Stupid question on resistive loads V Watts calculation.

[TimB]

Hi, sorry this is probably a stupid question

I have a resistive heater that is rated as 120v AC 400 watts
I want to run on 24v DC so my maths makes it 24v 400watts = 16.6 amps

So if I use a 24v 400 watt power supply I can use it (Although probably less amps as its DC)

Am I wrong. is there another issue that will prevent it being usable?

Thanks

Tim

[Pepe]

The heating power at 24v would be 16w because the resistance would be 120x120/400= 36 ohms and 24x24/36= 16w

[John Drew]

Hi Tim,
It's a pretty good question because it's more complicated than first look.
Firstly when we talk about AC we usually talk about RMS (the effective heating capability compared with DC). So we can do DC calculations with your heater.There is another hidden problem too, and that is when a heating element is cold it's resistance is lower than when it it hot so it will draw more current at first.
But let's look at the arithmetic.
Resistance = V²/Power
So resistance = 120 * 120 /400 = 36 ohms
So if you run it off a 24 volt supply
Power = V*V/R = 24 * 24 /36 = 16W
So your power supply will be fine but the heater won't be very warm.
Does this help?
John

Whoops, typing at same time as Pepe. As usual I'm more wordy but Pepe is right.

[chris_cb_uk]

Resistance is a constant in the equation
... Well relatively speaking, it will vary a little based on temperature but let's not split hairs!
Power is a variable based on voltage applied.
If you were splitting hairs you would also calculate the rms to give you the dc equal.

[Giuseppe MPO]

if it is powered at 120V ac it is already an effective voltage
in any case you have to see what type of heater it is, if it is a simple resistor
they have already answered you, you would have a power of 16W which would not be of any use.
Then you have to see if the heater has any parts that require 120 ac, (like fans, electromagnets,
various motors etc.)
You should explain what kind of heater it is.

[joesaliba]

When in doubt regarding formulas, (although formulas are spot on), connect the heater to two car battery in series, (24v) and measure current, before buying a power supply to suite the heater. And you can also see it's efficacy.

A small car battery will deliver 400A easily.

Regards

Joe

[TimB]

I tested it at 25v 5 amps and got ~16 watts
Resistance is ~38ohm

O well it really shows how bad my electronics theory is

[trastikata]

Don't forget that resistance is rising as temperature increases.

[John Drew]

I tested it at 25v 5 amps and got ~16 watts
Resistance is ~38ohm

O well it really shows how bad my electronics theory is

Tim,
I think you might be mixing up what a power supply is capable of providing (25 volts/5A) with the current drawn by the load. In your case it's drawing about 0.6 amps. As long as your supply can provide more current than the load, it is the load and its resistance that determine the power. In this case 25V and 38ohms.
John

[trastikata]

Just a thought - if the the heating wire is expendable, you can cut it in shorter pieces and/or connect them in parallel to decrease the resistance and increase the thermal output at 25v.

[John Lawton]

Just a thought - if the the heating wire is expendable, you can cut it in shorter pieces and/or connect them in parallel to decrease the resistance and increase the thermal output at 25v.

Of course. Ohm, sweet ohm.

John

[top204]

I've never fully understood why they use the "made up" term of the Watt, instead of the Amperage, which is part of the mathematically accurate, and factually accurate, Ohms Law?

It was originally made up for movements in Steam machines, when they did not fully understand the mathematics behind them, so why was it also adopted for Electricity as well? I've always considered it as factual as "Brake Horse Power". :-)

[trastikata]

I've never fully understood why they use the "made up" term of the Watt, instead of the Amperage, which is part of the mathematically accurate, and factually accurate, Ohms Law?

In electricity they describe two different things.

Amperage describes only the number of electrons moved from A to B but not their potential difference. Whereas the Watt includes their potential and thus the propensity to do work - hence, two different processes.   

[TimB]

I'm looking for a low voltage hot air source like in a desoldering gun. It's not going to be used in that way but I need to have a constant flow of hot air that I can control the temperature VERY accurately.

