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What is the cheapest way to get 5V(150mA) regulated from 24V unregulated?

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fuzzzy

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HI,

I was wondering what is the cheapest way to get 5V(150mA) regulated from 24Vunregulated? a linear regulator lm7805 would burn after some minutes. but a switching stepdown is too expensive.

thanks for help


Merry Christmas
 

24v to 5v

Hello

You can use a normal 7805 for 1 amp and this one can be pulled to 35V on the input.
Depending the current you need the dissipator.

You can use an lm2575 step down, equiped with an 500uH indutance and 100uF capacitor.

Regards
 

24v to 5v circuit

Moreover, you can step it down on two steps, i.e. 24V (150mA) -> 12V (150mA) -> 5V (150mA)

I do prefer this method rather than using the large 1A regulator!

I hope that helps u
Ahmad,
 

24v to 5v 7805

You can try a buck converter with MC34063, is really cheap, and I don't think the price will be over a linear's (plus dissipators) .

/pisoiu
 

7805 24v

To drop 20V accross the 7805 @150mA you must fit a small heatsink and with a heatsink the 7805 regulator is perfect for this job.
 

7805 switching regulator

With 24V in and 5V out, and a 150mA load, the lm7805 will need to dissipate 2.85W ((24-5)*0.15=2.85). The thermal conductivity from the die to the case (Rjc) is 5 C/W and the thermal conductivity from the case to air (Rja) is 65 C/W. The Rjc means the die will be 14.3 C warmer than the tab (2.85 * 5 = 14.25). If you do not heatsink the package then Rja means that the package temperature will be 185.3 C warmer than the air (65 * 2.85 = 182.25). The device will thermal shutdown at 125 C though. If possible add a heatsink to dissipate the 2.85 W well enough to keep the device from thermal shutdown. Even if you use a 12V regulator (lm7812) and then a 5V regulator (lm7805) you will still need to dissipate 2.85 W, 1.8 W in the lm7812 and 1.05 W in the lm7805. A switching regulator will allow you to get much better efficiency. If you build a switcher that's only 80% efficient you'll only need to dissipate a little over 0.7 W.
 

mc34063 5v supply -datasheet

Thanks for your answers.
This device should work in automotive applications and with 24V or 12V. what do you think if i use a zener diode like 6V drop in series with positive power supply and then a linear regulatore like 7805??
do you think it could work??
In case of 24V power supply linear regulator would dissipate 13V intead of 19.

Merry Xmas
 

mc34063a lm7812

In your case, using 7805 in TO-3 or TO-220 packages is the most feasible solution, as you do not want to use the simplest and most universal MC34063 switching regulator. Do not use zener circuit, as it will heat much and give the worst efficiency.

The problem you are facing is heat from 7805, of which 7805 would die? The solution is one of the followings:

1. You use a power resistor to share the heat with 7805, with calculation for dropout on it is about 10 to 14V, the rest 9 to 5 volts for 7805 dropout. Its power dissipation capability must be at least twice the power = dropout * current.

2. You use a pre-regulator with a power transistor with the output of about 8 to 9V, and then this 7805. For this pre-regulator, you can use a power transistor and a 8.2, 9.1 or 10V zener diode in its base circuit. So you can use this complex circuit in both 24V and 12V very well. Remember to use heatsinks for both transistor and 7805.

nguyennam
 

step-down 5v circuit

If I decide to use MC34063, do you think i can supply directly a MCU and other logic devices 5V supply?
is this supply clear as a linear 7805?
could I find some problems in MCU working??
 

24v to 5v converter

In general, a switching regulator cannot give a clean power as a linear does.

If you want a simple, maximum 150mA power supply of 5V, so in my opinion, my above circuit with 7805 and pre-regulator must be the most suitable for you. Although the switching regulator on MC34063 is the simplest in its family, but you need to have somewhat experience on them.

To use this MC34063 to supply MCU, you need large polar capacitor and a good bypass capacitor right next to the MCU.
 

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