Good diagram.
The only thing is, while you've got 2x 12V inputs shown on the diagram, I can't tell what your PSU is putting out as this doesn't show where on your PSU those inputs go to.
Are you using 12V power from the Main ATX12V 2.x connector plug (24 pin - I will just call this Main ATX from now on), or from the ATX12V (4 pin) or EPS12V (8 pin) connectors, or the 6 pin PEG connectors?
Not all PSU's do this, but the Main ATX, the ATX12V/EPS12V connectors and the 6 pin (or 8 pin) PEG connectors are "usually" on different internal rails on the PSU (or there is some grouping, so ATX12V + PEG may be grouped together). This means they usually have different current limits (see the link below about "Rails").
I would try connecting the 5A input to one of the PEG or ATX12V/EPS12V outputs from the PSU, and the heated bed to the Main ATX connector outputs.
Also, you usually need a load on the 5V ATX supply output, so if you don't have one, I would connect a 10 Ohm power resistor (one of the white ceramic jobs, rated to at least 5W) between the 5V and Gnd on the ATX connector. A lot of PSU's expect current to be drawn on the 5V line, and if there is too much on the 12V without a certain amount on the 5V, the PSU will simply shut down.
BTW: For a pic of the connectors so you know which is which, see:
[www.hardwaresecrets.com] - Main ATX, ATX12V & EPS12V connectors
[www.hardwaresecrets.com] - PEG connector (and others)
[www.hardwaresecrets.com] - Talks about the concept of "Rails", particularly near the end of the page.
[www.hardwaresecrets.com] - Pinouts and wire color codes for the connectors.
The only thing is, while you've got 2x 12V inputs shown on the diagram, I can't tell what your PSU is putting out as this doesn't show where on your PSU those inputs go to.
Are you using 12V power from the Main ATX12V 2.x connector plug (24 pin - I will just call this Main ATX from now on), or from the ATX12V (4 pin) or EPS12V (8 pin) connectors, or the 6 pin PEG connectors?
Not all PSU's do this, but the Main ATX, the ATX12V/EPS12V connectors and the 6 pin (or 8 pin) PEG connectors are "usually" on different internal rails on the PSU (or there is some grouping, so ATX12V + PEG may be grouped together). This means they usually have different current limits (see the link below about "Rails").
I would try connecting the 5A input to one of the PEG or ATX12V/EPS12V outputs from the PSU, and the heated bed to the Main ATX connector outputs.
Also, you usually need a load on the 5V ATX supply output, so if you don't have one, I would connect a 10 Ohm power resistor (one of the white ceramic jobs, rated to at least 5W) between the 5V and Gnd on the ATX connector. A lot of PSU's expect current to be drawn on the 5V line, and if there is too much on the 12V without a certain amount on the 5V, the PSU will simply shut down.
BTW: For a pic of the connectors so you know which is which, see:
[www.hardwaresecrets.com] - Main ATX, ATX12V & EPS12V connectors
[www.hardwaresecrets.com] - PEG connector (and others)
[www.hardwaresecrets.com] - Talks about the concept of "Rails", particularly near the end of the page.
[www.hardwaresecrets.com] - Pinouts and wire color codes for the connectors.