Electrical Engineering theory - need help

[metroplex]

I'm doing a senior project and my professor is being adamant about using a breadboard prior to using PCBs.

its a sound policy except my project involves building a 300W RMS subwoofer amplifier that requires +/- 35V DC with about 10A running through each supply lead max.

Will a breadboard be able to handle 300W RMS with jumper wires for the supply lines?

If the breadboard is not a wise way of testing/designing an amp, how should I go about convincing my professor to let me go straight to PCBs?

 

[corkyg]

Many printed circuit boards (PCBs) start out as breadboards. A breadboard can be anything. It is simply a non-permanent, lab version of a design. What your professor is interested in is seeing if you can design and develop the circuitry without using someone else's finished product. Play the game.

 

[metroplex]

I can design and build my own breadboards (not use someone else's design).

I have the software and I can buy the materials for etching them. My point was that a plastic? breadboard with 22-24AWG? jumper wire (from my perspective) isn't going to handle 35V DC/10A nor will the plastic handle the heat given off by the power transistors.

Heck, a 741 started to melt the breadboards we used in the lab.

So what you're saying is that I can successfully build a 300W RMS subwoofer amplifier (4 ohm woofer) using breadboards? I have to build a power supply that converts 12V-14VDC (car battery) to +/- 35V DC (i have an idea on how to do it) supply about 10A per lead to the amplifier board.

Do you have a picture of the breadboards that can handle this power? I know I have to be careful with trace widths because of this excess power. (i.e. motherboard fan power connectors can melt if you plop A Delta 8k on it)

 

[Keego]

I'd do what they did in the old days, have the wires go into a PCB, and just wire the back of the PCB so the front looks good and the back looks like crap


edit: here we go:

wire wrap

 

[sciencewhiz]

standard wire wrap isn't going to hold up any better then the breadboard (probably worse). However, Kyguy is on the right track, there are other ways to prototype a circuit without a breadboard. Considering the fact that you are a senior, you should be able to figure out a reasonable solution (part of being an engineer is being creative with the availible resources)

 

[dkozloski]

Think about the term "breadboard". Your grandmother used a board to cut bread on to keep from scarring the countertop. When your dad was designing his "All American Five" tube radio, he grabbed the breadboard, drove some brads in it and used them to make point to point connections to the components. Go to home depot, get a piece of plywood and some brads, get some hookup wire from Ace hardware and you have yourself a genuine breadboard to prototype your circuit. This is the way real men used to do it.

 

[WarCon]

Just place your control circuitry into the breadboard and take the rest off to something like dkozloski's wood block or an old heatsink to mount your amps anyway. Put a connector in the control and signal wires that go to the amp section (or even use a ribbon cable with alternated grounds).

Anything that works, but no you can't flow 10 amps through a little plastic breadboard............

 

[merlocka]

breadboard with 22-24AWG? jumper wire (from my perspective) isn't going to handle 35V DC/10A nor will the plastic handle the heat given off by the power transistors.

dude, your PCB traces ain't gonna handle that juice either. and your fet's had better be thermally coupled to some large heatsinks, regardless of what they are mounted too.

your professor's suggestion about getting a working breadboard is to simply urge you to get a working prototype prior to spending the time etching copper (and to save passes on copper). I'm pretty sure he's not trying to get you to pass 350 watts of power through hookup wire... or if that his intention then let me know what class he teaches because it would be fun to watch all the students go up in flames.

 

[metroplex]

Why wouldn't a PCB handle that juice???
I would make the necessary traces thicker.

The reason I want to go to PCBs directly is that its a cleaner way of doing things.
I think 12AWG is enough to handle the juice I'm working with. I just figured the professor was referring to the plastic breadboard sh*t we had in lab.

BTW No other prev seniors had dealt with such high amps/volts before. they were more involved with small signal circuits/signal processing/etc... (remote control devices, etc).

I think I'll go with op-amps instead of individual BJTs or MOSFETs (it's what I would do in the real world, source out parts that would cut down costs and R&D time)

I think I'll give the plywood + brads a shot - it sounds like a great idea. I just needed some brainstorming, thanks guys!!!

 

[merlocka]

Why wouldn't a PCB handle that juice???

