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I have a battery driven touch activated kitchen faucet. I am trying to convert it to DC adapter operated one to save on batteries. First thing I tried is just hooking up a 9V 2.5A DC adapter. It kind of worked but the faucet would turn on and off erratically without being touched. I figure that electronics are sensitive to fluctuations that are not present when battery operated.

  • Am I right by thinking that the erratic behavior is due to fluctuations or is it likely due to something completely different?
  • If it is due to fluctuations, would the following circuit smooth them out?
  • If yes, what specific diode do I need to make this work?
  • Is this a common problem? I did a conversion like this for some battery powered motion sensor lights and they work but once a month they need some kind of a "reset" to work again. Could that be also related?

schematic

simulate this circuit – Schematic created using CircuitLab

Note: as you can tell I'm a complete beginner at this. I kind of just googled stuff and figured out things in circuit simulator, but I have no idea what I'm doing so I would not be surprised if I'm on completely the wrong track in trying to accomplish this.

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  • \$\begingroup\$ You have one problem right off the bat: A 1F capacitor for that kind of input voltage (you'll want a 16V part) is going to be extremely hard to find at a reasonable price. Try 10000uF or less. Also, a 100mohm resistor might not be necessary since the capacitor itself may supply that resistance. \$\endgroup\$ – Los Frijoles Feb 4 at 6:07
  • \$\begingroup\$ I am going to say do not do this. There is a reason why your faucet, a big metal thing near water, is not connected, in any way, to the ac mains. Just don't do it. It's not worth risking someone's life. \$\endgroup\$ – Elliot Alderson Feb 4 at 15:25
  • \$\begingroup\$ @ElliotAlderson. I might just take you up on that, but then it means that there is a chance that there is a failure mode for DC adapter where somehow 110V AC gets through to the output side and into the equipment it's powering. How common is that? I know those DC adapters can catch fire and fail in other nasty ways, but I didn't know that passing AC through them is one. Or are you saying that under some circumstances 10V 2.5A DC can be lethal? \$\endgroup\$ – user3280964 Feb 4 at 15:53
  • \$\begingroup\$ If you found out that the odds were 1000:1 against such a fault, would you still do it? To your other question, 10V applied directly to your heart would probably be lethal. In general, 12V is considered to have a low level of danger when in contact with dry unbroken skin. I'm not going to even try to quantify the risk of these things, but I would never do what you suggest in my home. \$\endgroup\$ – Elliot Alderson Feb 4 at 16:19
  • \$\begingroup\$ @ElliotAlderson - your comment and answer from EugineK changed my mind about doing this at all. \$\endgroup\$ – user3280964 Feb 4 at 21:33
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First. Using AC/DC adapter near water and Earth-connected conductors requires excellent protective measures in that adapter. The risk is in breaking the isolations between primary (input) and secondary (output) sides of adapter. That can be lethal even in case of minor leaks.

Second. The reason of erratical behaviour of faucet is fluctuations. Not in differential voltage (across DC input), but rather in common-mode voltage of secondary side relative to the Earth. If you use switched-mode DC adapter, it (very likely) produces high-frequency (HF) common mode current, which may lead to significant HF common-mode voltage on secondary part, which is the reason of bad touch-detector behaviour. There are a number of ways to mitigate it, but taking into acount (First), I would not recommend doing that.

How about using rechargable batteries (may be low self-discharge, LSR version) to 'save on batteries'? This is safe and reliable (will not induce erratical behaviour).

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1)to be honest, I don't think the fluctuations would be causing that. Batteries themselves cause bigger issues when their own voltage drops with normal usage. I do not know the levels of your equipment though. It would be helpful if you mentioned what kind of batteries it needs.

2)the circuit you write will work, but the value for the capacitance is rather huge, as someone commented on your post for that voltage value it is a problem, and having that much of a capacitance might cause other issues(voltage might go above what you specified depending on the situation). I think a not so huge capacitor still can get the job done.

3) honestly 1N4148 should work, I don't know the voltage and current specifications exactly, but just look at the datasheet online and compare if your reverse voltage, current, etc do not go over the parameters, also consider that when current is going through the diode there will be about 0.7V between its terminals(ideal value, not exact)

4) I don't think it is related.

I have one thing to propose, I don't know what is the actual input voltage of the faucet with batteries. For example, if it uses 4 AAA batteries it could be 6V if they are in series, try to match this voltage the best you can. I guess since your circuit says 10V it could be close to that? If your adapter does not match you can use DC/DC converters or buy an adapter that is closer to the value you need. I believe this is most critical in general.

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  • \$\begingroup\$ It uses 6x1.5V AA = 9V ... the reason I made the circuit with 10V is because in the simulator it shows a drop from 10V to 9V by the time it gets through the components. I have a couple of adapters 9V and 10V. I tried both and they cause the same problem. Given your answer 1) and 4) ... what else could it be? 9V adapter should have worked because it's exact match and 10V should have worked too because with fully charge batteries I measured the battery bank output at 1.66V x 6 = 10V. I confirmed that the adapters do give me those outputs. \$\endgroup\$ – user3280964 Feb 4 at 15:48

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