Heat Sink is Positive ground, what does this mean?

[Billy Mayo]

At work , the heatsinks to all the power transistors, etc. is positive ground, mostly I have just seen heatsinks that are negative ground. My manger said that i should be careful when touching the heatsink with my hands plus it's easy to short out when the heatsink is positive ground.

Why would a designer use a positive ground for a heatsink?

Any reasons why the heatsink is positive ground?

 

[blueroomelectronics]

Because the tab or flange of the device is not ground.

 

[Billy Mayo]

Most power transistors, power diodes, power FETs' I have seen are ground

How do you know if the tab or flange on the device is not ground? any test I can do with my DVM meter or the datasheets will tell?

Any reason why a designer would choose components or devices that was not grounded or needed positive ground?

 

[blueroomelectronics]

Datasheets always tell you. Sometimes heat sinks are electrically isolated but if there is no real need to most engineers will not bother.

 

[Billy Mayo]

So why didn't the designer just choose the negative grounded devices?

I don't see positive grounded devices much or a heat sink that is positive grounded? what is a positive grounded heat sink used for?

Any advantage of using a positive heat sink?

 

[crutschow]

What do you mean by positive and negative ground? Generally there's only one ground and it has no polarity.

The heat-sink tab of a power transistor is connected to the drain/collector (not source/emitter) for MOSFETs/BJTs, unless the package is specifically isolated. Thus if you connect the transistor to a heat sink without an insulating washer, then the heatsink is at the same voltage and polarity as the transistor drain/collector. So depending upon the transistor polarity, the heatsink could be either at ground or at the positive or negative supply voltage, depending upon the circuit design. The polarity of the heat sink is immaterial as long as it's isolated from the chassis and it is not something a designer is usually concerned about.

Try not to generalize from a limited number of observations. Perhaps the transistors, diodes, and FETs you've seen have one terminal connected to ground but there are many circuits where they aren't.

 

[Billy Mayo]

the heatsink could be either at ground or at the positive or negative supply voltage

I have only seen heatsinks that is at ground zero volts

When is the heatsink at a positive voltage or negative supply voltage other than zero volts?

Isn't it dangerous to put your hands or work around a heatsink that is other than zero volts ground?

 

[dougy83]

When is the heatsink at a positive voltage or negative supply voltage other than zero volts?

Crutschow answered this above. Read his comment.

Isn't it dangerous to put your hands or work around a heatsink that is other than zero volts ground?

It would be dangerous to you if the voltage magnitude is high enough. It would be bad for your circuit if you touch a conductor between the heatsink and some other potential (e.g. short it out by touching a screwdriver between the heatsink and the chassis/ground).

 

[ChrisP58]

Most power transistors, power diodes, power FETs' I have seen are ground

How do you know if the tab or flange on the device is not ground? any test I can do with my DVM meter or the datasheets will tell?

Any reason why a designer would choose components or devices that was not grounded or needed positive ground?

I have never seen a transistor or diode that had a ground connection as it came out of the box. How a component is connected is purely a matter of what the design engineer wants it to do in his circuit.

As for how an engineer uses those components tend to be dictated by how components that he needs are commonly built by the manufacturers.

Mosfets and BJTs that come in TO-220 and similar packages usually have their drain terminal connected to the tab. (See crutschows post #6) For an N polarity device, that means the positive part of the circuit needs to be connected to the tab. If the tab is directly connected to the heatsink, the heatsink will be positive.

The next power component likely to be on a heatsink will be the rectifier. Again, most rectifiers in tabbed packages have the cathode on the tab. Since the cathode is going to be the positive output of the rectifier, the heatsink, if not insulated from the tab, will be positive.

But one thing that you need to be aware of, is that it really doesn't matter which polarity is connected to the heatsink in a high voltage circuit. A shock hazard is present anytime you have high voltage in a circuit. Even if there are no heatsinks. This is especially true if it is the front end of an off-line switching power supply. Even if the heatsinks were at 'ground' (the negative terminal of the bulk capacitor) it would still be at a lethal voltage with respect to earth ground.


