zigzagjoe
Well-known member
Primary fuse should match the output rating of whatever AC->DC 5v can provide + tiny overhead. I suggest that rather than going for the utter maximum the original can provide, instead design for a practical maximum (all slots populated, a couple drives, accelerator, etc.) as to save costs on requiring a 300 watt 5v PSU. This part of the "PSU" design should be sized to taste anyways - a IIci won't have the same power demands as an IIfx or Quadra 950, for example.
The original PSU fan needs to be preserved or replaced to cover cooling needs. Likely can just provide a JST connector (if the original fans use a connector) and/or a header that the fan leads can be soldered to off the +12v.
Yes, U1 is an abstraction for the DC-DC module(s) or circuits for +- 12 generation.
It is likely that finding a DC-DC that can do enough on the +12 will be well overkill on the -12. So two separate circuits/modules may be required. For cost effectiveness, I'd suggest finding reference DC-DC designs with known parameters (often found in datasheets for switching ICs) and copying that, rather than buying a module. Reputable good quality DC DC modules = expensive, and vendor lock-in.
Fuse on the 12v isn't essential, as AC/DC-DC switching regulators typically have built in over current/short circuit and over temp protection as a minimum. At the amperage we're talking on the 12v rail, regulator will shut down due to OCP on a short before a fuse could blow.
Common negative / ground: Any DC/DC happening inside the PSU needs to share a single common negative output as the main +5V PSU.
Some DC-DC designs provide a new common output that has a voltage differential as compared to the input common. This can cause problems if you join the input common to the secondary DC-DC's output common. Shouldn't be a big problem to deal with, but it is something to be aware of.
As a simple example to think of: you could get a -12v voltage differential by reversing polarities: +12v as -/common, -/common as your +. However, if you try to feed your "-12v" to any IC or other circuit referencing earth ground or the input common, you'd end up shorting +12v, which leads to fireworks.
Chassis ground off the AC input is not connected to PSU negative output inside the casing. This grounding is so there is ultimately the chassis is always grounded and you can't end up with a case where chassis becomes energized with +120vAC or +12vDC etc. Also helps to supress EMI. However, chassis ground doesn't see current from any AC->DC: if current is flowing into chassis ground, this is a Bad Thing and represents a shock hazard. It'll trip GFCI also.
Conclusion: PSU negative output to be kept seperated from chassis ground.
BTW: I am by no means an expert in this stuff, just have done quite a bit of tinkering. If I'm wrong in any of this, folks, please chime in to correct me
The original PSU fan needs to be preserved or replaced to cover cooling needs. Likely can just provide a JST connector (if the original fans use a connector) and/or a header that the fan leads can be soldered to off the +12v.
Yes, U1 is an abstraction for the DC-DC module(s) or circuits for +- 12 generation.
It is likely that finding a DC-DC that can do enough on the +12 will be well overkill on the -12. So two separate circuits/modules may be required. For cost effectiveness, I'd suggest finding reference DC-DC designs with known parameters (often found in datasheets for switching ICs) and copying that, rather than buying a module. Reputable good quality DC DC modules = expensive, and vendor lock-in.
Fuse on the 12v isn't essential, as AC/DC-DC switching regulators typically have built in over current/short circuit and over temp protection as a minimum. At the amperage we're talking on the 12v rail, regulator will shut down due to OCP on a short before a fuse could blow.
Common negative / ground: Any DC/DC happening inside the PSU needs to share a single common negative output as the main +5V PSU.
Some DC-DC designs provide a new common output that has a voltage differential as compared to the input common. This can cause problems if you join the input common to the secondary DC-DC's output common. Shouldn't be a big problem to deal with, but it is something to be aware of.
As a simple example to think of: you could get a -12v voltage differential by reversing polarities: +12v as -/common, -/common as your +. However, if you try to feed your "-12v" to any IC or other circuit referencing earth ground or the input common, you'd end up shorting +12v, which leads to fireworks.
Chassis ground off the AC input is not connected to PSU negative output inside the casing. This grounding is so there is ultimately the chassis is always grounded and you can't end up with a case where chassis becomes energized with +120vAC or +12vDC etc. Also helps to supress EMI. However, chassis ground doesn't see current from any AC->DC: if current is flowing into chassis ground, this is a Bad Thing and represents a shock hazard. It'll trip GFCI also.
Conclusion: PSU negative output to be kept seperated from chassis ground.
BTW: I am by no means an expert in this stuff, just have done quite a bit of tinkering. If I'm wrong in any of this, folks, please chime in to correct me