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