I have a 12v dc line for my refrigerator in my motorhome. it is powered by two 12v deep cycle batteries. I'm trying to put a computer fan in it to keep the fins from freezing up. The fan is rated at 1.6amps. So I bought a 7.5 ohm 25w resister. It was a huge rectangular block. I attached it and the fan ran great, but after just a few minutes the resistor was extremely hot to the touch. I took that out, installed a 50ohm 3 watt resister to lower the fan speed. That small resister got extremely hot very quickly. There is nothing else in the circuit, just the fan. I don't have to run it at the full 1.6 amps, but would want it to run at no less than 0.8 amps. What would you recommend for a resister that wont get so hot to the touch (don't want the motorhome to catch on fire )

 

I'm surprised that a 50 ohm resistor would get "extremely hot very quickly." Are you sure it is 50 ohms? I assume you connected one end of the resistor to your 12 volt source, the other end of the resistor to one end of the fan and the other end of the fan to the open end of the 12 volt source--right? Maybe try 100 ohms or even 120? Will the fan be inside the cooler?

 

I have a 12v dc line for my refrigerator in my motorhome. it is powered by two 12v deep cycle batteries. I'm trying to put a computer fan in it to keep the fins from freezing up. The fan is rated at 1.6amps. So I bought a 7.5 ohm 25w resister. It was a huge rectangular block. I attached it and the fan ran great, but after just a few minutes the resistor was extremely hot to the touch. I took that out, installed a 50ohm 3 watt resister to lower the fan speed. That small resister got extremely hot very quickly. There is nothing else in the circuit, just the fan. I don't have to run it at the full 1.6 amps, but would want it to run at no less than 0.8 amps. What would you recommend for a resister that wont get so hot to the touch (don't want the motorhome to catch on fire )

You would be better off buying a 12V rated fan at say 0.4A – you would be surprised at the airflow from such a low powered fan.

 

I'm surprised that a 50 ohm resistor would get "extremely hot very quickly." Are you sure it is 50 ohms? I assume you connected one end of the resistor to your 12 volt source, the other end of the resistor to one end of the fan and the other end of the fan to the open end of the 12 volt source--right? Maybe try 100 ohms or even 120? Will the fan be inside the cooler?

I soldered the resistor on one end to a wire that attaches to the source board. I soldered the other end of the resistor to the wire for the fan. The resister is about 6 inches from the power source board and about 2 feet from the fan itself. I attached the ground to the fins in the refrigerator, via a alligator clip.

 

I would just buy a pair of 120mm computer fans designed for 12 VDC operation and they will move a little over 40 CFM each quietly. They cost about $5 USD each and will last for years. Another option is to just buy a cheap PWM board from Amazon like this one for $7 USD and power your existing fan. Using a resistor to limit the fan current (reduce fan voltage) is always going to result in heat until you get a resistance so high your existing fan won't be moving any air to speak of. Here is another PWM controller rated at 10 Amps for under $7 USD.

Ron

 

X2 on the PWM...simple and cheap.

 

And if you want to stick with a resistor (not recommended) , try mounting the resistor in the cooling air flow from the fan. After all, you have a nice breeze there so make use of it.

 

Computer fans are typically rated at 12 volts. One drawing 1.6 amps means you're using (12 x 1.6 =) 19.2 watts of electrical energy. Since I don't know the resistance of the fan (can calculate a decent guess at it from the numbers) adding resistance still means you're going to be dealing with high wattages. 12 (v) ÷ 1.6 (a) = 7.5 ohms. Adding another 7.5 ohm resistor to the circuit brings the resistance up to 15 ohms. 12 ÷ 15 = 0.8 amps (800 mA). 12 x 0.8 = 9.6 watts. A 25 watt resistor should have no problem handling the heat. Keep in mind that no matter how big a resistor you get (watt wise) you still have the same amount of heat energy to dissipate.

You mentioned 50 ohms. That plus the potential fan's resistance (remember, it's only ballpark, and not even that good a ballpark figure) that's 57.5 ohms. 12 ÷ 57.5 = 209 mA (rounded up). That's 2.5 watts. That's a bit much for a 3 watt resistor. Yes, it's going to get hot fast. A 5 watt resistor would handle that energy better. In another post the discussion centered around wattages and I made the point that X watts is X watts no matter how big the resistor. Others argued (rightly) that a larger resistor has greater surface area and can expel the heat energy better. I still contend that X watts is still X watts. Anyway, that part not withstanding, if you want to control the speed of your computer fan then as others are suggesting, go with PWM (Pulse Width Modulation). It's a great way to cut power without wasting all that energy and making all that heat. In fact, PWM is more efficient than using a variable regulator or a bunch of large resistors. And since we don't know the fan's resistance (or impedance) the numbers I'm using are just fair guesses. But looking at the way the numbers behave you can see why you're developing a lot of heat. Resistors add friction to the flow of current. That friction generates heat. Hence, your resistors get hot. It's fairly straight forward. But as others suggest, PWM is the correct solution to your problem.

