I have instant hot water heaters which take 2 X 1.5 batteries = 3V. I want to replace the battereies with DC adapters we have. The adapters have various voltages. I tried to drop the voltage by adding a resistor to the circuit, but then the current drops too low for the unit to fire up. Is there anything I can do? The measurements are:
Batteries: 3.36V, 22.2mA
DC Adapter, no resistor added: 5.9V, 22mA (Why does the current not increase with the increased voltage?)
DC Adapter, resistor added: 5.9V, 15 mA and unit doesn't fire.
One unit is running OK using the DC adapter without a resistor, but I think it would be risky to use the higher voltage adapters without reducing the output voltage to near 3V.
There is something weird about these measurements. The battery measurement is giving 0.075 Watts, which is not much for an "instant hot water heater".
Did you do the measurement with the heater immersed in water? If not, I wonder if the heater unit can sense this (perhaps with a temperature sensor that gets above the boiling point of water when the unit is not actually in water?), and goes into a "low current" mode when this happens?
Try doing the measurements with the heater immersed in water.
Thanks for all replies. I was only using a 47 ohm resistor, and still the current dropped too low. I tried to measure resistance between the positive and negative terminals when the burner was firing and when off, but it justs gives an open circuit reading. I just found out that the power cables feed into some sort of inverter? It (the white box) says
Input: DC 3V Output >= DC 12 KV
Not sure if this changes anything.
I had read up about the first poster's zener soluti> Phil Allis>
Actually it only seems to ever draw 22mA, so I suppose the appropriate combo of reverse biased zener diodes and forward biased silicone diodes in series would reduce the voltage to the right level, without using a protective resistor.
4 or 5 1N400x diodes in series between the adapter & igniter should work. If you want precision, you can use an LM317 voltage regulator: (View in fixed font)
No it doesn't need a precise voltage, batteries between 3.3V and 2.8V make it go. In fact I tried a 15.5V adapter (12V nominal) today, and it worked as well, but the igniter kept sparking after it was lit. Also the current was about half an amp, luckily it seems OK. I will try the chained diodes as a last resort, but I'd need over well over 10 in this case. Here are some of the options I have tried:
15.5V adapter (12V nominal)
0.5A ??
------------------------------------------------------- Results worked but igniter kept sparking after ignition
15.5V adapter (12V nominal)
10V Zener diode in series
3.3V Zener diode in series
I have no idea where you live but sometimes it is better to have a battery ignitor because if the power is off (Storm etc) you may still want to use the heater. My daughter has a heater which has no provision for batteries and she was without hot water fo a week after a big tree brought down the power lines.
So, I'm guessing you want to be able to power your hot water controller with a DC adapter, powered by the mains, instead of with batteries. Am I right?
Why exactly would you want to do something like this?
Could you get two AA NiMH batteries, and charge them when they go down?
Another idea: get the correct size DC adapter, as close to 3.36V,
22.2mA as you can find. Is there a thrift shop near you? Here where I live, at the local Goodwill, they charge US $1 each for assorted used DC adapters.
Beacuse we have many hot water heaters, and replacing batteries all the time is expensive. I suppose I could try the rechargeable option, if they last long enough. We have no power supply issues in New Zealand.
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The OP\'s problem is that he doesn\'t understand that in order to bump
a few volts up to the 15kV or so required to strike the arc required
by the ignitor he\'ll need to keep the impedance of the substitute
supply down to what batteries look like, milliohms.
Because of that, his assessment of the current required to do the
job is flawed.
I have a lot of trouble believing these ratings for an "instant water heater"....Maybe you should consult the manufacturer and possibly a lawyer about faulty advertising....The ratings you give couldn't heat 10cc of water +10C in anything less than 45 seconds......
first off 15.5V is kind of high to reduce to 3 using zeners, and also it's not a regulated supply so the output voltage will change with the supply voltage (which fluctuates somewhat)
if you can't get a lower voltage plugpack you'll need to use the lm317 circuit to reduce the voltage.
yeah, that one. maybe a 10uF tantalum instead of the 1uf.
Fine weather recently, but cold with the onset of winter
Thanks for that information, I'll stop putting resistors in my circuit.
I haven't assessed the required current, only measured it as 22mA. However I put on a 4.7V DC adapter today, it made the unit work. The measured current this time was 32mA, this is a different unit though.
This attempt below, any ideas how to limit the current without inserting a resistor? Why is it drawing too much current anyway? Is that why it doesn't work?
15.5V adapter (12V nominal)
10V Zener diode in series
3.3V Zener diode in series
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