Stupid question on resistive loads V Watts calculation.

[rick.curl]

Small correction- my previous post was focused on 12 volt heaters, and now I see you need 24 volts.  You could get two of the cheap ones and connect them in series, or you could use something like this: 24 volt heater on Amazon

-Rick

[See_Mos]

or just some wire wound resistors and a fan?

[rick.curl]

Another possibility would be to get some Nichrome (Chromel) wire: https://www.mcmaster.com/wire/material~nickel-chromium/ and wind a coil that you place in the airflow.  Here's a calculator that will tell you how much you need: https://www.easycalculation.com/engineering/electrical/nichrome-wire-calculator.php

-Rick

[John Lawton]

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

This was for a low power heater. At high power levels phase control generates a lot of RF interference due to the fast switching, maybe not flickering of lights. With modern LED lights this won't be noticed.

[top204]

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

That used to happen when using a DIAC to control the TRIAC. These days, TRIACs can be purchased that have some noise suppression built in to them, and the drive of the TRIAC should always be through a Zero Cross opto-isolator chip. They still cause a bit of noise on the mains, but not as much as they used to do, and far less than some modern LED lights. :-)

I was busy the other day writing some serial coms code from one board to another at only 9600 Baud, and it kept failing intermittently. I was scratching my head for hours and re-writing parser code and making sure everything was OK, but all to no avail. I then switched off my LED lamps illuminating my desk because I did not need them at that time, and the coms started working as it should, and never failed! I switched the LED lamps back on, and the coms failed immediately. I got an MW radio and switched it on and the interference was incredible and wiped out the channel, even at over 1 foot away. So the noise was getting into the short cabling for the TX/RX and scrambling it. The LED bulbs were purchased from a "reputable" UK suppier with a CE mark on them. How is that for noise? Twisting the cables helped a little, but the LEDs still caused issues with the coms.

I've gone through CE testing myself for products and I know how utterly stupid and pointless it is, and it is red tape at its worse. When they fail a seperate PSU with a "valid" CE mark, and fail USB HID coms because it is "too noisy" from the screened cable coming out of a good quality Dell laptop. So how did the USB and laptop and PSU pass the pointless CE in the first place? And what did that have to do with the seperate product being tested? :-)

[m.kaviani]

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

Hi Tim
if your heater is that heater which was 120AC/400w, and you decrease the voltage it draws less amper because the resistance has not changed.
but if you want to run a 400w/24v heater it will draw about 17 Amper more or less.
in the electrical calculation, we must take the power fix. voltage and current are opposite. if decrease the voltage circuit will take more current to achieve the
rated power.
of course, in the ac circuit, we have another parameter as inductance that is opposite to circuit current based on LENs law. so the heaters in ac are like spring. 

[m.kaviani]

That used to happen when using a DIAC to control the TRIAC. These days, TRIACs can be purchased that have some noise suppression built in to them, and the drive of the TRIAC should always be through a Zero Cross opto-isolator chip. They still cause a bit of noise on the mains, but not as much as they used to do, and far less than some modern LED lights. :-)

I was busy the other day writing some serial coms code from one board to another at only 9600 Baud, and it kept failing intermittently. I was scratching my head for hours and re-writing parser code and making sure everything was OK, but all to no avail. I then switched off my LED lamps illuminating my desk because I did not need them at that time, and the coms started working as it should, and never failed! I switched the LED lamps back on, and the coms failed immediately. I got an MW radio and switched it on and the interference was incredible and wiped out the channel, even at over 1 foot away. So the noise was getting into the short cabling for the TX/RX and scrambling it. The LED bulbs were purchased from a "reputable" UK suppier with a CE mark on them. How is that for noise? Twisting the cables helped a little, but the LEDs still caused issues with the coms.

I've gone through CE testing myself for products and I know how utterly stupid and pointless it is, and it is red tape at its worse. When they fail a seperate PSU with a "valid" CE mark, and fail USB HID coms because it is "too noisy" from the screened cable coming out of a good quality Dell laptop. So how did the USB and laptop and PSU pass the pointless CE in the first place? And what did that have to do with the seperate product being tested? :-)

hi Top
the noise is because of switching in the power circuit. it is better to use a coil of about 100uH with sufficient current capability in series with load as a filter.
on the oscilloscope screen, you can see the spike when firring the SCR or Triac. also, snubber circuit is essential on your switching device.

[John Lawton]

I've gone through CE testing myself for products and I know how utterly stupid and pointless it is, and it is red tape at its worse. When they fail a seperate PSU with a "valid" CE mark, and fail USB HID coms because it is "too noisy" from the screened cable coming out of a good quality Dell laptop. So how did the USB and laptop and PSU pass the pointless CE in the first place? And what did that have to do with the seperate product being tested? :-)

Well it can be pointless but I've learnt a few things about design for good immunity and emissions too. I took a 500W LED chandelier system I had designed to a test house. It had a 24V psu with ENxxxx to suit EMC lighting  equipment regulations. We couldn't test the system because the psu output alone was so noisy. We had to power my system from some lab DC psu's instead. It passed.

John

[Giuseppe MPO]

That used to happen when using a DIAC to control the TRIAC. These days, TRIACs can be purchased that have some noise suppression built in to them, and the drive of the TRIAC should always be through a Zero Cross opto-isolator chip. They still cause a bit of noise on the mains, but not as much as they used to do, and far less than some modern LED lights. :-)

Actually my control was not done in diac but with zero crossing optocoupler, the flicker was not due to the AF disturbances but, since it was at full half-wave, only when a load of a certain power (1.6KW) is coupled and disconnected. That kind of disturbance would have been much less evident if the adjustment had been done in wave partialisation. I would have had other types of disturbance that I could eliminate with a special filter.

[See_Mos]

Tim has not yet told us what temperature and air flow rate is required , or the environment in which the heater will be used?