 

Given the currents of the fans that I have played with, that is a very big and extremely high powered fan. Or a terribly inefficient fan motor. A much smaller fan with a motor current of under 500Ma will be a better choice.
BUT, if the fins getting an ice build up is the problem, then something else is wrong, to begin with. Examine the refrigerator for an air leak. And also check the defrost timer and the defrost circuit.

 

Yes, My home aircon was icing up. It was low on gas. After a regas, it is working so much better. No ice and a lot less power used per day as it is not running flat out all the time.
So, as MisterBill2 says, get it checked out.

 

cheaper than power resistors- buck conveter modules eg
https://www.ebay.com/sch/i.html?_fr...e+5V~16V+12V+to+1.5V+1.8V+3.3V+5V+3A&_sacat=0

 

Low refrigerant would certainly cause the same symptom, but at such a distance I am reluctant to provide detailed diagnostics. Except that just a few months back I did diagnose correctly a refrigerator defrost problem for a missionary in Thailand. But that was an unusual situation in that he had the circuit and he was able to take meter readings as needed. I was in Detroit Michigan at thetime, so it would have been a very long-distance service call indeed.

 

1.6 A at 12 V is a much larger than normal "computer fan". As above, the best approach is to buy a fan that is sized for the job.

Separate from that, a brushless DC motor fan is not a normal electronic component. In terms of how to calculate a series resistor to reduce fan speed and power, the fan is somewhere between a constant resistance and a constant power device. Example, 12 V, 1 A fan. The equivalent resistance is 12 ohms. But if you limit the fan current to 0.5 A, the voltage across the fan will be greater than 6 V.

Also, many fans, particularly low cost ones, are not guaranteed to start with only 1/2 their rated voltage applied.

Finally, you might be misunderstanding resistor power ratings. If a resistor is rated to dissipate 25 W, that has nothing to do with personal comfort. At that power level the case might be hot enough to melt fingerprints and raise blisters, and melt the insulation of wires making contact.

ak

 

Was it really a 12 volt fan motor? Or was it a 5 volt motor? I don't recall seeing an explanation of why the resistor was needed at the beginning. If it is a 5 volt fan then put a second one in series and get twice the airflow, plus not needing a resistor. But if a fan really is needed, (see posts #9, #10, and #12), then get one for the 12 volts supply voltage. There are a lot of those available. But if the heat exchanger did not ice up previously but is doing it now, then something has changed. And if it is one of those gas-absorbtion models in an RV, when it develops a leak, the cost to repair or replace with the same kind will be totally shocking. It certainly was in my case. So I scrapped the ammonia cycle refrigerator and replaced it with a standard freon model and saved over $400, plus gaining a lot more refrigerator volume, and more cooling capacity.

 

I would just put the resistor in the refrigerator, where it will get cooled.

 

I would just put the resistor in the refrigerator, where it will get cooled.

Removing that many watts of heat will make a big reduction in the refrigerator's cooling ability and a serious reduction in the system efficiency as well. And every bit of the watts fed to the blower motor will be converted to heat, since no work is leaving the refrigerator. Keep that in mind as one of the secondary results of one action.

 

I would just put the resistor in the refrigerator, where it will get cooled.

Had a freezer with an automatic ice maker. When it was time to eject the ice the maker would heat up enough to melt the surface of the ice so it could be pushed out. On a couple occasions it got stuck and couldn't push out the ice. Even after the ice had melted to water. The freezer kept getting warmer and warmer until all the food in it was ruined. So I wouldn't recommend putting a heat source in the refrigerator. Especially if it's pushing a few watts of heat. That's like making heat for your refrigerator to pump out. And in the process it makes more heat. So don't put resistors inside the refrigerator. They can generate heat faster than the refrigerator can pump it out.

 

OK, I understand its a mistake.

Can you measure the voltage and current over the resistor as something seems not right.

 

You can measure the voltage across the resistor. Knowing the resistor value you can divide the voltage you just read and divide by the known resistance. That'll give you the current. Then you can multiply the current by that voltage and get the wattage.

 

I recommend making a 555 MOSFET circuit to generate a variable % PWM at a fixed, low frequency (100-200Hz). This will minimise inductive reactrance and get you close to 1.6A. Use a diode to prevent reverese current flow back into the PS. Use another one reverse biased with a resistor to clamp the reverse voltage to something reasonable but also so that it is quick. I would recommend maybe 80V, so about 50 ohms for 1.6A. MAKE SURE YOUR PROTECTION DIODE CAN HANDLE REVERSE VOLTAGES THIS HIGH. This allows for far more efficient and far better speed/power control.