Reduce voltage of DC adapter without reducing current

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.

Mark

"redbelly"

The OP is an idiot.

His heater is powered by gas.

The two batteries supply power to the ( electronic) spark igniter.

...... Phil

Well, shi-ite ... Never mind!

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>

Yes this inverter(?) does power the spark igniter.

"The two batteries supply power to the ( electronic) spark igniter"

suputnic wrote:

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.

suputnic wrote:

It's doubtful that precise voltage is needed.

Adapter -----

• ---+---Vin|LM317|Vout---+-----+---> To igniter | ----- | | | Adj [240R] | | | | | + [.1uF] +----------+ [1uF] | | | | [330R] | | | |

- ---+---------+----------------+---> To igniter

That will give you about 2.97 volts out to the igniter.

Ed

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:

------------------------------------------------------- Results worked but igniter kept sparking after ignition

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15.5V adapter (12V nominal)

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15.5V adapter (12V nominal)

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15.5V adapter (12V nominal)

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15.5V adapter (12V nominal)

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ehsjr wrote:

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.

weather out there?

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,

Michael

Diodes are made of silicon, bathtub caulk is made of silicone. ;-)

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.

snipped-for-privacy@gmail.com wrote:

Ah. I'd recommend Energizer NiMHs, 2500 mAh, 1.2V. They come in packs of 4 or 8. We have about 20 of them scattered throughout our house...

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......

"Electromotive Guru"

The two batteries supply power to the ( electronic) spark igniter.

....... Phil

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?

------------------------------------------------------- Results