tangent
Top Mall-Fi poster. The T in META42.
Formerly with Tangentsoft Parts Store
- Joined
- Sep 27, 2001
- Posts
- 5,969
- Likes
- 58
Beware that if you use the LM317 that way, you need to use a dual-secondary transformer or two independent transformers. I point this out only because it's not shown, and thus might not be obvious.
Regarding your layout, some comments:
1. Your bottom-side traces are fairly thin. This is a power supply, remember. Stiffen those up where you can.
2. The whole-top ground plane is probably a mistake, electrically. You'd have to build it and measure to be sure, but that's my sense from just looking at it. It's providing a coupling path between what you should be thinking of as three separate circuits: AC, unregulated DC, and regulated DC. It might be better to split the plane into those three separate parts, put a good 100 mils or more of space between them, and join them by short trace jumps.
EDIT: I say three circuits, but you only need two planes, unreg and reg DC. Get the plane entirely away from the purely AC side of things. This probably means rotating the bridges, and maybe running a short trace from the one grounded pin into the larger plane.
3. It looks like you're using the official EAGLE mounting hole parts in the corners, so you don't actually need to make those notches in the plane. EAGLE will flood the plane tightly around them, going no nearer than is safe, since it knows the size of typical mounting bolt heads for the shaft diameter you've selected. If you want more space, play with the DRC settings. You have been running DRC, right?
4. R5 and 6 might need to go to 1/2 W. Do the math.
5. The big C7/8 probably isn't very helpful. This is especially true if you're going to have a longish DC cable on the output of this thing, wiping out the low ESR you're buying with that part. Big caps here are only helpful if the downstream circuit doesn't have its own big power rail cap(s). I'd go with the lower values recommended in the LM317 datasheet. If you do, C11/12 go away, too, since lowering the ESR of this cap is again pointless. We're just optimizing local regulator performance here. The downstream circuit has to fend for itself unless you're using very short wire jumps to it so that the output ESR of the regulator actually matters.
6. Speaking of hookup wires, you're only allowing for about 22 ga here. If you can move to the 156 mil pitch Molex KK variant, you can use up to 18 ga, better for low impedance.
EDIT 7. Consider snubbing all of the diodes in your bridges, rather than just across the +/- pads as you're doing now. 10pF ceramics are cheap and may give a measurable improvement in noise.
Regarding your layout, some comments:
1. Your bottom-side traces are fairly thin. This is a power supply, remember. Stiffen those up where you can.
2. The whole-top ground plane is probably a mistake, electrically. You'd have to build it and measure to be sure, but that's my sense from just looking at it. It's providing a coupling path between what you should be thinking of as three separate circuits: AC, unregulated DC, and regulated DC. It might be better to split the plane into those three separate parts, put a good 100 mils or more of space between them, and join them by short trace jumps.
EDIT: I say three circuits, but you only need two planes, unreg and reg DC. Get the plane entirely away from the purely AC side of things. This probably means rotating the bridges, and maybe running a short trace from the one grounded pin into the larger plane.
3. It looks like you're using the official EAGLE mounting hole parts in the corners, so you don't actually need to make those notches in the plane. EAGLE will flood the plane tightly around them, going no nearer than is safe, since it knows the size of typical mounting bolt heads for the shaft diameter you've selected. If you want more space, play with the DRC settings. You have been running DRC, right?
4. R5 and 6 might need to go to 1/2 W. Do the math.
5. The big C7/8 probably isn't very helpful. This is especially true if you're going to have a longish DC cable on the output of this thing, wiping out the low ESR you're buying with that part. Big caps here are only helpful if the downstream circuit doesn't have its own big power rail cap(s). I'd go with the lower values recommended in the LM317 datasheet. If you do, C11/12 go away, too, since lowering the ESR of this cap is again pointless. We're just optimizing local regulator performance here. The downstream circuit has to fend for itself unless you're using very short wire jumps to it so that the output ESR of the regulator actually matters.
6. Speaking of hookup wires, you're only allowing for about 22 ga here. If you can move to the 156 mil pitch Molex KK variant, you can use up to 18 ga, better for low impedance.
EDIT 7. Consider snubbing all of the diodes in your bridges, rather than just across the +/- pads as you're doing now. 10pF ceramics are cheap and may give a measurable improvement in noise.