Dear all hello,

I am making an adjustable power unit, using a linear regulator and I will need a digital voltmeter and a fun for cooling the regulator. My SMPS I will use delivers 42 vdc.

The digital voltmeter needs 8-12vdc to operate with 10mA current. The fun needs 24vdc with 60mA current.

I could use step down dc converters to get the above voltages needed, but instead, for simplicity, I put a 300ohm resistance in series with my fun and got the 24V needed.

Also I did the same but with a 3Kohm resistance and I got the 12vdc needed for the voltmeter.

Probably is not the efficient way power wise to do it, but is very simple. Do you have any other proposal or comment?

Thanks a lot

Doros

 

Your simple circuits that you have shown in simulation will not work in real life. What you have shown are current sources with some unknown internal resistance. The voltmeter is showing the voltage across this unknown resistance (which can be determined after the fact using Ohm's Law).

Do you know what your real loads (i.e. resistance) will be?

What you need are two buck converters, one to output 24VDC and another to output 12VDC.

 

Low cost buck converters (pay attention to max Vin) -

https://www.banggood.com/search/buck-converter.html

Or use a simple series zener of appropriate voltage. That works if SMPS
fairly well regulated. Zener 1/2 W or 1W will do.

https://www.diodes.com/products/discrete/diodes-and-rectifiers/diodes/zener-diodes/

Regards, Dana.

 

Hello MrChips and Dana.

Sorry for the schematic, it was for illustrative purposes and not for simulating the problem. I could put a resistor of 400ohms in the place of the fan, (24vdc/0,06A), and a resistor of 300ohm in series, getting the voltage needed (24vdc) and current (60mA)

I can understand that the buck converter is the appropriate solution in this kind of situations, but for simplicity I thought the resistors.

Not efficient though, but I forgot that a zener which is also a simple solution and more appropriate than the resistor.

Thanks a lot for the input, I think I will go for a quick solution with the zeners

Doros