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Safe power for stamp project from 16/32 VAC transformer? — Parallax Forums

Safe power for stamp project from 16/32 VAC transformer?

qxoticqxotic Posts: 47
edited 2011-08-23 13:23 in BASIC Stamp
I have a keypad controlled door project. This will be the second one. The first one is in operation and working fine. For the first one I was able to repurpose a power supply from a broken DVD player.. it had a connector just like for a Hard Drive which I pulled the power from. It operated a 12VDC door latch.

Differences, this one will be to replace a commercial one which I would like to re-use some parts of. I want to use the door latch but this one is 24VDC. I would like to re-use the transformer supplied by 110 VAC and has both a 16 and a 32 VAC tap. I also have a large bridge rectifier from the old system I can use.

Given this as my supply... how can I safely connect the transformer to power my super carrier board with a BS2pe stamp?

My boards power everything on 5V except for a speaker/beeper and the latch. The latch for the 12VDC door was tripped by a 5VDC relay. Both the latch and the beeper were initiated through opto's.

I know the super carrier board itself can clean the signal up some, but rectified the 16V is still too high... what other precautions should I take?

Thank you,
Daniel

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2011-08-05 13:45
    The main problem is that the regulator on the Super Carrier Board is likely to overheat and turn itself off. What happens is that the regulator turns all the excess power (voltage at a given current) into heat and it has to get rid of the heat somehow. There is a heatsink on the regulator, but we're talking about a bit of heat. It all depends on how much current is needed from the 5V regulator. If it's just for the Stamp and a couple of optocouplers and LEDs, you're talking about 100mA to maybe 250mA. That would produce (20V - 5V) * 0.1A = 1.5W of heat up to (20V - 5V) * .25A = 3.75W. Either case could cause the regulator to overheat.

    The best answer would be to use a different power source for the Super Carrier Board using some kind of cheap "wall wart" that outputs 7V. If you really have to use the 16VAC transformer, put some kind of switching regulator between the DC output from the bridge rectifier and its filter capacitor and the Super Carrier Board's input. You can get a simple switching regulator from Dimension Engineering that'll do it.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2011-08-05 15:50
    I find it curious how Parallax made the power distribution different on the BoE and the Super Carrier.
    On the BoE, the BS2 runs on "V_in".
    On the Super Carrier, the BS2 (likewise the BS1) runs from VR1 ["V_dd"].
  • qxoticqxotic Posts: 47
    edited 2011-08-10 09:16
    Thank you for the input. I looked up a couple of Open-Frame power supplies I think might work well. One is Mean Well PD-65B AC/DC Power Supply Dual- Output 5 Volt24 Volt 3.5A/2A 65.5W The other is an EOS Power USA LFMVLT80 AC/DC Power Supply Quad- Output 5 Volt12 Volt-5 Volt-12 Volt 10A/3A/0.8A/0.8A 80W
    They are the same price. Which do you think would be a better fit? The first one has the full 24V but I worry if the tolerance is not good enough on it and the 5V line drops that it would not power the stamp, logger, RTC and things alright. The second one has a 5 and 12 I could cross for 7V if needed.. but does not have true 24V..although it does have enough current on both the 12V pos and 12V negative I think it would work to power the latch.
    What are your thoughts? Looking forward to the input.
    Thank you,
    Daniel
  • Spiral_72Spiral_72 Posts: 791
    edited 2011-08-10 12:13
    You could that 16V through a rectifier, filter and use a popular switching regulator right to 5V, or a switching regulator to 7-9V, then through the Vreg on your carrier board.

    Adjustable switching regulators are also available if you can't find a 9V unit, but these would be much more efficient than a linear regulator, develop much less heat and, cost a bit more than linear pieces.
  • qxoticqxotic Posts: 47
    edited 2011-08-23 13:23
    Solution... I have decided to use a salvaged computer power supply. I tested it using the -12V and +12V for the 24V needed to operate the latch. I have used salvaged computer PS's for camera systems several times in the past and they have worked perfectly. We'll see how this one goes after I get the rest of the parts for the project in. Still looking for answers on the thread I started about the keypad protocol.

    I am not sure how to edit my posting to show it is solved?

    Thank you.
    Daniel
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