To get an equivalent of 12AWG on a etchable PCB (which generally use 1Oz copper which is about 1.3 mils think), you would need trace widths of about 3-4 inches.

 

[metroplex]

I was talking about the supply leads (+/-35V DC, 10A) from the power supply to the amplifier connection.

I know it can be done on PCB because I don't see any fancy 1000W RMS amplifiers in cars that use plywood with thick braids..

In fact I found a few 250W-300W RMS designs that use PCBs and the largest trace width is like 1" TOPS and that's for the ground.

 

[merlocka]

Originally posted by: metroplex
I was talking about the supply leads (+/-35V DC, 10A) from the power supply to the amplifier connection.

I know it can be done on PCB because I don't see any fancy 1000W RMS amplifiers in cars that use plywood with thick braids..

In fact I found a few 250W-300W RMS designs that use PCBs and the largest trace width is like 1" TOPS and that's for the ground.

OK. Have fun with your project!

Post back periodically and let us know how it goes

 

[metroplex]

Are you absolutely POSITIVE that you can't use a PCB to make an amplifier???

 

[merlocka]

Are you absolutely POSITIVE that you can't use a PCB to make an amplifier???

I never said that, and of course you use a PCB to make an amplifier.

Although, I am pretty sure that with 1Oz copper you would need trace widths of 3-4 inches to approximate the copper in 12AWG wire...

You are looking for "yes" and "no" answers while asking "it depends" questions

Edit - If I had time I'd love to help out, but I think you would benifit by doing a bit of research on completed projects of this same nature.

Take a look here or here for a few ideas.

 

[metroplex]

Ok gotcha.

I chose 12AWG because i remember reading a book on amp design that said to use 12AWG for the supply leads (supply power frm the supply to the amplifier).

But the largest trace width I saw in that PCB design was maybe 1" thick TOPS (for the ground)

 

[merlocka]

Hint... you basically have 2 design projects, the amplifier and the power supply.

the amplifier can be prototyped on breadboard, with some consideration to outupt stages.

your powersupply is very critical, since depending on your amp (you mentioned using OP-amp's, take a look here for some idea's, I also added some links above) you will need about 400W of output? I'm actually not sure, but it will need to be alot. I never did power supplies.

The output lines of your power supply to the rails of your amplifier had better be huge. 12AWG seems about right, but these should be discreet wires, not PCB traces or hookup wires from a breadboard kit. And be careful. That kind of power can blow stuff up violently.

 

[metroplex]

Yes I'm familiar with the two separate stages.

I was taught to twist the supply lines together to reduce/eliminate cross-talk or something.

The power supply isn't the same as a domestic amp becuase that's really easy to do (120V AC to +/- 35V DC)

but the amp should be able to handle roughly 300W RMS itself - and I found a 250W RMS design that had at the very most 0.5" wide traces (rail traces, transistor traces, and near the filtering capacitor )

 

[Gosharkss]

The real lesson your professor is trying to teach you is simple. Most prototypes do not work the first time for various reasons especially if this is your first design. Going straight to PBC can be a very expensive lesson and odds are you will need to do rework on the board or relay all together before it works to your satisfaction.

Jim Witkowski
Chief Hardware Engineer
Monitorsdirect.com

 

[metroplex]

so what kind of breadboard would you recommend??

the plywood + braids?

 

[Gosharkss]

Since you are using large components that dissipate a great deal of heat and require hefty wiring to handle the power and current draw. The guy who suggested the wooden 1-inch thick breadboard with nails is on the right track IMHO.

 

[metroplex]

I think he said "brads" - I don't know what those are??

And w/ the nails, would that mean I wouldn't be wiring everything on the back - but instead have the wiring and components on the "front" ?

Thanks

 

[Gosharkss]

Brads / nails same thing, just use something you can solder to.

I have done it both ways, where I wire everything in top, or drill some holes in the board where you need them and wire on the back, its up to you.

 

[metroplex]

So the nails act as conductors? Will they allow for a clean signal? It won't matter much since it will be a subwoofer amp (20 Hz to about 200 Hz TOPS).

 

[merlocka]

So the nails act as conductors? Will they allow for a clean signal?

When you get it working, the asthetics will be the least of your concerns. A well designed messy "breadboard" with wires everywhere will work better than a poorly designed, pretty PCB.

Have you modeled the amplifier stages yet?