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I'm hoping you smart folks can help me. I'm puzzled by what I thought would be a simple DC battery to DC adapter swap.

The device I'm trying to mod is a a microchip activated pet door (SureFlap Microchip Pet Door on Amazon for about $150).

It normally takes 4x Alkaline AA batteries. The manual does NOT recommend using rechargeable batteries because they may trigger a battery replacement warning due to their lower voltage.

I thought, I could take a 6V 3A adapter (actual 6.3V on DMM) and splice on some alligator clips, clip them onto the battery springs and not have to worry about my cats being locked in/out due to a dead battery. I was WRONG.

This mod resulted in a powered device, but no longer would the door trigger to my pet's microchips. Switching back to the batteries eliminates the issue completely. therefore, I know the device is not broken.

I have confirmed power is being provided to the device, by taking my DMM probe to the springs on the device, and by pushing the "Learn" button on the device, which puts the door into the learn mode (unlocks the latch mechanism to allow a pet to pass through and register, it's supposed to re-latch to confirm a learned chip).

Issues while on DC adapter power:

  1. Putting the door into "Learn Mode" initiates the unlock mechanism for the pet to pass through, but does not complete the learn process (re-locking) no matter how many times I try. It just hangs perpetually unlocked in this mode until it is canceled. Again, going back on battery only mode I can learn a chip easily.

  2. The device has a memory function so microchips do not need to be re-learned after a battery swap. However, previously learned chips no longer trigger the door to open. I can no longer get the door to trigger for my cats. Again, going back on battery only mode I can learn a chip easily.

I thought initially I had an under-powered adapter, as I had a 6V 1A (6.0V on DMM) lying around. I bought a 3A adapter mentioned above and the problem persists. I am absolutely puzzled.

Is there something I am missing about the characteristic differences of DC batteries and power supplies? I've had success in the past doing the exact same mod with toy motors and the like, but I'm wondering if the issues is due to the device being a more complex electrical device.

4x Alkaline batteries are 6.3V on my DMM, so I don't think over-voltage is an issue. Also, my 6V 1A power supply is dead on 6V.

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    \$\begingroup\$ Are the four batteries in the device all in series? They probably are, but it's good to get the basic questions out of the way first. It could be running on three volts with two parallel sets of cells for longer battery life. \$\endgroup\$ – Hearth Nov 29 '18 at 2:41
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    \$\begingroup\$ What's inside this contraption? Is it a motor or a solenoid for example? Your power supply might not be able to handle the stall current. \$\endgroup\$ – replete Nov 29 '18 at 2:53
  • \$\begingroup\$ Can you get an ammeter on the 10A setting between the battery + and the device +, and measure the amps it draws when turning the motor? (you'll need to use a peak hold for this) \$\endgroup\$ – ThreePhaseEel Nov 29 '18 at 3:04
  • \$\begingroup\$ There can be a tap in the battery holder - some circuit needs less than 6V and that is made with an extra wire between the cells. Needs checking, if 2 wire only output isn't clearly visible. \$\endgroup\$ – user287001 Nov 29 '18 at 3:24
  • \$\begingroup\$ My batteries are in series. They are a in a 2x2 configuration, but the springs on one end doesn't tap out to feed a 3v circuit. It just loops around. \$\endgroup\$ – CMOS Kramer Dec 4 '18 at 3:17
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So I pulled the cover off one of these a couple of months ago with the same plan in mind, this is our second door, and the batteries only last 3-4 months.

The system seems to run the batteries both in parallel and in series.

The 6v of batteries in series seem to run the electronics and the 3v in parallel seem to run the rfid loop, so supplying just 6v DC gives the appearance that it all works, but failed to ever pick up tags.

I still want to hard wire the SureFlap. Plan is to make a pair of 3.3v supplied with different earths.

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Low frequency (125 kHz) RFID usually requires a very low ripple power supply or effective noise filtering. Switching power supplies are often a problem. Linears are better. Batteries are best.

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  • \$\begingroup\$ A capacitor will help anyway. \$\endgroup\$ – Ale..chenski Nov 29 '18 at 16:11
  • \$\begingroup\$ Ah, the wall-wart I have is a switching version. I'm going to try the 1000uF cap as suggested by another commentor before ordering a Linear version. How many amps should the supply be rated for? \$\endgroup\$ – CMOS Kramer Nov 29 '18 at 20:38
  • \$\begingroup\$ Hi @Warren, Not sure if you saw my response below, but I'd like to hear your further input. I tried installing 1000uF caps in parallel with the power supply near the battery springs (+ to +, - to -). Symptoms are the same as before; confirming power but the RF doesn't seem to be working. I'm looking into installing 250nH inductors right now, but wanted to hear your take as well. \$\endgroup\$ – CMOS Kramer Dec 4 '18 at 16:42
  • \$\begingroup\$ Instead of trying to quiet a noisy supply it would be easier to get a quiet one. Find a linear power supply instead of a switcher. Also make sure any wire pairs (power lines, antenna tails) are twisted together to reduce noise there. \$\endgroup\$ – Warren Dec 5 '18 at 20:58
  • \$\begingroup\$ Point taken, but 1. I'm having a hard time finding a 6VDC Linear Supply, and 2. are they normally this expensive? ebay.com/itm/… \$\endgroup\$ – CMOS Kramer Dec 6 '18 at 2:10
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You likely have too long wires, and your device likely uses powerful bursts of RF power to communicate with RFID chips, and the voltage sags momentarily. Try to solder a good 1000 uF electrolytic capacitor near the battery holder springs.

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  • \$\begingroup\$ What would be a good 1000uF? Would a $.50 16v Electrolytic be okay? \$\endgroup\$ – CMOS Kramer Nov 29 '18 at 21:06
  • \$\begingroup\$ @CMOSKramer, yes, anything might work. If it is "Low ESR", it will be better. \$\endgroup\$ – Ale..chenski Nov 29 '18 at 21:25
  • \$\begingroup\$ Thanks, ordered a few Panasonic EEU-FR1E102LB. Will see how they work. \$\endgroup\$ – CMOS Kramer Nov 30 '18 at 3:27
  • \$\begingroup\$ @CMOSKramer, don't you have a box with old junk electroncs in your garage where you could de-solder a cap and try it without ordering? \$\endgroup\$ – Ale..chenski Nov 30 '18 at 3:30
  • \$\begingroup\$ So, my caps arrived this evening, so I tried installing a 1000uF electrolytic in parallel with the power supply near the battery springs (+ to +, - to -). Symptoms are the same as before; confirming power but the RF doesn't seem to be working. \$\endgroup\$ – CMOS Kramer Dec 4 '18 at 3:20

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