Hello all,
I'm working on a small 12v circuit that I'd like to reduce the current flow of using an in-line resistor. My question is, am I calculating the needed resistance correctly?

The load is a 150w ceramic PTC heater with a small fan. Here is a link to the Amazon listing https://www.amazon.com/Maxmartt-Temperature-Incubator-Humidifier-Conditioning/dp/B081197DGH
By my calculations, at 13.8v, the current should be 10.9 amps. I plan on running two of these heaters which would be 21.8 amps. I'd like to reduce that to 20 amps due to the relay I'll be using.
Would something like a 0.050 ohm resistor accomplish this?

Thanks in advance,
Robert

 

Your math seems to be correct. Be aware that the resistor will dissipate around 20 watts at max power, so it will get HOT. I would use a 50 watt (or greater) resistor, and mount it and the relay on the firewall, under the hood, if possible.
Keep in mind that 10 ft of AWG 10 ga. wire will give you about 0.03 ohms.
Personally, I would probably omit the resistor and assume that the relay has been designed with a safety margin. The current will only be max until the heaters warm up.

 

Thank you Ron, that's very helpful. I was thinking that I needed a resistor sized to handle the entire load, i.e. 300w. That was proving difficult. The fact that it will only, "dissipate", 20w and not 300w is something I hadn't considered. A 50w rated resistor would be much easier to find.
Regarding your final thought about the safety margin, I am planning on running the heaters without the relay and monitoring the current draw through an entire heat cycle to determine if, like you imply, the resistor is actually needed.
Thank you very much for the assistance. I'll update this thread after I collect some real world data.
Robert

 

I would not worry too much about a 20A rated relay passing 22A although it might lead to an early failure – I’d be more worried that the fan is using a PTC (positive temperature coefficient) heating element.

When cold, the heating element will have a much lower resistance than when at operating temperature – drawing a far greater current (possibly more than double the 20A).

However in my experience, units like that in your Amazon link rarely live up to their specs – it would not surprise me if the actual power draw was less than 100W.