| Vivek |
Hi all,
I built an amp sometime ago and it has MJ2955/2N3055 at the output. The amp works normally (no hum etc.) but when working the output transistor heatsinks seem to be conducting some current. I can feel it when I touch the heatsinks. Is this a simple case of the collectors not being insulated from the heatsinks properly?
Thanks in advance for any help.
Vivek |
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| janneman |
Vivek,
Just for the sake of clarity: you feel that there is some VOLTAGE at the heatsink, I guess?
If you had BOTH transistors not well insulated, it wouldn't work. It can be that ONE transistor is not well insulated, and that is a disaster waiting to happen. It would mean that the heatsink is at either +V or -V, and if you connect it to gnd or another point on the amp, pooof!
So, to verify, measure the heatsing voltage with a multimeter against gnd. If there is a voltage, take care of the insulation of the power transistors before you blow up anything.
Jan Didden
PS It is possible that there is nothing wrong, that the heatsink is at zero volts, but that you touch a voltage point with your other hand. Same effect, what you feel is voltage difference, not absolute voltage. |
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| Vivek |
Hi,
This voltage I `feel' in the heatsink is there on both channel heatsinks. I have checked the insulation and the mica washers are all there. There is no voltage between the heatsink and ground.
Vivek |
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| janneman |
Vivek,
You can only feel voltage between two terminals. One apparently is the heatsink. What are you touching with the other hand? Are you maybe barefoot on the floor tiles? Arms on a chassis that is grounded or connected to mains neutral?
Jan Didden |
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| Vivek |
| You are right. Voltage is across two points. But I feel this even if I touch it with one hand with the other hand not touching anything. I might be sitting with this stuff on the bed, on the sofa or standing barefooted on the mosaic-tiled floor but it is still there. |
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| Jennice |
Vivek,
It may be the case that you feel some potential relative to "absolute" ground (your mains supply ground entering the building you live in.)
Potential can couple capacitively and inductively, so the entire amp is not at ground potential. Maybe the entire device is at a potential. Try measuring the heat sink or enclosure potential relative to mains "earth", but do so with a voltmeter that has a high input impedance (digital multimeter or similar). The old fashioned voltmeters with a scale and pointer have much lower input impedance, and thus you will measure a lower voltage.
Jennice |
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| phase_accurate |
| quote: | | This voltage I `feel' in the heatsink is there on both channel heatsinks. I have checked the insulation and the mica washers are all there. There is no voltage between the heatsink and ground. |
If you say ground, I assume you are talking about signal ground of the amp.
In this case there may be a voltage between the chassis and earth , that could be caused by an insulation failure. This can in fact be very dangerous ! :skull: :att'n:
If the chassis is earthed it could even becaused by the electrical installation of your home. So be very careful.
It could also be something less severe (like capacitive coupling in a transformer or mains filter), since such things are difiicult to judge remotely, but it always pays to be careful.
Regards
Charles |
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| Vivek |
Jennice,
I will measure heatsink to mains earth and let you know.
Charles,
I am talking about the signal ground of the amp. Actually, the heatsinks are not mounted on any chassis. I was just testing the amp and the heatsinks are kept separately. The PCBs are just kept in a metal chassis but on a piece of wood. Maybe I should replace all mica washers and check again. |
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| phase_accurate |
Jennice was a little quicker than me. Basically I wanted to say the same but maybe I was alittle less clear.
Regards
Charles |
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| wuffwaff |
Vivek,
are you shure you are feeling a voltage? Sometimes touching the sharp edged heatsinks can give the sensation of feeling a voltage, especially when you let you fingers run over the edges.
William |
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| Vivek |
I had been measuring using the DC scale. I tried it with the AC and between the chassis and 0V rail there is about 40V and between the heatsink and 0V rail there is about 14V.
As far as I can see the only metal parts in contact are the chassis and the mounting clamps on the transformer.
So, William, this probably rules out the sharp edges thingie.
Vivek |
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| wuffwaff |
Vivek,
do you have you chassis connected to the mains earth?
Didnīt you connect the power supply 0V to the chassis?
since you donīt have a chassis the answer to the questions above is probably :whazzat:
You should at least connect the 0V from the power supply via an bridge rectifier to the chassis and the chassis to the main earth. Otherwise you chassis is floating around somewhere between 0 and 230V AC
William |
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| Vivek |
William,
I will try that but tell me one thing. If I connect the chassis to the mains earth, I will be using a 3 pin plug. But commercial systems never have a 3 pin plug. How do they manage to earth the chassis, etc?
Vivek |
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| wuffwaff |
Hi Vivek,
they donīt earth the chassis. Thatīs why you can measure a quite high voltage at the in or output connectors of some components (you normally do this if you want to get the phase of the mains plug right).
As long as the case is made of plastic or is laquered you wonīt feel anything.
William |
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| Vivek |
I connected the 0V supply rail to the chassis and all the voltage is gone from the heatsinks and the chassis. Looks like the problem has been resolved. Thanks to all for your help.
Cheers,
Vivek |
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