calculating resistors needed for a 0-5V E/P controller

[robbm]

i have a circuit that i need to switch between a couple different voltages to give an E/P 0-5V controller. right now, i am planning on using a 5VDC supply wired to two different resistors and a 2 position switch (to select either resistor), in order to feed the E/P with two different voltages. is the E/P current draw the only number i need in order the calculate the resistor size needed to feed it 2 and 4V for each switch position? i also have a 0-5VDC transmitter that will be loop powered to a display unit, so do i need to add that current draw as well when calculating the resistor size needed?

thanks in advance.

 

[IRstuff]

That probably will not work AT ALL, and might actually damage whatever part you're trying to program. Programming requires a constant voltage, precisely because the current draw is non-constant.

TTFN

FAQ731-376

 

[robbm]

ok, well i was just given the instruction sheet for the unit, which shows that first off its actually 0-10VDC, and has a separate 24VDC supply voltage required, and it states a command signal impedance for the voltage input of 4.75K-ohms. does this help at all in figuring it out? can i put a resistor in series with the 0-10V supply and the E/P to give the E/P a specific voltage as its 'input'?

 

[Skogsgurra]

Excuse me, I can think of many meanings for E/P. What does E/P mean in your world?

BTW,the 4.75 kohms you mention is the input impedance. Not something you should have in your supply (or control) circuit.

Leave this to someone that knows how.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[robbm]

what we are working with is a voltage-pressure transducer, so by feeding it a signal voltage (separate from the supply voltage) of an amount that varies between 0 and 10V, we'll get a specific output pressure.

 

[IRstuff]

How do you even know that it's voltage command device? The datasheet indicates that it could be either voltage or current commanded.

Moreover, that datasheet also indicates that it's supposed to be a in closed-loop system, so a fixed voltage will probably not work correctly, since there's nothing to correlate with the actual pressure produced.

TTFN

FAQ731-376

 

[zappedagain]

Robbm, this is in response to your second post, not your first. It looks like your requirements changed quite a bit between the two.

Power and signals are very different. It gets extremely tricky to try and mix the two. Provide 5V to your micro and 24V to your sensor, and make sure you have a bypass capacitor at the sensor and your micro.

First, make sure you have a voltage output (OB1TFE) and not a current output (OB1TFI) unit.

All you need is a voltage divider to convert the 0-10V signal to a 0-5V signal, so something like 20K in series with 20K will work. If you don't know what I'm talking about then Google 'voltage divider'.

A capacitor in parallel with the lower resistor will help knock out noise. Just be careful you don't slow your response down too slow.

The 4.75K is actually the output impedance of the sensor, so that will add to the top resistor in the divider and actually give you a slightly lower output. If you aren't driving an A/D input you may need to tweak your values a bit to stay at the proper logic levels; watch out that the 4.75K may not be constant though.

John D

 

[zappedagain]

Ignore that last paragraph. The 4.75K is the load of the control signal, not the monitor output resistance.

John D

 

[robbm]

ok, so let me know if i'm understanding this:

the load resistance is 4.75k, and i want a 6V signal to the load, so from a 10V supply, i need a voltage drop of 4V across the 1st resistor, so a resistance ratio of 4:6 or (2:3) for the resistor and the load. this would need a 2.3k resistor in front of the load. (see attached for what i mean)

would that be right?

 

[robbm]

dammit, got my numbers displaced, i meant to type 3.2k resistor, not 2.3k. sorry

 

[zappedagain]

You've got the concept. Welcome to electronics design!

John D

 

[robbm]

haha, thanks. designing industrial control panels, i don't mess much with resistors and such anymore, the PLCs do all the work now! and remembering anything is always in the 'use it or lose it' department it seems.

thanks again.