Billy, some of the questions you have asked have been of the "why is a product built this way", or "why component X was chosen". There are no universal answers to those questions. If you give the same product spec to 10 different engineers, you will end up with 10 different designs. Granted, 8 or 9 of them may be similar as there are 'common' ways to approach many tasks. But they won't all be the same down to each individual component.

Also, engineering is always about trade-offs. Product safety, cost, size, efficiency, component count, ease of assembly, components already in use in other products, etc. All of these affect each other in sometimes very complex ways. An engineer will take lots of factors into account when working through a design task.

 

[Billy Mayo]

My manager was saying that a lot of times Heatsinks are FLOATING because they want to separate and isolate low voltages and high voltages or digital grounds from analog grounds

Why would a designer or circuit want a bunch of heatsinks to be Floating?

My manager said that the Heatsinks that are Floating can either be negative ground or positive ground

Any other reasons why you would want a heatsink to be floating?

 

[ericgibbs]

Why would a designer or circuit want a bunch of heatsinks to be Floating?

hi,

Higher voltage isolation can be achieved by isolating the heat sink from external voltages.

For grounded/common heat sinks the isolation of the semiconductor from its heat sink is limited to what the thermal/isolation pads on the semiconductors can withstand

 

[atferrari]

negative ground or positive ground

Actually I do not care why you insist in using those expressions but what is the meaning of them? Ground is just ground AFAIK.

 

[Billy Mayo]

If the heatsinks aren't floating or isolated from each other what will happen? it will cause what?

 

[arunb]

most laser diodes are positive grounded, which means there will be some voltage between the body and the ground (-ve).

 

[ericgibbs]

If the heatsinks aren't floating or isolated from each other what will happen? it will cause what?

hi,
Surely you can imagine what would happen IF the devices on the heat sinks required isolation and the isolation was not present.??

Its not rocket science.

Tell us what you think would happen in the above case.?

E.

 

[WTP Pepper]

My manager was saying that a lot of times Heatsinks are FLOATING because they want to separate and isolate low voltages and high voltages or digital grounds from analog grounds

Why would a designer or circuit want a bunch of heatsinks to be Floating?

My manager said that the Heatsinks that are Floating can either be negative ground or positive ground

Any other reasons why you would want a heatsink to be floating?

One reason is that in mass production, mica isolating washers cost money and if there is no reason to isolate a heatsink then why bother.
Sure it maybe sat at a high voltage, but that's what cases are for.
Last time I designed a MOSFET power amp for home I left the heatsinks floating voltage wise as there was no reason to do otherwise.
Heat flows through the air, not the ground connection.

Also an incorrectly grounded heatsing could cause unwanted feedback into sensitive input stages to an amp as the high power output(s) could be capacitively coupled back via the mica washer.

As for positive and negative grounds...try questioning your manager or get him to post here....

 

[Billy Mayo]

I have no idea what will happen or cause

maybe it will cause hum , noises, oscillations?

 

[KeepItSimpleStupid]

Let's use the pos/neg ground concept correctly and appropriately. I'll let you use the conceps of your heat sinks being above groud, below ground, floating or grounded, but not positive ground UNLESS all voltages are referenced to this heat sink.

Pos ground is a term used in older automobiles where the pos terminal of the battery was connected to the car chassis.

The concept is not even used in telephony. TIP and Ring are the concepts. TIP is at -48 VDC relative to ground. Why? Because corrosion is less.

It does seem like "ground" is a misunderstood concept.

 

[Billy Mayo]

Why would u want a heatsink above ground?
Is a floating heatsink above ground? Why would u want a heatsink to be floating and not grounded?

 

[kubeek]

A floating heatsink is floating wherever it´s pushed by electric fields, so it could be basically anywhere.