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Old 1st November 2012, 08:20 AM   #11
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Is this purpose very different from the kevlar sleeves found on some power resistors? Those usually surround the component body, whereas these ceramics surround the leads.
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Old 1st November 2012, 10:45 AM   #12
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Quote:
Originally Posted by DRC View Post
The heat issue is less of a problem with modern PCB materials. The old paxolin type scorch really easily. The downside of standoff is both mechanical (just look at those ceramic coffins on the OP image) and a slight increase in the loop area for inductive pickup.

dc
But still a very good idea to let there be some distance to the PCB, as the benefit is prolonged service time anyway.
Se very very few downsides with standoffs.
Ceramic ones are very good as the also provide some mechanical stability aswell.
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Old 1st November 2012, 11:50 AM   #13
RJM1 is offline RJM1  United States
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Also metal spacers.
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Old 2nd November 2012, 09:06 AM   #14
djk is offline djk
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"Those usually surround the component body, "

Usually because the sleeve is cheaper than buying a flame-proof resistor.
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Old 19th November 2012, 12:25 PM   #15
freax is offline freax  Australia
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I prefer to mount all resistors vertically as a matter of practice, this saves space and permits many more 90 degree turns on prototyping board than would mounting them flat to a board.

Mounting them vertically is my personal preference but what you see done in commercial equipment isn't necessarily the best practice to do at home.

I wouldn't for example mount an IEC socket to a PCB board, or a pot, or a heatsink (if I can avoid it), mechanical stresson a PCB is a serious issue with me because I want my equipment to be transportable and survive a very rough road trip which could be as long as 1,000km. And the way I want to do that is by reducing on-pcb potential/kinetic energy when a sudden stop is encountered.

But thats my own paranoia.

I'm sure that with larger 5w-10w resistors that are driven hard that mounting them vertically would scorch the pcb if not soon, in time and that my friends can result in dry joints.

Resistors get HOT if they are dissipating a lot of power.

Powering up an electric resistor to 1.5 Watts - YouTube

Last edited by freax; 19th November 2012 at 12:36 PM.
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Old 19th November 2012, 02:12 PM   #16
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Back then many components had coating running onto the leads. It was common to use spacers to avoid the coated lead getting into the pcb hole and causing a dry joint.

We also had beads on tantalum capacitors and little plastic frames for low power vertical mount resistors for the same reason
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Old 20th November 2012, 06:35 AM   #17
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Hi Freax

As for mounting axial components vertically, you get a design that is prune to vibration stresses on every component mounted. Actually more stress to the solder joint than most PCB-mounted pots and heatsinks will ever make.
And a strip of leads from a chassis-mounted pot down to suitable place on the PCB wich often have a stretch about 100mm or so will cause much more trouble.

I have a number of TANDBERG- Radioes from the 60, 70 and 80 here, ant the elder of them have shown a remarkeable stability as for the pots and other PCB-mounted components. Some has had a need of resoldering of the lugs of the pots, some not.
But this is due to bad solder in the manufacturing process. Becvause theese solder joints that failed often was some where the components actually was mounted rigidly and stable. The solder itself was the problem.

I have in many amplifiers and other equipment seen vertically mounted Hi power resistors having scorched the PCB. Especially in hybrid equpiment from the 60.

But I agree on Your statement that power resistors driven HARD gets awfully hot sometimes. Luckily we doesn't see designs involving such hard driven resistors too often.
And mounding them with sleeves to get good air-distribution around them is always a good idea. And horisontally mounted they have a slightly better dissipation of the heat.

The very best way to ensure good solderjoints anyway is to have PCBs there is plated through on every critical solderjoint.
This ensures the very best reability any way one mounts the components.
And as one ensures that hot devices has as long leads as nessecary to keep the heat away from the joint, we'll never have to bother if the solderjoints will dry out due to big heavy hot components.

Tested and tried out in PA-systems built over a 35 year period.
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Old 20th November 2012, 01:08 PM   #18
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At this amplifier that you have in your hands many things depend on ""voltage drop"" which obviously produces heat ... now days modern designers use current mirrors and sources and sophisticated regulators in replace of classic zener and heat producing resistors . ( generally also designs become far more effective now days )

hot resistors will require more protection and dissipation methods ...

By the way nice Marantz you have in the photo ... wonderful big VU meters ...

kind regards
sakis
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Old 21st November 2012, 01:22 AM   #19
freax is offline freax  Australia
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Quote:
Originally Posted by TANDBERGEREN View Post
Hi Freax

As for mounting axial components vertically, you get a design that is prune to vibration stresses on every component mounted. Actually more stress to the solder joint than most PCB-mounted pots and heatsinks will ever make.
And a strip of leads from a chassis-mounted pot down to suitable place on the PCB wich often have a stretch about 100mm or so will cause much more trouble.
Thank you for that valuable information about vibration resistance and induced stress on soldered joints.

Yes, I was going to use those little plastic terminal thingies to cater for wiring for chassis mounted pots and use pure copper strand for wiring. aka CAT5 cable.


Quote:
I have a number of TANDBERG- Radioes from the 60, 70 and 80 here, ant the elder of them have shown a remarkeable stability as for the pots and other PCB-mounted components. Some has had a need of resoldering of the lugs of the pots, some not.
I always put that down to a chemical reaction in the soldered joint to the atmosphere over time (oxidation), but you're saying that its a vibration induced crack/stress fracture in the soldered joint?


Quote:
But this is due to bad solder in the manufacturing process. Becvause theese solder joints that failed often was some where the components actually was mounted rigidly and stable. The solder itself was the problem.
Yeah, as I said above.

Quote:
I have in many amplifiers and other equipment seen vertically mounted Hi power resistors having scorched the PCB. Especially in hybrid equpiment from the 60.
Yes, I only really vertically mount the 1 watt and below resistors vertically.

Quote:
But I agree on Your statement that power resistors driven HARD gets awfully hot sometimes. Luckily we doesn't see designs involving such hard driven resistors too often.
Yep, I've seen plenty of tv-sets that are like this. You could pickup a set from the 1960's here and replace the capacitors in it and have a large spool of solder to do almost every joint in it and it will spring back to life, thats how durable the resistors are when they're mounted in free-air even when running on high-B+ voltages.

In a cabinet with vent holes on the bottom and the top, which in combination with the hot tubes creates a nice slow convective flow of air.

Kinda wished I didn't toss out that tv set now... *stares at his CRO on the bench*

Quote:
And mounding them with sleeves to get good air-distribution around them is always a good idea. And horisontally mounted they have a slightly better dissipation of the heat.
I tend to think that perfboard has the best chance of cooling resistors, with all of those holes in perfboard it acts as a sort of barrier between the cool air below the PCB board and the warm air above, and this generates currents of cool air that if holes are drilled (as in perfboard) in right underneath the resistor it will cool the resistor far more effectively than any component mounting scheme above on a sealed (no holes) pcb.

Quote:
The very best way to ensure good solderjoints anyway is to have PCBs there is plated through on every critical solderjoint.
Could I get clarification on 'plated through'?

You know one thing that I don't figure, is why ceramic insulators on the legs of sensitive components like transistors aren't more common place.

I mean if you have a power transistor or a diode even that is designed in a circuit to get blazing hot even when mounted to a heatsink, why not put these ceramic heat insulators on its legs to improve the longevity of the soldered joint beneath it?

Last edited by freax; 21st November 2012 at 01:30 AM.
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