# How to drop 2-40V down to 1-1.5V?

• posted

Hi, I've run into a problem that I have worked on for quite a while. I was trying to find an inexpensive way to drop a 2-36V source down to around 1-1.5V. I had done it with a zener and a LDO voltage regulator before to drop sources from 5-18V, until I found a particular source that had 36V. I had been using the zener to drop the higher voltages down to a range that my voltage regulator could handle. However, when I tried the same circuit on the 36V source, the zener drew too much current causing the source circuit to "misbehave". The source is a DC source and the less current I draw to drop it down the better. The source isn't a battery and has some output impedence (which is why I used the zener), but depending on the product I test on, the voltage varies between 2-36V.

Is it possible to do this inexpensively? I am not an expert by any means but have done the best I can researching on my own and trying my own solutions. Any help is much appreciated.

Thanks, Gabe

• posted

As with all power supply questions posted here, I'm going to have to ask: How much current do you need coming out (at 1.5 V)?

• posted

try something like this for your preregulator for the LDO.

use a current limiting diode (1n5299) instead of the resistor to provide about 1mA constant current.

Chose a transistor beta large enough to handle your currents. If biasing a zener cause erratic operation it doesn't appear to be much available current.

• posted

I could probably get away with around 2mA but 5-10mA should be plenty.

• posted

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And how efficient does it need to be?

One approach is to use an LM317 voltage regulator, which, I think, outputs

1.25V if you use it like a 7805 (no resistors, sense pin directly connected to ground).

Another is to build a current source -- say 10 mA -- and drive the current through an infrared LED, which should give 1.6 V across the LED, or a pair of silicon diodes, which should give 1.2 V.

• posted

Thanks guys,

I'll have to order some samples and try them out... I think both will work. I think the sites I had searched didn't have a regulator that could handle the spread in voltage (at least not cheaply), and I didn't know about the current limiting diodes. I'm a software guy, but I think I have chosen the wrong field as I am fascinated by electronic design... so all I can do is tinker for now!

Thanks again!

Gabe

• posted

normal diodes can be used to limit voltage, but there are (or were) diodes out there that limit current.

or maybe a JFET could be used...

--+-[50R]---. .----+--- 1.2V | | | | | ---- | | ^ | | | | | V | `---------[100K]--+-->|-->|-- ground

Bye. Jasen

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