I'd appreciate some ideas on a precision, temperature-stable current
to voltage circuit to convert e.g. a 4-20mA signal to a 0-5V signal
(which I will then feed into a Voltage-to-freq converter and read
with a uP).
Rob BRUCE-BRAND wrote:
>
> I'd appreciate some ideas on a precision, temperature-stable current
> to voltage circuit to convert e.g. a 4-20mA signal to a 0-5V signal
> (which I will then feed into a Voltage-to-freq converter and read
> with a uP).
>
> Many thanks
> Rob Bruce-Brand
> Durban Electricity
What sort of temperature swings are you expecting? If not too severe, a
resistor will do the job just fine.
You could also charge a cap and measure the time it takes to 'trip'
(change to high) your uP's input. This will work well on a PIC. (Of
course)
--
Friendly Regards
Tjaart van der Walt
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Rob BRUCE-BRAND <spam_OUTBRUCE-BRANDRATakeThisOuTDURBAN.GOV.ZA> wrote:
>I'd appreciate some ideas on a precision, temperature-stable current
>to voltage circuit to convert e.g. a 4-20mA signal to a 0-5V signal
>(which I will then feed into a Voltage-to-freq converter and read
>with a uP).
Rob BRUCE-BRAND wrote:
>
> I'd appreciate some ideas on a precision, temperature-stable current
> to voltage circuit to convert e.g. a 4-20mA signal to a 0-5V signal
> (which I will then feed into a Voltage-to-freq converter and read
> with a uP).
>
If your planning to ultimately send your current signal to a "voltage-
to-frequency" converter, then all you will need are two resistors.
This is because most V/F converters are really current-to-frequency
converters (eg AD652, LM231, etc). Let's take the AD652 for example.
It has a full range current input of 1ma. So you would need to
divide your full range of 20ma by 20. (Actually, I would divide by
a slightly larger number to allow overanging).
4-20ma-->
----------+------+
| |
3.8 to 19 / / 0.2 to 1ma
ma \ \
R1 / / R2
\ \
| |
---- +---- To "virtual gound" in V/F converter
/ / /
The other equation relates to the maximum drive voltage of the
4-20ma circuit. In other words, the voltage developed at the input
is
V_in = I_in * (R1||R2)
= I_in * (R1*R2)/(R1+R2)
= I_in * 19/20 * R1
And, Vin must be less than the maximum voltage of the 4-20 ma driver,
so:
V_max > V_in(max) = 20ma * 19/20 * R1
> (19 volts)*R1
or
R1 < V_max/(19 volts).
There are obviously other design considerations that I'm intentionally
ignoring, but I think this will get you going. (The other considerations
are 1: temperature stability of the resistors, 2: over voltage
protection
especially when your circuit is not powered, 3: Calibration, 4: Compen-
sation for the low end 0.2 ma offset current (software). I'll be glad
to address these issues off line, Rob.)
> I'd appreciate some ideas on a precision, temperature-stable current
> to voltage circuit to convert e.g. a 4-20mA signal to a 0-5V signal
> (which I will then feed into a Voltage-to-freq converter and read
> with a uP).
>
I think you are looking for a resistor: U = f(I) = R * I :-)
If you want to convert 0-20mA to 0-5V you choose a value of about
250 Ohm. But be sure to get a resistor with a low TEMPCO (oh, what's just
the english expression for 'Temperatur - Koeffizient') ?). I think
precision metal-film resistors have a TEMPCO of about 20 ppm/(degree
kelvin) or even lower. Hope that's good anough.
Q'uapla
PS.: I hope you understand what I wrote, because I couldn't find all the
words I wasn't sure about in my 'Langenscheidts'-dictionary :-(
>> I'd appreciate some ideas on a precision, temperature-stable current
>> to voltage circuit to convert e.g. a 4-20mA signal to a 0-5V signal
>> (which I will then feed into a Voltage-to-freq converter and read
>> with a uP).
>>
>
>If your planning to ultimately send your current signal to a "voltage-
>to-frequency" converter, then all you will need are two resistors.
>This is because most V/F converters are really current-to-frequency
>converters (eg AD652, LM231, etc).
Scott took the words out of my mouth.
I would recommend the AD654 for this purpose (it's an
8-pin package). The data sheet is on the web.
Use a fixed voltage as your supply or exciter voltage. 24 V seems to be
standard. Then calculate the resistance value to get a current flow of
20mA. Read the voltage drop across the resistor. The drop will vary
according to the current drain. Your transducer should be in line with
the resistor and the other end of your supply voltage. That is the current
to voltage converter portion of your circuit. The use a 4046 P.L.L. for
a V.C.O. and read the freq on your PIC.
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At 05:17 PM 10/28/96 +0200, you wrote:
>I'd appreciate some ideas on a precision, temperature-stable current
>to voltage circuit to convert e.g. a 4-20mA signal to a 0-5V signal
>(which I will then feed into a Voltage-to-freq converter and read
>with a uP).
>
>Many thanks
>Rob Bruce-Brand
>Durban Electricity
>
>Rob BRUCE-BRAND <EraseMEBRUCE-BRANDRAspam_OUTTakeThisOuTDURBAN.GOV.ZA> wrote:
>>I'd appreciate some ideas on a precision, temperature-stable current
>>to voltage circuit to convert e.g. a 4-20mA signal to a 0-5V signal
>>(which I will then feed into a Voltage-to-freq converter and read
>>with a uP).
>
>A 249 ohm 1% resistor works for me.
>
>--
>Bob Fehrenbach Wauwatosa, WI bfehrenbspam_OUTexecpc.com
>
>
DURBAN.GOV.ZA> wrote:
