Regulators or Resistors??

[Carnivore]

I'm very new to electronics in general. Having said that, I have run into my first real newbie snag.

I would like to convert, or split off and reduce an existing DC voltage from a higher, unregulated voltage to a lower voltage. See following:

Existing voltage:
Min 24 VDC
Nominal 34 VCD
Max Peak 44.5 VDC

Desired Voltage:
5 VDC

I was thinking a regulator would do the job but from what I have read, you should try to keep input voltage within a few volts of the output voltage.

So I thought maybe resistors in series but then I read that using resistors in this fashion can produce a lot of heat.

If I’m asking this question wrong, or seem generally uneducated on the topic please don’t be too hard on me, I really am new but am trying diligently to learn.

 

[kchriste]

As long as the current is fairly low, then a linear regulator will be fine. What's the max current you need to draw at 5V?

 

[Carnivore]

As long as the current is fairly low, then a linear regulator will be fine. What's the max current you need to draw at 5V?

According to the datasheet for the part I'm trying to power only needs
0.04mA.

would you happen to have any suggestions on a paticular Linear Regulartor?

 

[dknguyen]

I'd go for a switching regulator since you are stepping down the voltage so much. If the output was 100mA, you'd be dissipating 2W of power...at minimum input voltage!
====================
If you really wanted to use a linear regulator (an industry standard one is the LM78XX, where XX is output voltage) then you would have to attach one of two things to the input of the regulator since it can't dissipate that much energy and can only handle 12V input max-

(1) A zener diode and BEEFY resistor as a voltage chopper to dissipate the large amouts of energy so the regulator doesn't have to, and bring the voltage down to a level the regulator can handle
Regulator- http://www.fairchildsemi.com/pf/LM/LM7805.html
Zener Diode+Resistor Voltage Chopper - http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/zenereg.html

(2) Lead the linear regulator with a regulated, or unregulated (semi-regulated) DC-DC converter, which will do the same thing as the method above without all the energy dissipation. If you choose a regulated converter, you might as well go all the way and get one that steps it straight from 34V to 5V.
===================
That said, you are wasting a lot of energy if you directly clamp the voltage from 34V to 5V with a linear regulator. At 40mA it's probably okay if you use the zener clamper -> linear regulator method. But a minimum of 760mW-1.58W is a lot of power wasted.

I would say a step-down charge pump IC, but that's probably more than you want to do (since it's probably surface mount and it's something you probably have to order from Digikey or Mouser, although not hard to wire up). If you want some efficiency, but don't want to have to huntdown a 34V>5V switching converter, I would recommend using a zener voltage to 28V or so and following with this switching regulator (or another one):
http://www.dimensionengineering.com/DE-SW0XX.htm

What are you powering and what are you powering it from? It's a bit strange the source voltage is so high.

 

[kchriste]

That said, you are wasting huge amounts of energy if you directly clamp the voltage from 34V to 5V with a linear regulator.

40V max drop @ 0.04ma = 0.0016 watts hardly a lot of power....
Unless the Op meant 40V max drop @ 0.04A then it would be 1.6 watts of power which would require a heatsink on the regulator.

According to the datasheet for the part I'm trying to power only needs 0.04mA.

If it is only 0.04mA you need, then a simple 5V zener and a 8.2K (Or higher depending on the min current the zener needs for good regulation) 1/2watt resistor will be all you'll need.

 

[dknguyen]

Oh, it is 0.04mA. I see. Then zener clamper leading a linear regulator is probably best (you'd be hard pressed to find a 5V linear regulator that can handle 24V, let alone 44.5V input.)

 

[kchriste]

you'd be hard pressed to find a 5V linear regulator that can handle 24V, let alone 44.5V input.

Correct. If he doesn't need accurate regulation he can just use a 5V zener and resistor. Otherwise, a 12V zener regulator followed by a 78L05 would work OK. Or he could use the more exotic LM2936HV–5.0 which is rated for a max input voltage of 48V

 

[Carnivore]

Thanks a bunch!

im not sure what half of what you said is, but i believe you have given me enough information google me way through it.

 

[on1aag]

High voltage regulator.

Hi Carnivore,

There's a voltage regulator which can handle the 45 volts.
If you choose 240 hm: for R2 the output voltage will be 5 volts.

on1aag.

 

[Hero999]

An LM317 would even do it providing you don't short circuit it and if you're worried about that add a resistor in series with the regulator (about 100hm: or so), something that will drop the input voltage to a safe level when it's passing the short circuit current.