AC to DC / mA Converter


Thread Starter

Shah, Dharmesh

I have a requirement for conversion of 0.1 V to 200 V AC into either 4-20 mA or 1-5 V DC output. This conversion is expected with galvanic isolation and maximum within 150 msec. If resolution is applicable, it should be 0.1%

Can anyone suggest me the suitable means for achieving this?

Dharmesh Shah
go to "": or call them American Aerospace Controls, Inc. 516-694-5100. The model number you want is 108X-270 this will take .1-270 VAC and give you a 4-20mA signal. they have other ranges also. The cost for this one is around $210.00

Johan Bengtsson

Do you want an off the shelf-ready solution or do you want to build it yourself?

If building it yourself is an option then here is some ideas:

Since you have AC in and want galvanic isolation start with a transformer While you are at it shoose a transformer bringing down the voltage to
some suitable value (like 10-15V max). You don't say anithing about frequency of your AC voltage but if it is around 50-60Hz or not too far away from that almost any small power transformer with suitable voltages would be good.

Take the output voltage and feed it to an OP (+ pin) and put a diode from output to the - pin, put a capacitor and a resistor to GND. Should also have a resistor between the transormer output and GND to give the transformer some small load.

| \
Transformer secondary ----*-------------|+ \
| | ----
| --|- / |
--- | | / |
| | | |/ |
| | | |
--- *---|<|--*
| |
| *-------*-----------> Vout
| | |
| --- |
| | | -----
| | | -----
| --- |
| | |
Transformer secondary ----*-----------*-------*------------ GND

The idea is to rectify (half wave inthe above example) without loosing the forward voltage drop of the diode. Tha capacitor is of course
expected to hold the voltage between pulses and be slowly discharged.

what you have now is a voltage 0-something (depending on your transformer) and converting that to either 1-5V or 4-20mA is something you either can do with some more OP:s or some ready made solution for that. Note that you can't load the above output with much before affecting the
reading, but that is probably not a problem anyway... Also note that the OP need to have a power source giving full +- voltage exceeding the peaks of the AC voltage as it appears on the secondary coil enough for the OP to work with and allowing for diode voltage drop as well
as voltage drop internal to the OP.

/Johan Bengtsson

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