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I initially designed a circuit that would take a 12V battery as its power supply and power up a comparator circuit that would turn on/off a 12V fan based on temperature. This 12V battery could have been anywhere between 11.58-14.4V due to dis/charge so I regulated it with an LM7812, no worries. Comparator circuit uses LM358, NTC thermistor and a BJT for turning fan on/off.

Now my requirements have changed: The circuit will be supplied from a 48V battery that will have a voltage range of 43.2-67.2V. The load is now 48V (x4 12V computer fans in series). I have no idea how to regulate the power supply that is this high. Everything I look at, e.g. LM2576HV only goes up to 63V and Vout possibility is only ~15V.

I'm thinking of powering the comparator circuit with a 9V battery so that I don't have to drastically change all my components.

My questions are: 1. Do you know of an IC that can take this kind of Vin (43.2-67.2V) to have a Vout of 48V? Is there a simple alternative solution to this? 2. Is the 9V to power the comparator circuit a good idea? Will have to adjust the resistors so the LM358 opamp can turn on/off at values I want and pretty sure it can be supplied by 9V. 3. With the increased supply and load, will my current BJT be able to handle the change. It's a BDX53C and the load will be 1.11A.

Thank you.

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  • \$\begingroup\$ Assuming 4 fans in series work OK. Use a high side FET or bipolar to drive them.Place a 2 resistor voltage divider across the 4 fan string to give say 3V or whatever when Vfans = 48V. Feed 3V signal to a comparator to drive a transistor to drive high side control transistor. Op-amp supply is only a few mA and can be supplied by eg resistor plus zener post regulated if desired by a 3 terminal regulator. Op-amp operates at low voltage. Only need 2 transistors able to handle Vmax - which are easily sourced. at say 5 mA and ~60 V opamp supply takes 60C x 5 mA = 300 mW. \$\endgroup\$ – Russell McMahon Feb 17 '14 at 14:34
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Firstly, your use of a 7812 voltage regulator for you initial circuit may have seemed fine when you tested it but the 7812 needs at least 14V (or above) for it to provide a regulated output of 12V - if you check what the output is when the input drops significantly below 14V you will see that I'm correct. At 11.58V, it's likely that your "regulated" output is less than 11V.

I'm saying this because you have a misconception as to how linear voltage regulators work and you are in danger of believing you can apply this to higher voltage supplies like the 48V in your question.

You have a desired input voltage range of 43.2V to 67.2V and your required output is 48V. Without compromising your intended requirements, the only way this can be achieved is with a buck-boost switching regulator.

In addition to this, powering fans in series will likely destroy one fan - it will go open circuit (or burning) and then nothing will work. To simplify this, consider using a switching buck regulator that converts your 43.2-67.2V to +12V, then power your fans in parallel.

Maybe try one of these: -

enter image description here

I'm assuming you don't need more than 1A at 12V output as per your original design.

EDIT A higher output version is this and now, you will have to use external transistors on the device: -

enter image description here

I would also seriously consider the LTC3810. Here's a picture and also see figure 19 in the data sheet: -

enter image description here

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  • \$\begingroup\$ Wish I could give this more than +1 for pointing out multiple very good points. \$\endgroup\$ – John U Feb 17 '14 at 10:13
  • \$\begingroup\$ Thank you for your answer. I did notice the other day that the 7812 was not giving a 12V output on discharged batteries. Looks like the LM2576-12 has a lower limit of 15V too. I see from the datasheet that Vin for LM7812 should be 14.5-27V (Fairchild). \$\endgroup\$ – Dan Iim Feb 18 '14 at 2:02
  • \$\begingroup\$ Looks like the LM2576-12 has a lower limit of 15V too. Had a look at the LTC3637 and looks great. Am I reading the graph correctly: when load current is higher the efficiency is better? Also, with such a drop in voltage is there any need for a heat sink. I could not tell from the datasheet and am inexperienced in the use of step-down regulators. <br/> Also, I would need more than 1A. I should explain my load more: I have 12 fans that are 12V, 0.37A each. They will be in 3 sets of 4 (I have 3 'windows' where exactly 4 fans fit) so I thought the best way was to series 4 and then parallel 3 sets. \$\endgroup\$ – Dan Iim Feb 18 '14 at 2:39
  • \$\begingroup\$ Having 4 in series gives me 48V, 0.37A and then paralleling them gives me 48V, 1.11A requirement. I think that there are many points of failure for all the possible configurations of this load. Now I'm not sure what to do. I guess I'll try and find a regulator that can do >1A. Sorry for all the comments but looks like there's a character limit. \$\endgroup\$ – Dan Iim Feb 18 '14 at 2:39
  • \$\begingroup\$ @DanIim - take a look at the other devices I've added. These can give you 12v at up to 10A. I wouldn't put fans in series. \$\endgroup\$ – Andy aka Feb 18 '14 at 8:15
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I dunno, can you run those fans in series? That might not be such a hot idea. And what if one fails such that current no longer flows through it - then all of them shut down. You may want to consider putting them in parallel and using a '48V' - to 12V DC-DC to make them go. I just had to spec something like this from Vicor, which has modules that run in inputs over the range of 35-75VDC in the '48V' range. There are likely more cost effective options; you'll want to consider other requirements when selecting a converter.

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You could consider using a linear regulator similar to the 7812 that can handle those voltages. It's an inefficient way of doing it, compared to switching regulator, but it is comparatively simpler to get going quickly. It would depend on how much loss you are willing to tolerate.

The TL783, with connections similar to the LM317, could be a regulator you could consider using if efficiency isn't an issue. Make sure you include some reasonably heavy heat sinking and use resistors with sufficient power dissipation ratings. Note that regulators like the TL783 and the now obsolete VB408 with essentially identical connections have a not insignificant minimum load requirement (though most switching regulators will also impose that condition)

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