Electric curcuit help

[ricer]

I am trying to set up a snow melting grid on my front walkway out of some low voltage heating elements. I would like to control the amps through the heating elments by adjusting the voltage across the heaters. I am trying to do somingthing similar to a system that I saw years ago where hoses for a painting system were heated and the power source had to be adjustable depending upon the length of the hose being used. I plan on using a multi tap transformer to get from 240 volt down to 24 volt but somehow that painting system used a multi tap transformer to get close to the needed voltage but also had a fine tuning adjustmet to 'zero' in on exactly the right amout of power for a particular length of hose.

I am looking for what that curcuitry is for the fine tuning.

Thanks for the help

 

[Colt45]

few options

1. variac - works good, but it's expensive, and big. (variable transformer)

2. big variable resistor - brute force method, horribly inefficient

3. Triacs could work, but I don't know a whole lot about them and you would need a trigger of some sort

4. switchmode PSU, with variable duty cycle - the voltage would be full all the time, but you can control how much of a cycle is high, therefore reducing or increasing overall power/cycle

5. a few big pass-transistors with a variable refrence.. this might not be too feasible(sp) depending on the current the pad draws at full power

what is the normal voltage and current for this thing?

 

[KMurphy]

Why do you want to control the voltage? You don't need to maintain a constant temperature to melt the ice; just run it at full power until the ice melts and turn it off, then repeat as necessary. You could use an electric water heater timer as a cheap and easy controller for your heater; adjust the pegs for duration and cycles per day to optimize your efficiency vs. ice melting.

Heater controlers are on the market already (Watlow makes decent small ones) that regulate heater output. You need temperature feedback to maintain your desired temperature though. This is expensive and way beyond your requirements.

EDIT:
Are you planning on using heated water under your sidewalk? You could use a standard water heater element with thermostat to regulate temperature in the hose. No need for complicated/expensive voltage control. Electric heat requires a lot of power.

As you reduce the voltage to resistive heaters, the power output (watts) goes down exponentially. If you half the rated voltage, power drops to one quarter (Power = V^2/R; R stays relatively constant).

 

[AbsolutDealage]

Originally posted by: Colt45
4. switchmode PSU, with variable duty cycle - the voltage would be full all the time, but you can control how much of a cycle is high, therefore reducing or increasing overall power/cycle

This is definately the road I would go with it. Look around for DC-DC Converters with PWM controllers that put out the power you need, and you should be set. This is probably your best cost/efficiency option.

 

[FrankSchwab]

Just use a standard AC dimming circuit, and don't worry about it. You can buy a standard 120VAC light dimmer at the store; it should work fine at 24VAC also. See for example This page for theory and circuits.

 

[KMurphy]

Here is a pretty good technical document for how to size a heater for different applications.

http://www.heatersplus.com/heat-req.pdf

 

[kanigo2]

This is my very first post here: I hope it comes off well .

You are concerned with not snow but ICE and this can be controlled by temperature.

A simple circuit to control temperature can be found here http://www.techlib.com/electronics/ovenckts.htm , with variable control to a transformer.

Concerns are line length to the circuit(figure to sense proper readings you need to run some distance outside to measure temperature)
and small relays to turn on the circuit.

If you would like that info ,I will take the time to find, but the control circuit is what you wanted now.

Merry christmas.