Question About Resistor Wattage Required For A Project

Thread Starter


Joined Jan 15, 2013
Hello All!

At our home, we have a heat pump. When the cooling call ends, the reversing valve is de-energized. When de-energized, the system pressures equalize so quickly that there is a very loud scream which bothers the neighbors (and wakes me up when I'm pretending to be working outside).

The reversing valve is basically a multi-directional solenoid. The coil causes a slide to move inside the unit which directs the flow of refrigerant as needed for heating or cooling, per the room thermostat's call.

My solution is to keep the reversing valve energized for several seconds after the cooling call ends. This lets the system pressures equalize through the expansion valve and there is no scream. Manual testing shows 20 seconds suffices.

To implement, I chose a "Fan Post Flow Delay Timer" which is typically used in furnaces to keep the indoor fan running for a time period after the burners turn off. The reversing valve coil is 24 vac.

The problem is that the reversing valve's solenoid coil draws 5 mA. The timer requires a load of between 40 mA and 1 amp.
I think, possibly incorrectly, that I can install a resistor parallel with the solenoid coil to increase the load on the timer to 45 mA. (A rough sketch is attached.)
My Math = 24 Volts divided by 600 ohms equals 40 mA. Add in the coil & we have a load on the timer of 45 mA.

The conundrum is what wattage does the resistor need to be to prevent it from destroying itself. Is a standard 1/4 watt sufficient?
Is there a better solution to my situation?

Thank You All for your advice & sharing of knowledge. It's much appreciated.



Joined Feb 20, 2016
5mA is a pretty low value for a coil.
What output is the timer? It is a bit odd to have a minimum current requirement for that.
Can you post some info on the parts in question, and how you intend to wire it all up please?


Joined Nov 13, 2015
Dendad's comments are correct, but just for the record....

P = E*I
P = E²/R
P = I²R

You have voltage (E), you have current (I), and you have resistance(R). use the formula of your choice. Then chose a resistor that has a rating at least 2x the expected dissipation.

Thread Starter


Joined Jan 15, 2013
Good Morning & Thank You Both for helping me with this.

5mA is a pretty low value for a coil.
What output is the timer? It is a bit odd to have a minimum current requirement for that.
Can you post some info on the parts in question, and how you intend to wire it all up please?
The 5 mA figure comes from the machine manufacturer's manual. It is the only documentation available, as there is no manufacturer's name for the valve to try to get more details. I was called away, but when I can return, I'll measure the amp draw of the coil while in use.

Dendad, you asked "What output is the timer?"
I don't think I understand your question. If you're asking the voltage out, it will be 24 volts.
I also was curious about the minimum requirement for the output load, so I asked the manufacturer. The person who replied verified the 40 mA requirement. Another document for the same timer says 20 mA. I'll attach a copy of that pdf to this post.

My quick sketch drawing diagram of how I will wire it up was supposed to be attached to the original question post. I apologize that it didn't show up Dendad.
I'll re-attach it with this post.

Thanks for the reminder formulas Yllli. How quickly I forget they are available when needed. It looks like my load will be 0.96 watts.
Doubling it, per your good suggestion, leaves 1.92 watts. A 2 watt resistor shouldn't be hard to find. Or, since I have leeway for total load between 40 mA & 1 amp, I can adjust the resistor value to lower the wattage requirement.

But, as Dendad mentioned, and YIIIi confirmed, the 5 mA is curious. Before I do anything, I'll measure the actual draw. Perhaps it is a typographical or translation error in the documentation for the machine.

The installation manual for the is attached, but their wiring diagram is for use with a contactor for a fan, so please ignore that & see my drawing.
Also, since the pinouts for the timer aren't on their manual's photo, a photo of the unit is also attached. BUT- The drawing on the photo is a guide for the external wiring of the timer. At first glance, one could think it is the timer's internal wiring.

Thank You Both Again. Your help & sharing of knowledge is very much appreciated.


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Joined Sep 24, 2015
How to size a resistor for wattage:

Know the voltage going through it.
Know the current going through it.
Multiply the voltage and the current to get the wattage.
Choose a resistor wattage rated at least 1 1/2 times higher or the nearest HIGHER wattage resistor available.

For instance: if you draw 950 mW (milli-watts) you would need at least a 1425 mW resistor per my preferred calculations. Since you can't get a resistor in anything other than 1 or 2 watt (or 3 or 5 or 10 or higher) you would simply choose the next higher available 2 watt resistor. Suppose you're drawing 450 mW. You'd need a 675 watt resistor. Since none exist, a 1 watt would suffice. If you draw (for instance) 550 mW, a 1 watt resistor will suffice under the 1 1/2 times rule I like. At 1 1/2 times 550 you get 825 mW. A 1 watt resistor will do just fine (my opinion - others will vary). Choosing a 2 watt resistor will cause no harm whatsoever, so you CAN go that route if you choose. I don't believe in engineering way over what's needed. You certainly wouldn't put a 5 watt or 10 watt resistor in circuit - it's just way overkill.

Whatever you choose, 1 1/2 times or 2 times, I've seen some recommendations for 3 times (which I think is utterly ridiculous - but again it's just an opinion). Bottom line, do what you believe is in your best interests. It's likely you won't have a space issue; but higher wattage resistors ARE larger. This is why I prefer the 1 1/2 times rule. It comes from a long line of industrial engineers who typically engineer commercial products at 1 1/3 times service rating and life support critical or mission critical engineering is done at 1 1/2 times service rating. Mother-in-law had a washing machine with a fusible link in a component that was rated at 106% service rating. It failed within a few years, and the machine wasn't heavily used.

Thread Starter


Joined Jan 15, 2013
Thanks Tony for your great explanation & helpful details. Someday I hope to re-learn all that I once learned and forgotten. (Yeah- Sure I will...)

The problem of choosing a resistor to ass load to the timer may have just become a non-problem. The manufacturer's specification sheet was way wrong on the coil's amperage load. As Dendad mentioned, the load was much higher when I measured it in use this morning.

The load of the coil turned out to be 387 mA, well above the lower limit of the timer's requirement & still below its maximum.
I look forward to obtaining & trying out the timer to see if it solves the scream problem.

Thanks Again to All of you for helping me out and for the much appreciated advice and knowledge sharing. I learned a lot with all of your help.

Enjoy Today!