DIY AIR BRAKE WITHOUT THE RHEOSTAT?

[dehughes]

Well, ALL the parts came today...enough to build 4 Airbrakes...(group buy with come local friends).

One question: How do the mounting brackets work for the 100w resistors? Do they snap in on the sides, or....? Also, they are kinda tall brackets for a 10"x6"x2" case....nearly touch the base plate. Maybe they are better mounted on the sides?

[JoeyE]

The brackets can be shortened by folding it twice which is how i did mine.

[dehughes]

Nice....good idea.... I'm still inclined to mount them on the sides of the box, a la what Ken Fischer did on his Airbrake. We'll see what the final layout is.

Also, do the clips just rest partially inside the resistor, or is that bent, V-shaped part supposed to be totally inside the body of the resistor?

[JoeyE]

I bent the v-shape a bit more and put it all in. It just gave me peace of mind that it was holding the heavy resistor better.

[dehughes]

Cool. THANKS.

Also, what gauge/type wire are you supposed to use with this? I have 18AWG PTFE...will that do?

[mhuss]

That's fine. PTFE is actually a good idea, as those power resistors can get HOT.

--mark

[dehughes]

That's fine. PTFE is actually a good idea, as those power resistors can get HOT.

--mark

Cool, thanks....I didn't know if 16 gauge would be required....

[dehughes]

One other question....how hard does one need to tighten those variable tabs for the 100w resistors? I know if you over tighten them you can damage the resistors...so I wager, just tighten them until you feel it snugly attached and get a solid ohm reading?

[horn man]

Back to the wire question, what about 20 AWG solid core PTFE? That's all I've got on hand. I do have some low quality 18 AWG solid core but its just standard plastic insulated (and melts like crazy with too much soldering gun heat!)

[mhuss]

1) Tighten until snug (i.e., it does not move) but no more. The ceramic used in these type of resistor cracks pretty easily, but the weak metal used in the clamp might strip it threads first.

2) I used 22 gauge! This is an attenuator, right? We're not worrying about power losses in the wiring, and the short lengths and relatively low currents involved means no danger of overheating.

--mark

[dehughes]

1) Tighten until snug (i.e., it does not move) but no more. The ceramic used in these type of resistor cracks pretty easily, but the weak metal used in the clamp might strip it threads first.

2) I used 22 gauge! This is an attenuator, right? We're not worrying about power losses in the wiring, and the short lengths and relatively low currents involved means no danger of overheating.

--mark

THANK YOU, MARK!

[horn man]

Yes, thanks for clearing up that wire gauge question, MHuss. JoeyE, I like that graphic you posted earlier. How did you arrive at the attenuation levels by 2 at each step? Also, does anyone know of some down loadable graphics of this sort floating around on the web? (of course I could always "borrow" yours, JoeyE )

[horn man]

My parts for the lite Brake came in from Mouser a little while ago on the FedEX truck. And now it hits me, how do you mount the resistors? And then something in the back of my mind says, yeah they were talking about that earlier in the thread. I guess I assumed the resistors came with brackets. Can someone give me the width of the brackets so I can fabricate something out of sheet metal?

[JoeyE]

Yes, thanks for clearing up that wire gauge question, MHuss. JoeyE, I like that graphic you posted earlier. How did you arrive at the attenuation levels by 2 at each step? Also, does anyone know of some down loadable graphics of this sort floating around on the web? (of course I could always "borrow" yours, JoeyE )

It was made with corel draw and you can use it if you want. It is actually more like 1.6-1.8db attenuation per click but i just made it a whole number 2db per setting total 10db's for 5 positions.

[dehughes]

I found that by using two "slider" 100 watt resistors and an added 20W resistor, I could keep the load impedance as seen by the amp much closer to the load value (8 ohms in this case) vs. the stock Airbrake design.

Note that this is a simple resistive L-Pad design, no rocket science. Perhaps using the wire-wound resistors adds some inductive mojo.

In practice I never use the "-1 db" setting, which is the worst mismatch of the bunch; You could skip this one altogether.

You could do even better with extra sliders, I was working within the constraints of the four sliders I had on hand.

N.B., PTFE-insulated wire or careful layout is a good idea in a high-temp project like this.

--mark

Nevermind, Mark. Figured it out. Your schematic is right on...