I generally use a through bolt. I use a 316 stainless bolt. The head is below the PCB. Between the PCB and the core, a place a thin silicon rubber sheet. Above the core goes a thicker silicon rubber sheet, then a large "washer" that is punched to match the core size, after that a store bought washer and a "stop nut" AKA "aviation nut".
BTW: the customer was asking about "repair".
-- -- kensmith@rahul.net forging knowledge
That bolt reduces the Q of the inductor. Its losses are especially large if there is a gap in the center of the pot core.
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I guess Ken was speaking about toroids.
-- Thanks, Fred.
Hello Ken,
A bolt through the hole in a pot core usually isn't a good thing. It's in the magnetic path, to some extent. Anyway, in aeronautics that might be an option, in consumer or even in low cost med electronics the cost associated with mounting all that usually isn't in the cards.
Oh man. A friend told me about an aircraft salvage yard. He said the sights when peeking into the cockpit of a crashed aircraft are really gruesome.
Regards, Joerg
In article , John Popelish wrote: [....]
Yes it does but I've never seen it reduce the Q enough to matter.
-- -- kensmith@rahul.net forging knowledge
In article , Joerg wrote: [...]
316 stainless is about as non-magnetic as you can get for something with iron in it. Try hanging one on a string and pulling it with a strong magnet. It hardly moves. I've seen brass that is more magnetic.A lot depends on what your customer base is, I agree.
[...]I've had two opportunities to know the details and passed on both.
-- -- kensmith@rahul.net forging knowledge
When used through the pot cores of power components, I have seen it make a spot hot enough to push the ferrite above its Curie temperature. At first glance, one might think it would have acted as a heat sink.
Being non magnetic helps a bit, but it is the conductivity of the bolt, surrounded by that di/dt that circulates current around the bolt that makes the hot spot. The bolt, in effect, is in an induction heating coil.
Hello Ken,
It still messes with the inductor, especially with the Q. Yesterday I needed some hardware for a rig and thought stainless would be nice. Went to the local hardware store and almost froze when I saw what that stuff costs. Had left the credit card at home and the cash in my wallet wouldn't have covered it. Went for Cat-8 galvanized...
Regards, Joerg
Was it 316 stainless? I've never seen anything close to that happen.
I can see it with a magnetic type of steel.
-- -- kensmith@rahul.net forging knowledge
I've just never seen the problem. The bolt is very poorly coupled to the inductor in this situation. I find it hard to believe that you see so much rise when I don't.
-- -- kensmith@rahul.net forging knowledge
In article , Joerg wrote: [...]
The good stuff costs more. This is often the way in the world.
BTW: 316 isn't all that good mechanically. 70 series aluminum is stronger in many applications.
-- -- kensmith@rahul.net forging knowledge
SAE grade 8?
Someone once said that every mech eng should be given a chunk of SS and told to machine a part from it. Then they'd be a little less eager to specify it where it isn't absolutely necessary.
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
I think so, but it was a long time ago. I do remember that we had little trouble with half bridge, full bridge and forward converters. The big problems showed up with flyback converters that had significant gaps. This fringed the field in around the bolt. With one example, we had a pretty good working converter that showed signs of falling inductance when load tested. We finally put a thermocouple down along side the bolt and saw the temperature getting to the curie temp. The bolt would burn you. Pulling the bolt, and just poking the thermocouple down into the core showed a much lower temperature rise.
At 100 kHz, the permeability of the bolt is not very significant, and it is essentially an eddy current, skin effect problem. The high resistivity of stainless helps a bit. Using a small size bolt with a thick piece of plastic tubing around it, to keep it away from the edges of the gap helped too.
Is your core gapped? Is the flux swing high and fast?
Hello Ken,
Sure, but that makes it less enticing to use it in a design.
What can I say, we live in the country. So one has to live with what one's gitten at this here trading post.
Regards, Joerg
Hello Spehro,
My father has a degree in metallurgy and one of the first things their professors told them: Don't ever specify a screw of less than 16mm thread size or every worker in the steel industry would be able to torque its head off.
Then one day I (MSEE, not metallurgy) visited my parents with badly busted knuckles. Had just torqued off a 16mm crane hook.
Regards, Joerg
Hummmm..... It sounds like you were pushing things a lot harder than I usually do. This could explain why I've never seen this happen. It could also be that the pot core designs are generally at lowish frequencies.
The mechanically biggest one I did was a current fed forward converter that ran from a car battery and powered a 9 track tape deck. Being a forward converter means that the core was not gapped at all.
[...]I was thinking that the hysteresis loss could explain a temperature rise much larger than the 316 was seeing. In the area of the gap, a magnetic bolt could look like more of the core.
You always have a gap around the bolt. The holes in pot cores never seem to be a standard clearance hole size. You also don't want a tight fit for fear of cracking the core. I've only ever added plastic over the bolt if I worried about electrical problems.
-- -- kensmith@rahul.net forging knowledge
On the smaller design the core is gapped. On the biggest one there is no gap. The running frequencies are from 5KHz to 140KHz. The newer design is using the EFD cores for height reasons.
I do tend to over design the inductive elements. The latest design is the first where I'm really tight on space.
-- -- kensmith@rahul.net forging knowledge
OCXO ===== $350 CPLD ====== $130 Micro ===== $60 Bolt ====== > in many applications.
-- -- kensmith@rahul.net forging knowledge
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