You can see online replacements for desoldering stations but they are bare wire. Then I saw one in a 1/2" tube for heat sealing or plastic floor welders. 110v at 400 watts. It was not inexpensive and purchased it on a whim. Before I really thought about the issues. 24v at 400 watts is ~16 amps. Ok LED power supply and I have a DC feed I can PWM. I have developed a 240v speed controller using a triac and hated it.

But as has been pointed out the wattage is set by the voltage and the resistance. So I have now an expensive paper weight...

Issue is that I cannot find a low voltage coil to generate the hot air I need. The nearest was a Dewalt hot air gun battery powered. No parts seem available.

Tim

[trastikata]

Issue is that I cannot find a low voltage coil to generate the hot air I need. The nearest was a Dewalt hot air gun battery powered. No parts seem available.

Tim, you should have started with that. Nichrome wire is sold in different gauges on internet and there are some calculators online that can help you figuring out the power-amperage-voltage-gauge relationship just google "nichrome wire calculator".

[Giuseppe MPO]

If you want to use what you bought, I think there are two ways, the first is to use a triac directly in AC controlled,
for example, by a PIC16F1619 that has a CIP peripheral that can calculate in HW the phase shift needed to control the triac,
perhaps by implementing a PID routine on the PIC to have a precise temperature control.
Alternatively, you could also drive your resistor in PWM by rectifying 110V and using a MOSFET that can withstand at least 250-300V.
But whatever you use to drive the resistance you have to check that if on your appliance if there is a fan or other
you have to separate it from the resistance and power it separately.

[John Lawton]

If it's of interest, some time ago I designed a temperature controller where the heater was mains AC powered. I used a zero crossing circuit and pulse density modulation using half-sine waves rather than phase control. This worked fine as a heater is relatively slow to respond and the method was low noise - unlike a phase controller. Circuit of temperature controller

The power control system used these triac control patterns:

Code Select Expand

Pattern[0]  =  %0000000000000000
Pattern[1]  =  %0000000100000000
Pattern[2]  =  %0000000100000001
Pattern[3]  =  %0000100000100001
Pattern[4]  =  %0001000100010001
Pattern[5]  =  %0010010010010001
Pattern[6]  =  %0010010010010101
Pattern[7]  =  %0001010101010101
Pattern[8]  =  %0101010101010101
Pattern[9]  =  %0101010101010111
Pattern[10] =  %0101010111010111
Pattern[11] =  %0101110111010111
Pattern[12] =  %0111110111010111
Pattern[13] =  %0111110111110111
Pattern[14] =  %0111111101111111
Pattern[15] =  %1111111111111111



[Giuseppe MPO]

.

[Giuseppe MPO]

If it's of interest, some time ago I designed a temperature controller where the heater was mains AC powered. I used a zero crossing circuit and pulse density modulation using half-sine waves rather than phase control. This worked fine as a heater is relatively slow to respond and the method was low noise - unlike a phase controller. Circuit of temperature controller

The power control system used these triac control patterns:

Code Select Expand

Pattern[0]  =  %0000000000000000
 Pattern[1]  =  %0000000100000000
 Pattern[2]  =  %0000000100000001
 Pattern[3]  =  %0000100000100001
 Pattern[4]  =  %0001000100010001
 Pattern[5]  =  %0010010010010001
 Pattern[6]  =  %0010010010010101
 Pattern[7]  =  %0001010101010101
 Pattern[8]  =  %0101010101010101
 Pattern[9]  =  %0101010101010111
 Pattern[10] =  %0101010111010111
 Pattern[11] =  %0101110111010111
 Pattern[12] =  %0111110111010111
 Pattern[13] =  %0111110111110111
 Pattern[14] =  %0111111101111111
 Pattern[15] =  %1111111111111111
 


I also tried, for a heating plate, to do such a check, in this way the resistance works at full half-wave but, at least in my case, I had an annoying flickering of the lights in the house and, in certain conditions, it was very annoying

[rick.curl]

Hi Tim- Try searching for an electric windshield defroster or an electric golf cart heater.

Just Googling "12V heater" turned up a few interesting hits.

-Rick