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Lead spacing for through hole resistor

--- Ok, but the conducting path for the horizontal case will be removing heat from both ends of the resistor through _short_ leads and transferring the heat to the traces. The vertically mounted case will have the heat transferred from one end to its trace through a long lead which will also be radiating toward the body of the resistor as well as giving rise to its own convection currents.

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--- Yes, but in the vertical case the convection currents will carry the heat from the bottom of the resistor along the body until it leaves at the apex of the resistor. In the horizontal case, convection carries the heat from the line closest to the PCB around the body and then to ambient, without creating a temperature gradient up the "pole" standing there in the vertical case.

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--- Probably only where it touches the board, on the bumps over the resistor end caps. There will be some reflection back to the body from the PCB, but how much is going to depend on the solder mask. I think convection, bringing cool air in to the side of the resistor closest to the PCB, will largely negate that, and If I get some time I'll measure both configurations to see what's _really_ happening.

Don't hold your breath though! ;)

--- I think mostly the reason is not to burn up the PCB since those beasties are _designed_ to be abused!

-- John Fields Professional Circuit Designer

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John Fields
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I think that\'s what I said, no?
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John Fields 

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Yup!
Reply to
John Fields 

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Yup, and they dissipated next to zero power.
Reply to
John Fields

that

--- Back in the late '50's, as I recall, MIL-E-5400 (the spec that described how to build military electronic equipment back then) specified that diode lead wires be formed into loops before the free end of the lead was soldered down to its terminal. Isolation from shock was the reason.

-- John Fields Professional Circuit Designer

Reply to
John Fields

On Sun, 04 Jun 2006 16:47:10 -0400, Spehro Pefhany Gave us:

Yes, and not a single one of those resistors were being taxed at even half their rated power, which was my entire point.

Good design is key. One does NOT incorporate a 3 Watt resistor into an application where a resistor that can dissipate 3 Watts is needed. One would call for a five or ten Watt resistor if one had any brains at all.

Reply to
Roy L. Fuchs

On Sun, 04 Jun 2006 21:47:44 +0100, Pooh Bear Gave us:

The kinked lead is for maintaining enough bare resistor lead in the hole to form a proper solder joint. It has nothing to do with any form of stress relief.

Any good QC will tag any resistor that sits in a hole where the body of the resistor is actually butted up against the hole.

Kinked leads are used on horizontally mounted resistors as well, if the application calls for the body of the resistor being elevated off the surface of the PCB. The kinked leads are for maintaining a space between a part and the hole the part's lead slips into.

Reply to
Roy L. Fuchs

somewhere that

have a

That rings a bell. I suspect they are indeed the parts I was thinking of.

Graham

Reply to
Pooh Bear

the

They do but they *also* help support the part. Think single sided pcb.

Graham

Reply to
Pooh Bear

insert

you

boards.

Vibration essentially. Also shock in transport.

Graham

Reply to
Pooh Bear

It's all about component temperarure.

I've seen the same body size resistor rated at 0.25, 0.5 and 0.6 W depending on the materials used.

Graham

Reply to
Pooh Bear

On Sun, 04 Jun 2006 16:09:56 -0500, John Fields Gave us:

The added lead length *almost* sounds like a better deal at this point. :-]

Yes, but it isn't a smoke stack, it is a half inch long (example) ceramic tube. The difference in the micro realm we are in is much smaller than the difference of those big foot long power resistors (which get mounted vertically in a case... all the time.) Trolley braking resistors come to mind.

Not really. When IR emissions are that close to a surface, they do a lot of work, and the heated surface will kill the dissipation rate of the underside of the resistor quite well.

A nice warm blanket of it, in fact.

The basic emissivity of the medium is the main factor, and a matte finish mask will be slightly worse off than a glossy mask.

Of course it will. Air currents will also cool the vertical scenario quite well.

I only do that when the media in my lungs warrants it. :-]

True, but the good designer keeps hot stuff away from his printed circuit cards... for so many reasons.

I even place a stop kink in the ones I installed to elevate it off the PCB a bit more.

Reply to
Roy L. Fuchs

On Sun, 04 Jun 2006 16:28:04 -0500, John Fields Gave us:

Yes. I did not remark poorly on it.

Think about it. The currents heating up the resistance medium (carbon slurry usually) will do so evenly across the part. Since the emissivity of the enamel they use is low, the entire part will likely be at nearly the same temp, regardless of orientation, because the heat can only leave the part as fast as its emissivity allows.

I agree. I am, however, attempting to express the fact that the difference is very little, and far too little to worry about.

I doubt it. The reason why SOME mil spec wirewounds were painted matte black addressed this issue. A high emissivity part will be more efficient at giving up its heat. A high gloss enamel is NOT very emissive. It is almost like putting a blanket on it.

I suppose. They are cheap though. It is really just a lesson that the good designer would remember to think about any power that is actually being dissipated by a part. If that power is significant enough, heat abatement, and management should be in their thoughts.

For most small signal apps, though, I would say it is not a problem, so my initial remark about no situations can be stabbed at... a little bit... Ouch.

Reply to
Roy L. Fuchs

On Sun, 04 Jun 2006 21:58:52 GMT, Roy L. Fuchs Gave us:

the

Actually, a resistor ALL the way into a hole IS a stress problem, but only for the basic idiot that pushes on the part, causing solder creep and tensile load lead breakage issues, so stress relief is part of the reason. Proper solder joint formation is the other part.

Reply to
Roy L. Fuchs

On Sun, 04 Jun 2006 23:05:12 +0100, Pooh Bear Gave us:

the

Not likely one would find SS in a mil environment. Particularly not anything that is mobile or subject to inertial shocks.

But yes, once the solder joint is formed, the kink stops movement on top, and the solder on bottom, making the assembly more reliable.

If it were fully inserted, it would have the same fixturing, but a simple push on the part will snap the trace free from the PCB on the bottom. So yes, it is for stress relief in such settings.

Reply to
Roy L. Fuchs

On Sun, 04 Jun 2006 23:08:21 +0100, Pooh Bear Gave us:

I have seen a quarter watt resistor from a british supplier that looks like our 1/8 watt form factor.

I'll stick with the US made form factors, thank you.

125 mW pushed through either get either just as hot, so I wouldn't be placing much faith in their double power, half sized declarations.
Reply to
Roy L. Fuchs 

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Agreed.
Reply to
John Fields

Never had the 'luxury' of designing to mil spec. Many of my designs have ended up on ss.

We seem to be agreed on that then.

Graham

Reply to
Pooh Bear 

--
I made some time and here\'s what I set up:

I took a standard 100 ohm 5%  1/2 watt carbon film resistor and
connected steel Mueller "mini-gator" clips to its leads about 1/4"
away from the body, suspended it in air horizontally, and put 3V
across it (900mW) in order to heat it up nicely.  Then I let it get
stable and measured the temperature where the leads just exit the
body with an Omega 450AKT thermocouple thermometer and a type K
probe which ends in a 0.050" diameter welded bead. I got 52C on one
end and 53C on the other.

Next, I bent one of the leads over, connected the clip to that lead
so that it was next to the other clip, and oriented the rig so the
resistor was vertical.  I let it stabilize again with 900mW in it,
and measured the temperature at the ends of the resistor again.

This time I got 68C at the bottom of the resistor and 70C at the
top, so it looks like we were both right since I said the temp of
the vertical arrangement would be higher and you said the gradient
across it wouldn\'t be bad.
Reply to
John Fields

Except for the output transistors, yes.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

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