Simple voltage limiter question

[libertydog]

Hello,

I need to find the best way to limit, not regulate a voltage input to +5VDC.

Basically I have a signal input that will fluctuate from 0 - 19VDC (250 mAmps) but I want to insure that if the signal goes over 5V it stays at 5V. My sensor doesn't like voltages over that but I *don't* want it to cut out (go to zero).

The linear regulators I see seem to all want to produce 5VDC no matter what.

-vb

 

[CDRIVE]

If your load requirements aren't high a 5V Zener and resistor will do what you want. What's your signal source and what's the output Z? What's your load? Are you feeding this signal to a microcontroller, TTL logic block, other?

Edit: Oops! Missed your 250mA requirements

 

[CocaCola]

You could use a voltage divider and scale the 0-19 to about 0-5

 

[KrisBlueNZ]

I think you're misunderstanding what regulators do. A regulator will do what you want. If the input voltage to the regulator is low, the output voltage will be low as well. A regulator can't create voltage from nothing.
You should also consider the regulator's dropout voltage. If a regulator has a dropout voltage of say 1V this means you will need at least 6V at the input to get 5V at the output, and when the input voltage is 3V the output voltage will only be 2V. (It's not quite that simple, and the dropout voltage also depends on how much current you're drawing from it.)
In other words, the output of a 5V regulator with 1V dropout voltage will track the input voltage (with a drop of 1V) until the input voltage exceeds 6V, when the output will stay at 5V.
The simplest way to do this is with a resistor and a zener diode or a shunt regulator (e.g. TL431) and this could work quite well if your load current is low.
You need to explain your application in more detail.

 

[CDRIVE]

Hello,

<snip>
Basically I have a signal input that will fluctuate from 0 - 19VDC (250 mAmps) but <snip>
-vb

How did I miss this in my first reply? Anyway, what's your load that your voltage can swing from 0-19V but I=250mA, or is that 250mA @ 19V? A resistor and Zener, as I previously stated, isn't going to handle 250mA.