Electronics > Beginners

Newbie understanding joule heating

(1/5) > >>

rthorntn:
Hi,

Sorry for the newbie questions.

Say I have a hard (fixed) requirement of 48V power, up to 20A, it must be DC.

Similar to a hairdryer I want to create hot air, assume the fan speed is independently controlled.

There would be a microcontroller and a PID.

Questions:

How do you design a resistive heater to use 960W?
Does equal wattage of AC or DC in to a resistive heater result in the same temperature?
How do you best switch the heater on/off (960W at 100% duty cycle, that would be an issue for MOSFET because it would have to dissipate that)?
Is there a way you can work out the maximum temperature of the air without building it, I guess it's a function of heater and fan duty cycle (lowest fan speed plus the maximum heater power)?

Thanks.

Richard

vk6zgo:
 :box:
--- Quote from: rthorntn on July 20, 2018, 04:27:35 am ---Hi,

Sorry for the newbie questions.

Say I have a hard (fixed) requirement of 48V power, up to 20A, it must be DC.

Similar to a hairdryer I want to create hot air, assume the fan speed is independently controlled.

There would be a microcontroller and a PID.

Questions:

How do you design a resistive heater to use 960W?

--- End quote ---

As you quoted 960W with the given voltage & current, you already know
P=VI.

Knowing the needed voltage & current, you can derive the resistance.
R=V/I
In this case, 48/20 = 4.8 ohms.

960 watt is a serious amount of power, so that 4.8 ohms needs to be pretty rugged, or it will "have a short life but a happy one."

There are almost certainly ways of deriving the specs for such an element, but I would suggest looking at the elements in devices that already have ratings of around 1kw.
Perhaps elements in parallel rather than series, or something like that.


--- Quote ---Does equal wattage of AC or DC in to a resistive heater result in the same temperature?

--- End quote ---
Yes, this is why we use RMS values for AC, rather than the more intuitive Average.
48v RMS AC will produce the same power dissipation in the element as 48v DC.

--- Quote ---
How do you best switch the heater on/off (960W at 100% duty cycle, that would be an issue for MOSFET because it would have to dissipate that)?
--- End quote ---

The switching device doesn't dissipate the 960W, its dissipation is entirely due to its own resistance.
If you had to dissipate the entire devices operating power, if you switched off a bar heater, you would vaporise the switch on the GPO it is plugged into.

It isn't a matter of power, it is really the current handling ability of the switching device.
Electric kettles used to (maybe still do ) use bi-metal switches, but,of course, they are operating at full Mains voltage, so the current is a lot lower.

If the 48v is ultimately derived from the Mains, I suggest doing the switching there, or perhaps as a function of a SMPS.

--- Quote ---Is there a way you can work out the maximum temperature of the air without building it, I guess it's a function of heater and fan duty cycle (lowest fan speed plus the maximum heater power)?

Thanks.

Richard

--- End quote ---
You could measure the air temp of a similarly rated Mains operated heater, but, depending on how you organise your 4.8ohms, the elements may not be all exposed to the air in the same way as the comparison device.

Nusa:
You might find this useful on the subject of heat transfer:
https://www.engineersedge.com/heat_transfer/heat_transfer_table_content.htm

Even if you've managed the math for your situation, confirming your answers empirically with a prototype is still a really good idea.

Don't forget safety considerations. What happens if the fan fails? What happens if the air flow is blocked? Unless the heating element will also fail before something *really bad* happens, you likely need a thermal shutoff. Depending on the type of device, there may be other legal requirements. Tilt switches for space heaters, for example.

rthorntn:
Thanks!

The 48V DC is not derived from the mains.

Is there a way to safely switch the 1KW heater on/off with a microcontroller to control the temperature output accurately?

vk6zgo:

--- Quote from: rthorntn on July 20, 2018, 11:22:04 pm ---Thanks!

The 48V DC is not derived from the mains.
--- End quote ---
Bummer! :(

--- Quote ---Is there a way to safely switch the 1KW heater on/off with a microcontroller to control the temperature output accurately?

--- End quote ---
You can control almost anything from a micro controller, with an appropriate interface.
Devices you could use are:- heavy duty automotive (truck) relays, suitably rated power MOSFETs, or
complete modules specifically designed for the purpose.

I wouldn't expect any solution to be very compact, as they are power devices, which tends to make them a bit "clunky".

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod