What size core for choke?

Prune · 13th September 2004, 06:34 AM

I need a choke between 3 and 5 H, 400 mA, couple hundred volts ripple. I can't afford to pay $70 for a Hammond 193p, so I want to wind my own. My question is, what size core would be appropriate? Do I need something the size of a MOT, or will something smaller work? Heat is less of an issue since the PSU case will have a fan, so I'm using 23 AWG.

Geek · 13th September 2004, 06:53 AM

About 2-1/4" square centre peg (1.5" x 1.5") will do. Here's an article if you're interested:
http://geek.scorpiorising.ca/windingchokes.html

Prune · 13th September 2004, 07:03 AM

Aha, I've come across that site, but I didn't connect the name to your username before...

In your transformer winding page you say that up to three laminations can be thrown out. I'm rebuilding a large microwave oven transformer as its windings were welded together and it was buzzing too much. It looks like I'll have to throw out four, maybe five laminations (out of 108 layers total) if I'm to be able to fit the original windings on (and rewinding the thousands of turns of the secondary is beyond what I can do); do you think that will be a problem?

Geek · 13th September 2004, 07:15 AM

Quote:

Originally posted by Prune
...do you think that will be a problem?

Nah

Microwave trannies are made of very good silicon steel to take the abuse. You'll do fine missing five or so.

I had to generalize my article, 'cause I have seen some transformers made of *REAL* bad iron and still made UL certification

Prune · 13th September 2004, 08:16 AM

I don't understand why the laminations were welded together, and why they were not interleaved (separate E and I blocks). It doesn't seem to save any construction cost.

I'm using a cap about 135 uF for PFC; I hope it's not too far off as these large ripple AC caps are not cheap. I've seen people use from 20 to 200 uF for MOTs.

Sch3mat1c · 13th September 2004, 09:04 AM

Seems easier to me. The air gap between the E and I segments also softens saturation, which is probably just 5% away on those overrated transformers. (Take a bare MOT, monitor primary current with a scope and shunt resistor, then increase primary voltage to maybe 130-140V. Watch the peaks fly by! Now try the same without the air gap (interleaved)...)

Tim

Prune · 13th September 2004, 09:40 AM

Interleaved will still have gaps between the E and I laminations, as the nail polish prevents electrical contact.

Geek · 13th September 2004, 10:10 AM

Interleaved E-I will have a much higher inductance because of the short magnetic path.

Welding of the tranny is faster and creates an energy-limiting "Class-2" transformer. This is needed because microwave trannies are so temporarily overloaded, that your bacon wouldn't be the only thing burning in 5 minutes

Prune · 13th September 2004, 10:25 AM

So Geek, in that case what about my PFC question?

Prune · 13th September 2004, 12:24 PM

And also, does the direction the windings face when putting them back on the transformer core matter? I can't remember how they were oriented before I took it apart.

13th September 2004, 06:34 AM	#1
Prune Account Disabled Join Date: Mar 2003 Location: Vancouver	What size core for choke? I need a choke between 3 and 5 H, 400 mA, couple hundred volts ripple. I can't afford to pay $70 for a Hammond 193p, so I want to wind my own. My question is, what size core would be appropriate? Do I need something the size of a MOT, or will something smaller work? Heat is less of an issue since the PSU case will have a fan, so I'm using 23 AWG.

13th September 2004, 09:04 AM	#6
Sch3mat1c diyAudio Member Join Date: Jan 2003 Location: Milwaukee, WI	Seems easier to me. The air gap between the E and I segments also softens saturation, which is probably just 5% away on those overrated transformers. (Take a bare MOT, monitor primary current with a scope and shunt resistor, then increase primary voltage to maybe 130-140V. Watch the peaks fly by! Now try the same without the air gap (interleaved)...) Tim __________________ See my Electronics webpage -- the home of Vacuum Tube Drag Racing. The key to being a successful Audiophile: "I reject your reality and substitute my own!"

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13th September 2004, 06:53 AM	#2
Geek diyAudio Member Join Date: Sep 2004	About 2-1/4" square centre peg (1.5" x 1.5") will do. Here's an article if you're interested: http://geek.scorpiorising.ca/windingchokes.html

13th September 2004, 07:03 AM	#3
Prune Account Disabled Join Date: Mar 2003 Location: Vancouver	Aha, I've come across that site, but I didn't connect the name to your username before... In your transformer winding page you say that up to three laminations can be thrown out. I'm rebuilding a large microwave oven transformer as its windings were welded together and it was buzzing too much. It looks like I'll have to throw out four, maybe five laminations (out of 108 layers total) if I'm to be able to fit the original windings on (and rewinding the thousands of turns of the secondary is beyond what I can do); do you think that will be a problem?

13th September 2004, 08:16 AM	#5
Prune Account Disabled Join Date: Mar 2003 Location: Vancouver	I don't understand why the laminations were welded together, and why they were not interleaved (separate E and I blocks). It doesn't seem to save any construction cost. I'm using a cap about 135 uF for PFC; I hope it's not too far off as these large ripple AC caps are not cheap. I've seen people use from 20 to 200 uF for MOTs.

13th September 2004, 09:40 AM	#7
Prune Account Disabled Join Date: Mar 2003 Location: Vancouver	Interleaved will still have gaps between the E and I laminations, as the nail polish prevents electrical contact.

13th September 2004, 10:10 AM	#8
Geek diyAudio Member Join Date: Sep 2004	Interleaved E-I will have a much higher inductance because of the short magnetic path. Welding of the tranny is faster and creates an energy-limiting "Class-2" transformer. This is needed because microwave trannies are so temporarily overloaded, that your bacon wouldn't be the only thing burning in 5 minutes

13th September 2004, 10:25 AM	#9
Prune Account Disabled Join Date: Mar 2003 Location: Vancouver	So Geek, in that case what about my PFC question?

13th September 2004, 12:24 PM	#10
Prune Account Disabled Join Date: Mar 2003 Location: Vancouver	And also, does the direction the windings face when putting them back on the transformer core matter? I can't remember how they were oriented before I took it apart.

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