Photoshop's not the first tool I'd use to do something like this (Illustrator would be my choice), but you can achieve those results by stroking a path using a square brush with the right settings.
- Create your circular path using the Ellipse tool.
- Select the Brush tool and load the Square Brushes brush set. Select one of the square brushes.
- Open the brush options palette and change the Brush Shape settings:
- First, change the spacing so that there are gaps in the brush stroke (ex. 240%).
- Then change the square to a rectangle so it looks more dash-like by adjusting its "Roundness" (ex. 50%).
- Next, adjust the size of the brush as appropriate for the thickness of the stroke you want (ex. 12px).
- Then change the Shape Dynamics, setting Angle control to "Direction".
- Now select the Path Selection Tool (A) and right-click on your circular path and select "Stroke Path..." and choose "Brush" as your tool.
You should be able to come up with something like this:
The above method is fairly quick, but because of the way the brush is transformed, the results are somewhat crude. So here's a better method using vector paths:
- Use the Ellipse tool to create a circular path.
- Duplicate the circular path and then using Free Transform, shrink the path to create the inside edge of your eventual stroke.
- Combine the 2 paths by subtracting the inner circle from the outer circle.
- Use the Polygon tool to create a triangle (set number of sides to 3 and hold Shift to make sure it's straight) that points down. Move the triangle so that the bottom vertex snaps to the center of your circles.
- Free Transform the triangle and stretch it vertically, pulling the top edge of the bounding box past the top of your circular stroke. Adjust the width of the triangle to the length of the dash (or gap) you want your stroke to have. Apply the Transformation.
- Free Transform the triangular path again. This time set the reference point to the bottom center and rotate by an even angle (e.g. 10°, 15°, 20°, 30°, etc.). Apply the transformation.
- Hold Alt and hit Ctrl+Shift+T to repeat the transformation with a new copy of the path. Do this until you've made a full circle with the triangular paths.
- Select all the triangular paths and combine them additively. Then either subtract (if you want to use them as the gaps) this new path from the stroke path, or combine it with the stroke path via intersection.
- Now you should have a vector path of a dashed circle, and you simply have to go to "Layer"->"New Fill Layer"->"Solid Color..." to add it to your composition.
This method takes longer, but the result will be cleaner. You can also resize the path easily, as well as save it as a custom shape. Or you can just save the triangular paths as a custom shape so you can re-use them to intersect/subtract them from a circular stroke path of different thicknesses in the future.