Keeping Heating Element. . .not so hot. . .

[CircusPig]

I am making an attempt at brevity for the sake of clarity:

I have a 1500w water tank element @120v.
When the element comes on, the power is at 100%, essentially boiling the water immediately around the element. I need a lower heat output. The idea is lower heat output for longer periods of time rather than a crazy amount of heat all at once.

Is there a cost effective way of reducing the harsh heat output of the element?

I need to maintain a water temperature of about 75 to 80 degrees. Can this be done with a simple household water tank thermostat?

Thanks in advance for your help. You are some pretty brainy individuals. lol

 

[KMoffett]

"Can this be done with a simple household water tank thermostat?" That just turns full on and full off.
What is the problem with boiling water around the element? That would tend to circulate the water in the tank. Is this a special application?

Ken

 

[NorthGuy]

You can use a dimmer with Triac. It may not be easy to find a ready-made dimmer powerful emough.

 

[CircusPig]

Ken, thanks for the reply.

I'll clarify: I know this regulation can be done with the thermostat in question: I'm currently doing it in my bench testing.
"When the element comes on, the power is at 100%. . ." --I understand how the element works (generally speaking) but thanks for that additional information.
Circulation is inconsequential.

From your post, should I assume that what I'm looking for isn't easily accomplished?

All I need is for the temperature of the element to be governed/reduced/scaled--whatever the terminology may be. My apologies for seeming curt.

 

[CircusPig]

NorthGuy,

I've found some information to address the situation I have; however, none of the solutions are really applicable. I was hoping to get some details since I'm not familiar with the materials and/or terminology. Are there any schematics you know of that employ a triac? Building wouldn't be a problem so long as I had plans and parts list.

Thanks, NorthGuy

 

[NorthGuy]

I don't have a drawn schematics. There must be dimmer schematics on the Internet.

You can look at the outocouplers, such as this one. The data sheet has typical schematics with the optocoupler and the triac connected to it. You need a big triac, such as BTA24, and a big heat sink.

You will also need some circuit that will need to drive it, which is not straightforward because you need to detect zero crossing and drive triac at the specific time in the AC cycle, which IMHO is best done with a microcontroller.

To simplify, you can switch the triac on and off once a second or so, which doesn't need zero crossing detection and doesn't need microcontroller, but can be controlled by 555 timer running at 1Hz rate. However, it's not a very small project either.

An even simpler way would be to buy two heating elements and connect them in series. This will produce 50% reduction of power compared to the single element.

 

[picbits]

I would personally use a microcontroller and use it to switch a triac at every n'th cycle. I'd also use the microcontroller to do the zero crossing sensing.

If you were to turn on the element for 2 out of 3 cycles then you'd be running at 2/3rd power and if you turned it on for half the cycles you'd be running at half power.

Even consider using a diode (with a large enough rating) in series with the heater - that would only heat the element at half the power but if you wanted full power you could just short out the diode.

 

[CircusPig]

NG,
I found a few 1500w dimmer switches online but the price makes the entire project prohibitive. I must heed the parameters of my bank account.

What would have to be done if I had access to an 800w dimmer switch?

 

[KeepItSimpleStupid]

An aquarium heater will do just that. About a 100 W element for 10 gallons if I remember and the thermostat was bimetalic.

The setpoint was trial and error because there is no PI or PID control. P is proportional to error I is the integral of the error and D is the derivative of the error. The P term is primarily for control/stability, the I term makes the setpoint and measured value agree and the D term works on overshoots.

 

[alec_t]

temperature of about 75 to 80 degrees

Is that F or C?

 

[KeepItSimpleStupid]

Good point. Virginia - probably F is my guess unless your doing some scientific work.

 

[Pommie]

The simplest way to reduce the power would be a diode in series. This will reduce the power to a half of normal. Google 15A diode for examples.

Mike.

 

[ChrisP58]

Sounds like you just have too much element for the amount of water you're trying to heat. Another way to reduce the power in the heater would be to use a transformer. Driving it with 24V would drop the power by about 80%. 96% But you'd need a 300 65VA transformer.

How much water are you working with?
What is it's cold temperature?
How fast do you need to heat it to 80 degrees?



Corrected as per picbits comment below.

 

[ronv]

I'm not sure I understand the problem with the boiling either.
All the other suggestions will work with the diode being the easiest, but all waste power.
How about a low watt density element or is it already?

 

[picbits]

Sounds like you just have too much element for the amount of water you're trying to heat. Another way to reduce the power in the heater would be to use a transformer. Driving it with 24V would drop the power by about 80%. But you'd need a 300 VA transformer.

How much water are you working with?
What is it's cold temperature?
How fast do you need to heat it to 80 degrees?

Actually when you do the maths, it works out a little differently

1500w @ 120v = 12.5 amps
R = V/I so the resistance of the heater is 9.6 ohms
I=V/R so at 24VAC the current would be 2.6 amps
Therefore the power consumed at 24v would be P=IV = 2.6 *24 = 62.5 watts - only around 4.2% of the original.

 

[picbits]

All the other suggestions will work with the diode being the easiest, but all waste power.

Using a triac and switching off alternate cycles or even burst heating i.e. 1 second on and one second off will not waste power

 

[ronv]

When the triac is on there will be a volt or so of drop across it so you still loose some power.

 

[CircusPig]

When I could

An aquarium heater will do just that. About a 100 W element for 10 gallons if I remember and the thermostat was bimetalic.

When I couldn't find exactly the answer I was looking for I actually went ahead and ordered an aquarium heater. The wattage is actually pretty high and should do the trick. Great idea! Wish you had found me earlier. Lok

 

[CircusPig]

I'm not sure I understand the problem with the boiling either.

How about a low watt density element or is it already?

The problem is the initial shock to a proprietary photo chemical, Ron. The element is too hot when powered and renders the chemical useless (to me). I'm not certain what the temp the element itself is, but it boils the water around it and dissipates the heat from there.

The only wattage info I have was provided in my first post: 1500w 120v (if I remember correctly).

 

[CircusPig]

Thanks to all of you who took the time to try helping. The easiest and cheapest solution to my problem was an aquarium heater for 20 bucks. 300w with a built in thermostat.