# Lower Voltage of 15 VDC Power Supply to 13 VDC for DC Blower

I have a centrifugal blower, model 007-A42-32D, that uses 12 VDC at 17.5 amps. The performance information provided by the manufacturer, SPAL Automotive, was acquired at 13 VDC.

I have a power supply that provides 15 VDC at 20 A. I feel that applying 15 VDC to the blower, given that it will be subject to continuous duty, may cause premature failure.

Question #1: Is applying 15 VDC to a blower that was tested by the manufacturer at 13 VDC problematic?

Question #2: Will someone please provide to me the simplest method of reducing the voltage of the power supply using electronic components?

Thank you to all who help for your time.

edit: I understand Voltage is the product of Current and Resistance as in V = IR. Since 12 VDC power supplies capable of 17.5 A minimum are not cheap, if the placement of a resistor can allow my 15 VDC power supply to work, that is the most sensible choice.

Given V = IR, if I = 17.5 A, then R must equal the equivalent series resistance of the blower combined with additional resistors such that V will equal 12. Is that correct?

• Your fan isn't really like a resistor. For example, it probably needs a lot more current to start up than it does after it gets running. If you size your resistor for the running current, you might prevent it from starting up. If you size your resistor for the start-up current, you might overheat the fan after it gets running. Commented Mar 9, 2013 at 14:59
• @TheP: Blowers don't need much torque to start up. A resistor in series would probably work on that account, but I also agree with your conclusion that this is a bad idea. During motor startup, which will be prolonged with a resistor there, that resistor will dissipate much more than the 50W it will dissipate during the best scenario normal operation. Again, really bad idea. Commented Mar 9, 2013 at 15:13
• Depending on the design of the power supply, it may be possible to reduce the output voltage. Some have an adjustment potentiometer. Commented Mar 9, 2013 at 15:18

1. Will 15 V hurt a 12 V blower? Maybe. What does the datasheet say about absolute maximum and operating maximum voltage?
2. Reducing the voltage at this power level is the wrong way to go about this. Get the right power supply. 12 V is a common power supply voltage, so this will be easy.
• I have considered purchasing another power supply but 12 VDC power supply with at least 17.5 A is not cheap. That is why I am trying to use the one I have. I don't feel there is a right and wrong way to do this, there is simply possible and impossible. Commented Mar 9, 2013 at 14:42
• @jordan: Handling 18 A is not a beginner project. By the time you get some sort of 15V to 12V switcher working, the 12 V supply would have paid for itself. Also, what's the cost of a blown motor? A 12V power supply will be cheap in comparison. If you're willing to waste a lot of power and deal with the heat, you can put a big power resistor in series with the 15V supply, but we're talking about dissipating over 50 W. A 12V supply should start to look pretty attractive about now. Commented Mar 9, 2013 at 15:00
• Whoever downvoted this, it is useful to explain your objection. As far as I can see, everything I said is correct. Perhaps the OP wishes the answer were something else, but that doesn't make the answer wrong nor bad advice. Commented Mar 9, 2013 at 15:20
• @OlinLathrop I orignally downvoted it because I didn't feel it offered a valid solution, given he wanted to stay with the original supply. Now that someone else has suggested getting a PC PSU, which are not terribly expensive, your second item makes more sense and I tried to reverse my vote. However the system won't let me. Commented Mar 9, 2013 at 17:26
• I upvoted @OlinLathrop 's response for you, and for several reasons of my own, including the simple fact that sometimes the question we ask isn't the question we should or could be asking in seeking the most efficient solution to a problem. So even though an answer can appear non responsive because it does not speak toward the solution proposed by the question, it can be a valid answer all the same, because it does speak to the resolution of the problem. I also upvoted Olin's response due to his additional follow up in the comments, which helped explain the reasoning for his recommendation. Commented Jan 2, 2021 at 12:15

Is applying 15 VDC to a blower that was tested by the manufacturer at 13 VDC problematic?

Like Olin says, only the data sheet of the fan can answer this for you.

I understand Voltage is the product of Current and Resistance as in V = IR. Since 12 VDC power supplies capable of 17.5 A minimum are not cheap, if the placement of a resistor can allow my 15 VDC power supply to work, that is the most sensible choice.

Your fan isn't really like a resistor. For example, it probably needs a lot more current to start up than it does after it gets running. If you size your resistor for the running current, you might prevent it from starting up. If you size your resistor for the start-up current, you might overheat the fan after it gets running.

Will someone please provide to me the simplest method of reducing the voltage of the power supply using electronic components?

If you're determined to hack this, 2 or 3 series diodes would be a better choice to drop the voltage than a resistor. Diodes will drop roughly the same amount of voltage over a wide current range. It's up to you to find a combination of diodes that can carry 18 A continuously and drop close enough to the right amount of voltage. Also, these diodes will be burning about 36 W, so you will need to be sure they have heat sinks or other means of cooling.

Edit

Olin's comment points out that your question title says the fan needs 13 V, but your text says 12 V. To drop 2 V, your diodes (or any other thin you put in series) will need to burn 36 W, to drop 3 V, they'll need to burn more than 50 W. Careful consideration of cooling will be needed.

• Note that simply dropping the voltage by putting something in series will dissipate over 50 W, whether a resistor or diodes or something else. That level of power is not only wateful, but dealing with the heat will not be trivial. The power diodes and heat sink will cost a good fraction of a proper 12V supply. Commented Mar 9, 2013 at 15:09
• @OlinLathrop, you're right. I was reading the title of the question, not the text. Commented Mar 9, 2013 at 15:12

You could use two of these STTH30L06 power diodes in series; they have a current rating of 30A, and a maximum forward voltage drop Vf of 1.55v at 25 deg C and, and a typical Vf of 1.0v at 150 deg C. So two of them in series would give you a drop somewhere between 3.1v to 2.0v, which fits your requirements. Make sure you use hefty heat sinks on each diode.