I'm upgrading to a tankless water heater in my home and the one I got requires 2 d batteries to function. I have 120v and 12v available at the location so I was thinking that it would make sense to put in a converter or transformer to step down the 12v to 3v and hardwire that in so I'm not replacing batteries all the time. I estimated that about 3 amps of 3v DC would be similar to 2 d batteries. Are there any unforeseen consequences of doing this and would 3a be reasonable or would dropping it lower make sense.

 

If the supply is DC then you can use a LM2596 buck converter, or if it's AC then use a Transformer and bridge rectifier..

 

One thing you need to check is do you need to provide an isolated 3 volt supply as a battery supply will not have any connection to either side of the 12 vols supply. If you use Dave's suggestion of an LM2596 you MUST confirm that connecting the negative of the 12 volts supply to the negative of the part that uses the 3 volts does not cause a problem. Are you sure that the D cells are expected to provide 3 amps. They would only last for a very short time supplying that current. Also are you sure that the 12 volts supply is capable of providing the current to the buck converter. ( This would be just under 1 amp assuming 80% efficiency.)

Les.

 

One thing you need to check is do you need to provide an isolated 3 volt supply as a battery supply will not have any connection to either side of the 12 vols supply. If you use Dave's suggestion of an LM2596 you MUST confirm that connecting the negative of the 12 volts supply to the negative of the part that uses the 3 volts does not cause a problem. Are you sure that the D cells are expected to provide 3 amps. They would only last for a very short time supplying that current. Also are you sure that the 12 volts supply is capable of providing the current to the buck converter. ( This would be just under 1 amp assuming 80% efficiency.)

Les.

I was going with 3a only because I thought that would be a top end of what the batteries could put out. Would using a power converter designed for 12v to 3v step down with a 3a rating and protecting that with a 1 or 2 amp protection device make more sense?

 

We would be able to give better answers is you provided more information. I am making some VERY WILD GUESSES. I guess that it is heater that uses gas to heat the water and I am guessing that the 3 volts is required to ignite the gas using a heated element or a spark generator. Are the 120 volt supplies AC or DC ? What current are they capable of providing ? Are these supplies being tapped off from within the heater or from some external source ? If they are tapped off from within the unit can you post the schematic of the unit ?

Les.

 

I was going with 3a only because I thought that would be a top end of what the batteries could put out

FYI Duracell coppertop D cell battery current capacity is 17,000 milli-amp hour, that 17 amps, in one hour and it will drain very quickly at 17 amps just making the point you got to know your current draw.

 

These are good points. The 120 is ac and the 12v is DC and both are isolated circuits that I could run at least 15-20a. I haven't found a schematic but the batteries are used to ignite the gas but not sure what it uses to do that. I'll update when it gets here and I can be more specific

 

Dave's suggestion of using a LM2596 will probably solve your problem. You can get a ready built buck converter such as this on ebay to save having to build one. (It is also probably cheaper than just buying the components.) I suggest that you measure the current taken from the from the battery as it may be quite high for a short time while the igniter is firing. It is also possible that there is a continuous small current to drive some electronics. again I am guessing that there is some kind of flow detector that turns on a gas valve and fires the igniter when the tap is turned on. (Or you may have to press a button to perform those actions.) As I said the more information you supply the better the answers will be.

Les.

 

Still no schematic but I was able to find more information. It requires a surge of 12a for ignition and a low current to sense the water and use the led display. The batteries also appear to be in series since a picture of the battery compartment showed + connected with -. Something like https://www.amazon.com/DZS-Elec-Con...verter+step+down+3v+10a&qid=1581403497&sr=8-3 dc DC 12a step down buck converter running on a dedicated 12v dc circuit off a 3a thermal fuse with output protected with a 12a breaker should be pretty stable. You were all very helpful thank you for this and feel free to criticize my plan if I missed something. I am still going to wait until it gets here to verify before ordering parts

 

12 amps surge at 3 volts (36 Watts) seems very high to ignite what I have been assuming to be a gas flame. Did you actually MEASURE this peak current ? Is this an oil fired heater ? Can you provide us with a model number or some more information on the heater as I have been imagining a small over the sink instant waster heater but I am now wondering if this is some large industrial heater.

Les.

 

I am trying to imagine what sort of water heater would utilize 2 "D" cell batteries. The last few water heaters that I have installed use a thermocouple arangement and flash a blue LED to show that they are working properly. But the heater you have may be in a trailer, and that is an entirely different situation, because those units do not include a pilot light, because they run from a small propane tank. Theyhave a spark-ignition system that does draw a bit of power but only to generate the spark. So 3 amps for 2 milliseconds will not draw down the battery very fast.
Thus the power supply to replace the battery arrangement must be able to deliver the current for a short time, which implies a larger value of output capacitor and adequately sized wires. My suggestion is find a regulator IC made to provide the 3.3 volts for a processor, and use that device and a larger output filter capacitor.

 

This might be a case for using a super-cap as the pulse current source. The power supply could then be low continuous-current rated and have a small series resistor to limit inrush current when charging the cap.