Direct Voltage Adapter

[jtexas]

I have a need to measure peak AC voltage, but since I'm too cheap to buy a DVA, plus which I prefer fabricating my own tools anyway just for the challenge, I'm looking for some advice. A design involving a bridge rectifier/smoothing capacitor wouldn't work because I need to measure voltages as low as 0.5V. Unless there's a way around the diodes' voltage drop. My upper bound would be 150V.

Does anyone know how a DVA works? Have a schematic? Any ideas at all?

Thanks!

 

[Sceadwian]

A rectifier + smoothing capacitor would give you average voltage not peak.
If you really need to measure .5V the answer is simple, create a multi-range device, where the voltages above 5 are fed directly into the measurement section and voltages bellow 5 were fed through an op amp with a fixed gain of say 10. That would give you a detection range down to about .05V's of input signal, depending on the diode drop.

 

[kchriste]

You would use something like this:
**broken link removed**
To scale it up to 150V input you would need to use a voltage divider on the input.

A rectifier + smoothing capacitor would give you average voltage not peak.

Yes it would give you peak not average. Well sort of; Vout = Vpeak - 0.7V with just a simple diode and cap.

 

[ericgibbs]

hi,
Download the CA3240 datasheet from www.datasheetarchive.com
[its the dual version of the attached cct]

 

[Hero999]

That looks like a single wave precision rectifier not a fulwave precision rectifier to me.

 

[ericgibbs]

That looks like a single wave precision rectifier not a fulwave precision rectifier to me.

I have never heard of a single wave rectifier! perhaps you could explain?
Unless you have it confused with 3 phase?

Do you mean a half wave rectifier?

I would take a closer, careful look, its a FULL wave rectifier!

 

[Hero999]

I think I've figured it out.

For the negitive cycles the op-amp acts as an inverting half wave precision rectifier and for positive cycles R1+R2 and R3 act as a potential divider.

This circuit can't amplify, it will always make a loss of greater than 2 and the load always needs to be a specific impedance.

 

[ericgibbs]

I think I've figured it out.

For the negitive cycles the op-amp acts as an inverting half wave precision rectifier and for positive cycles R1+R2 and R3 act as a potential divider.

This circuit can't amplify, it will always make a loss of greater than 2 and the load always needs to be a specific impedance.

Hi,
You are right.
Thats also why I suggested to the OP he gets the dual version of the 3140 ie;[3240] and then he can use the second stage as a non-inverting amp.

 

[Hero999]

I normally use this circuit as it amplifies too.
**broken link removed**
**broken link removed**

 

[jtexas]

this some great stuff...

"If you really need to measure .5V..." LOL, my application is, troubleshooting the magneto-driven capacitive discharge ignition on my 1979 outboard motor. The trigger is an inductive pulse that needs to measure >= 0.5V peak AC. Why an RMS or average AC measure isn't good enough, I'm not sure. Output from the capacitor to the coils is 150V up to 400V or more. Don't why this would be an AC measurement, but that's another can of worms altogether.

thanks!

 

[Leftyretro]

this some great stuff...

"If you really need to measure .5V..." LOL, my application is, troubleshooting the magneto-driven capacitive discharge ignition on my 1979 outboard motor. The trigger is an inductive pulse that needs to measure >= 0.5V peak AC. Why an RMS or average AC measure isn't good enough, I'm not sure. Output from the capacitor to the coils is 150V up to 400V or more. Don't why this would be an AC measurement, but that's another can of worms altogether.

thanks!

A O-scope would work great to tell if the inductive pick-up probe is developing enough voltage to meet specifications. One usually sets the probe gap to insure amplitude is greater then minimum specification required for reliable triggering.

We use passive inductive pick-up probes extensively in our refinery as speed detectors (RPM) for electronic over speed trip protection on large compressors and such. Usually this in a 3 probe/channel system with 2 out of 3 voting to trip the machine off. The manufactures of the passive probes we use supply a chart showing AC RMS value (not peak AC values)Vs physical gap and recommend a .005 to .025 inch gap. Most of these systems use comparator ICs to read the probes. At normal operating speed these probes will typically output around 2-3V AC RMS.

Lefty