I'm designing an Open Hardware door-lock-controller for our hackerspace, to these basic requirements:

  • Be made exclusively from available and inexpensive parts.
  • Read external Wiegand keyboard
  • Read external Wiegand RFID reader
  • Control various door-locks and other actuators.
  • Read 125 kHz EM4100 RFID directly with an onboard reader.
  • Passive POE powered, to make cabling easier.
  • Run off unregulated 15-30V DC (cheap and easy high-capacity UPS via SLA)
  • Provide a regulated 12V DC out @ 300 mA to 1.5A peak for external gear.
  • Talk Ethernet to the server.
  • Keep a local list of allowable keys, so only power is needed to let us in.
  • Control external 230V gear using two relays.

This is the second spin of the PCB with a few changes to fix the problems I've found, the biggest change I've made is to throw away the LDO that regulated the 3.3V rail, in exchange for a simple MC34063 based switcher.

I'm pretty sure it's going to work, but I'd like some comments to check if I've overlooked something.

Compromises I've had to make in the design are:

  • The primary (35V) capacitor is stacked on top of the Ethernet transformer to save space.
  • The Ethernet transformer is huge, but I have a bunch of them and they were cheap, so I'm going to use them, even if they are huge.

Main questions

  • Is there a cheaper and better way to implement the two power supplies (I need around 200mA@3.3V and 300mA@12V with peaks to 1.5A) ?
  • Does the MC34063 switcher circuit/layout look sane?
  • I have no idea how inefficient the MC34063 is so I've tried to heat sink it as well as possible using the copper I have available, any hints for improvements?

Schematic in pdf

Click to embiggen PCB image alt text

  • 2
    \$\begingroup\$ What does "passive POE" mean? Some sort of non-standard wiring that looks almost exactly like standard Ethernet that may cause severe damage to anything that is inadvertently plugged into it? 802.3af POE has intelligence for good reason. \$\endgroup\$ – Nick T Nov 11 '10 at 6:58
  • \$\begingroup\$ passive POE is the non-standard way of doing POE that simply uses the two extra pairs for power, it's very common in certain fields because it's cheap and efficient at both ends of the cable. \$\endgroup\$ – dren.dk Nov 11 '10 at 8:02
  • \$\begingroup\$ How did it work out? \$\endgroup\$ – Russell McMahon Feb 5 '12 at 4:43

I'd be really surprised if this operates, at least with any reliability. You desperately need a power plane and a ground plane with components like these.

For instance the ENC28J60SS is a current mode driver, those traces from it to the pulse transformer to the jack are going to be carrying high currents switching at high speeds with no reasonable supply or ground path for them. I'd also be concerned with the crystal as its tuning capacitors have a very long high inductance path to ground. I would guess that this design will emit a ton of EMI, possibly enough to interfere with any nearby electronics.

The layout for the switcher is also huge, i would expect you to have serious amounts of noise and supply ringing throughout this design coming from the switcher and the ethernet components.

You also mentioned POE but there is no POE controller on this board. If your planning to just plug into a POE switch there is a negotiation procedure to request power (in various levels). The only way you could power this off the ethernet port as is would be to find a mid run injector that just dumped voltage on the unused pairs in 10/100 Base-T. If you plugged this into a gigabit switch that could cause major problems. Those pairs also aren't center tapped as they should be for POE. Dumping pins 7/8 directly to ground could cause major problems for both this device and the switch.

There is a lot i could go into but 90% of it is a result of not having power/ground planes, clear that up first.

Also the mid point of R5 and R6 should not be connected to the center tap of the pulse transformer and the jack side of the Output transformer should have a small, high voltage cap to chassis ground at its center tap

  • \$\begingroup\$ I'm with Mark on this one. it's a neat (as in tidy) design, but it's not a very good one. I recently designed a PoE board and used the LM5071 PoE switchmode controller; it was a breeze to build, works very well and would give you the types of voltages at the current levels you are asking for. Also, your large electrolytic caps are overlapping other parts/access screws, it might be tricky to mount. \$\endgroup\$ – akohlsmith Nov 10 '10 at 22:21
  • \$\begingroup\$ About the power planes: I'm not going to use more than two layers of PCB, so I can't spare two layers for power, but I will certainly reorganize the Ethernet components to allow me to get a largeish local groundplane under them, tighten up the power supply around it and to get the crystal+caps tightened up. \$\endgroup\$ – dren.dk Nov 11 '10 at 8:17
  • \$\begingroup\$ About the POE: Yes, I'm not using standard POE, but the, in some fields, more common non-standard passive POE. \$\endgroup\$ – dren.dk Nov 11 '10 at 8:20
  • \$\begingroup\$ About R5&R6: They aren't connected to the center tap, I'm just sloppy at drawing, sorry. \$\endgroup\$ – dren.dk Nov 11 '10 at 8:22
  • \$\begingroup\$ About the switcher: Yes, it's sort of spread out, but other than to turn the MC34063 90 degrees to get a shorter wire from pin 2 to the coil and the diode I don't see how I could possibly tighten it up. \$\endgroup\$ – dren.dk Nov 11 '10 at 8:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.