dc-dc booster, how do I ensure I don't draw too much amperage.

[jleslie48]

I have a bunch of 24v dc fans I want to power from a marine battery at 12v for an rv. each fan draws 110ma I wired them in parallel, so my load is 440ma (I think) Now If I power this directly from the 12v, the fans run slow, so I bought:

**broken link removed**

150W DC-DC Boost Converter 10-32V to 12-35V Step Up Charger Power Module

which by black magic converts the 12v to 24v. but there is a warning on the ebay listing:

Specifications:
Input voltage: DC 10V-32V.
Output voltage: DC12V-35V(adjustable).
Output current: 10A (MAX).
Input current: 16A (MAX) (more than 10A please strengthen heatsink).
Output power: natural cooling 100W (MAX), strengthening cooling 150W (MAX) real power.
Conversion efficiency: 94% (when Input 19V 2.5A Output 16V, Reference).
Output ripple: 2% (MAX).
Working temperature: industrial (- 40 ° c to +85 °c) ( environmental temperature, more than 40 degrees, please reduce power to use, or - enhance cooling).
Full-Load temperature: 45 degrees.
No-load current: 25mA typical.
Voltage regulation: ± 0.5%.
Load regulation: ± 0.5%.
Dynamic response speed: 200uS 5%.

Short circuit protection: No (Please installs the fuse or protection circuit at input parts).
Input Reverse protection: No (please comply with a reverse protection or connect a diode at input parts.)
Size: 65mm x 35mm x 21mm

I believe putting this DC-DC power boost inline will work fine but I have some theory questions if I add more load and
push the input current beyond the 10A suggested limit.
being an amateur, I figured it was like LEDs, all I would have to do iis put a resistor inline (something like a 2-ohm 25watt resistor) and that would "current limit" the input connectors to around 4-6A. The double E guys say that won't work, but can't explain to me why in laymen's terms. While researching this I see some reasonably simple circuit boards that say they will do this, but none seem quite right, and besides, I found it suspicious that I can't find one of these fairly simple "current limiting circuits" pre-built to buy from china for about $2, can somebody explain:

1) what is wrong with my inline resistor Idea and

2) what kind of circuit do I need if the inline resistor doesn't work, or what is wrong with my idea? I don't want to overheat the dc-dc booster.

Here is a typical website that talks about a dc current limiting circuit board:

https://freecircuitdiagram.com/2008/08/27/variable-adjustable-current-limiter-circuit/

or

https://www.radio-electronics.com/i...urrent_limiter/transistor_current_limiter.php

 

[FvM]

Assuming a converter efficiency of 85 %, the 12 V input current with 24V/0.44A load will be about 1A. Far below the maximum converter current, no risk to overload it. To be prepared for a possible short or defective fan, you can connect a fuse, e.g. 2 or 5A in series with the converter input line. Using a current limiter would be just overengineering.

 

[jleslie48]

Assuming a converter efficiency of 85 %, the 12 V input current with 24V/0.44A load will be about 1A. Far below the maximum converter current, no risk to overload it. To be prepared for a possible short or defective fan, you can connect a fuse, e.g. 2 or 5A in series with the converter input line. Using a current limiter would be just overengineering.

thank you for your response, and yes over-engineering is something I am very guilty of in the past The fuse is definitely a good idea.

I'd still like an explanation as to why a resistor in-line would not be an effective amperage limiting solution.

 

[D.A.(Tony)Stewart]

THe current for Fans starts at a fairly constant current and then rises slowly to perhaps double the current from 12 to 24V as more work is done pushing air. Typical 4" muffin fans take about 5W and you would get perhaps 20% of the RPM at 12V.

Adding resistors would only reduce the fan speed and dissipate more power in the series R's and be ineffective for efficiently regulating the speed and current consumption but possible. You can regulate the fan speed by adjusting the Vout trimpot will will some some effect on current consumption but not in a linear way as current starts fairly constant at low V then rises from stall speed to Vmax .

I see no problem running a dozen of these fans but if you are trying to cool by circulation, keep in mind you are also adding a bit of heat.
Using a fan for forced air speed over the fins for cooling of the boost regulator when attached to a heatsink will allow you to use the rated max power.

 

[FvM]

I'd still like an explanation as to why a resistor in-line would not be an effective amperage limiting solution.

The bad thing with a current limiting resistor is the voltage drop and respective power loss in normal operation. It may be a reasonable option if the current in normal operation is only a small fraction of the maximum current, e.g. 1 A in case of the fan load. But even in this case, the overall efficiency is considerably reduced.

The shown transistor current limiter circuits are much better in this reagrd. Also a non-linear resistor like a filament lamp or a PTC fuse.

 

[jleslie48]

The bad thing with a current limiting resistor is the voltage drop and respective power loss in normal operation. It may be a reasonable option if the current in normal operation is only a small fraction of the maximum current, e.g. 1 A in case of the fan load. But even in this case, the overall efficiency is considerably reduced.

The shown transistor current limiter circuits are much better in this reagrd. Also a non-linear resistor like a filament lamp or a PTC fuse.

Yes, in a battery system, reducing effciency is a bad thing, and also any heat generation is counter productive. So continuing with the thought experiment (I concede the KISS it solution is simply an in-line fuse,):

1) which of the links are you referring to for the "shown transistor current limiter circuits" ?
2) why would such a circuit not be readily available pre-assembled? is would seems to me to be a very useful component?
3) what is the difference between a regular resistor and a "current sense resistor"
4) difference between a regular transistor and a "current sens resistor" (is the author just referring to there functionality or is it a different component?
5) can a BOM list be made? I would like to experiment with this circuit. (I have that kind of time. )

 

[Audioguru]

1) Which of the links are you referring to for the "shown transistor current limiter circuits" ?

They are both the same. They both explain how they limit the current.

2) Why would such a circuit not be readily available pre-assembled?

It is built-in on many circuits. It is simple to make.

3) What is the difference between a regular resistor and a "current sense resistor"?

That is what it is.

4) What is the difference between a regular transistor and a "current sens resistor"?

The resistor senses the current and turns on a transistor when its voltage reaches about 0.6V.

5) Can a BOM list be made?

The schematics have transistor part numbers and resistor values. They are usually changed to produce the desired amount of current limiting.