily handle 300VDC without trouble, but I'd be wary of using a $7-on-sale multimeter for more than 50VDC. Many do just fine with them, but I'm paranoid as I've seen a cheap meter fail dramatically even though it was used properly and within rating.

If you test a cap for charge, be very careful that you don't short the probe tips while they're across the cap, or it'll happily discharge through your probes! Caps discharging over a low resistance (any metallic conductor) will happily push as much energy out as their internal resistance allows, and this could easily be tens or hundreds of amps. This leads to melted probe tips, vaporized metal, and occasionally flying chunks of screwdriver, and it'll scare the hell out of you if you don't see it coming.

Hope that helps some, and thanks for letting me indulge in babbling on and avoiding my studies for a while.