Immersion Heaters - reducing kW drain

[Strathglass]

Extremely.

I was an engineer in IBM in Gourock before they started manufacturing the PC. In those days a flip-flop was a separate plug in card and 4k was a very big memory.
That was more than thirty years ago, i'm afraid.

All changed a bit now.

Iain

 

[claudio]

And I remember when an 'AND' gate was a plug in card, using DTL (Diode Transistor Logic) /forums/images/graemlins/smile.gif

 

[DaveS]

I would agree with others that a replacement immersion heater of lower rating is probably the simplest fix.

If, for whatever reason, you did want to keep the current heater, could I suggest an alternative, more elegent, electronic solution? You can obtain from RS for around £2 or so a small IC called a "zero voltage switching burst fire controller". With a few passive components (2 resistors, a potentiometer and a small capacitor, from memory) and a triac rated at 15A or more this makes up a small unit with which you can vary the effective power output between 0 and 100%. Circuit on free RS data sheet. It operates like a dimmer, but there is an important difference: instead of chopping the mains waveform - which generates lots of RF interference, it lets through a varying number of complete cycles. (This technique can only be used for heaters - lights would show flicker.) Triac volt drop is about 1.5 volts, so with a 3kW immersion running flat out the heat sink will have to dissipate 20W; proportionally less as the knob is turned down.

You would have to calibrate the knob to give a scale with kW or A (or both) markings, but if you have access to a clip-on ammeter this should be fairly easy. Normal caveats, of course, about knowing what you are doing before connecting DIY circuits to mains voltage while surrounded by salt water!

 

[claudio]

The instantaneous current would still be 13A though with burst firing, this may still trip the breaker

 

[DaveS]

The instantaneous current will indeed be 13A but for thermal tripping it is the average that matters.

For magnetic tripping, it depends on the relative time constants of the breaker and the burst firer. I can't remember offhand what the cycle period of the RS module is - but well under 1 second, 0.2s maybe? If this is less than the response time for the breaker (which will be slugged to prevent nuisance tripping: consider the short duration magnetic inrush current of equipment with transformers or motors) there should be no problem.

 

[claudio]

Wouldn't a 600V 25A silicon diode in series with the element reduce the power by half then ?

 

[VicMallows]

That was my first reaction to the 'problem'.....but at the time couldn't get my head around whether it would solve the Magnetic-Breaker question. In any case, not quite enough reduction in power in this instance.

My experience of marina supplies in France (original question) is that they seem to use slow thermal breakers, or possibly as DaveS says very slow magnetic ones.

Vic

 

[DaveS]

Well yes it would, but...

Apart from any other considerations, drawing a half wave rectified 13A load from the marina supply would result in the neutral adopting a fluctating DC voltage wrt earth, polarity dependent on which way round you connected your diode, and voltage dependent on the cable impedence from source, but likely to be several volts. All other connected customers would experience this to a greater or lesser extent. This could potentially (groan!) cause a number of interesting effects including vigorous electrolytic action, depending on exactly what was connected to what.

 

[Anonymous]

We find that the single most useful thing on limited marina supplies is an ac ammeter situated where the cook can see it.

 

[claudio]

I believe that in the US they use a single diode connected in series with a lamp in order to provide full / half lamp intensity. (Full intensity by shorting the diode).

So lookout for US boats moored adjacent to you as they may be making your anodes fizz!