bleeding caps
Moderators: pompeiisneaks, Colossal
bleeding caps
Could you help a very inexperienced builder understand the proper methods to discharge the caps on an amp after power up. Also, what are the proper methods to probe a live amp for voltages.
Re: bleeding caps
A lot of amps will have a 'bleeder' resistor across one of the power supply caps to slowly run the voltage down when the power is off, but you can also make your own for those that do not.
I use a 220K 2W resistor with leads soldered on to each end, and then the assembly is heat-shrinked over to completely insulate it. The ends of the wires have insulated crocodile clips, so I can clip one end to the chassis and then the other end to the positive side of the cap I want to drain.
Most amps have the caps arranged in a voltage ladder with dropping resistors between them, so draining one will drain all. However, some amps have the standby switch in between banks, so it pays to have the standby switch on when you're doing this, and certain other amps have diodes in the voltage ladder so draining one cap won't necessarily drain the others.
The key is to measure the voltage across all of the power supply caps before getting your hands inside the chassis, whether the amp has a built-in bleeder resistor or whether you've used your temporary one.
Get yourself a set of multimeter leads with clips on the end, and use the black one to clip to the chassis. You can then use your regular red lead with its probe one-handed to measure voltages with your other hand away from the amp. This means that, should the worst happen and you get shocked, you won't have your other hand providing a nice path to ground across your chest.
Also ensure that the plug is removed before working on the amp. There's no excuse for leaving it in, as it then exposes you to mains voltage within the chassis.
The only time the plug should be in the wall is for measuring voltages when the amp is running, and then you should be exceptionally careful about where you're probing. Again, use only one hand and have the other lead clipped to the chassis. Also, resist the temptation to rest your measuring hand against the chassis edge for stability or you're likely to find the edge of your hand brushing against the power switch or the IEC inlet.
You may find that for certain areas where it's difficult to get a probe in safely while the amp is running it's better to use a clip lead instead of the probe. Just make sure you've got the amp disconnected and the caps discharged before connecting and disconnecting it.
It sounds a bit frightening, but it's all just common sense stuff once you start getting familiar with amps. Just don't let familiarity breed complacency and you'll be safe.
Paul
I use a 220K 2W resistor with leads soldered on to each end, and then the assembly is heat-shrinked over to completely insulate it. The ends of the wires have insulated crocodile clips, so I can clip one end to the chassis and then the other end to the positive side of the cap I want to drain.
Most amps have the caps arranged in a voltage ladder with dropping resistors between them, so draining one will drain all. However, some amps have the standby switch in between banks, so it pays to have the standby switch on when you're doing this, and certain other amps have diodes in the voltage ladder so draining one cap won't necessarily drain the others.
The key is to measure the voltage across all of the power supply caps before getting your hands inside the chassis, whether the amp has a built-in bleeder resistor or whether you've used your temporary one.
Get yourself a set of multimeter leads with clips on the end, and use the black one to clip to the chassis. You can then use your regular red lead with its probe one-handed to measure voltages with your other hand away from the amp. This means that, should the worst happen and you get shocked, you won't have your other hand providing a nice path to ground across your chest.
Also ensure that the plug is removed before working on the amp. There's no excuse for leaving it in, as it then exposes you to mains voltage within the chassis.
The only time the plug should be in the wall is for measuring voltages when the amp is running, and then you should be exceptionally careful about where you're probing. Again, use only one hand and have the other lead clipped to the chassis. Also, resist the temptation to rest your measuring hand against the chassis edge for stability or you're likely to find the edge of your hand brushing against the power switch or the IEC inlet.
You may find that for certain areas where it's difficult to get a probe in safely while the amp is running it's better to use a clip lead instead of the probe. Just make sure you've got the amp disconnected and the caps discharged before connecting and disconnecting it.
It sounds a bit frightening, but it's all just common sense stuff once you start getting familiar with amps. Just don't let familiarity breed complacency and you'll be safe.
Paul
Re: bleeding caps
angelodp,
You are building a D'Lite Kit right?
If so and you are following the Brown Note layout, it has bleeder resistors in the power supply circuit.
Once the amp is turned off for a few minutes, check across the big filter caps for DC voltage I think you will find it in the millivolt range and draining rapidly.
You are building a D'Lite Kit right?
If so and you are following the Brown Note layout, it has bleeder resistors in the power supply circuit.
Once the amp is turned off for a few minutes, check across the big filter caps for DC voltage I think you will find it in the millivolt range and draining rapidly.
Tom
Don't let that smoke out!
Don't let that smoke out!