Very low current isolated voltage detection - Page 1
Forum section: electronics / projects

author: envisionelec
author_rank: Frequent Contributor
timestamp: December 30, 2017, 03:09:48 am
content: I’m tasked with finding a low cost way to detect a 120V, 400Hz voltage into an op amp through a resistive divider. The current design uses a standard 2VA 50/60Hz laminate transformer but a size reduction is forcing us to look at alternatives.Would an audio coupling transformer do the trick? They’re often 10:1 ratio and have a substantial breakdown voltage rating. There is no human interaction with the secondary side - isolation would merely be an incidental luxury when using test equipment in the field.

author: David Hess
author_rank: Super Contributor
timestamp: December 30, 2017, 03:30:25 am
content: An audio coupling transformer could work if its voltage rating and inductance is sufficient.Exactly what about the 120 volt AC 400 Hz signal are you trying to detect? Amplitude? Frequency? Presence?If a load of a couple milliamps is acceptable, then an optocoupler is often used.

author: envisionelec
author_rank: Frequent Contributor
timestamp: December 30, 2017, 03:33:39 am
content: Voltage amplitude.What kind of optocoupler can output a ratiometric voltage - do they have solar cell types?

author: David Hess
author_rank: Super Contributor
timestamp: December 30, 2017, 03:45:26 am
content: Quote from: envisionelec on December 30, 2017, 03:33:39 amThey make linear optocouplers which use one LED to drive two photodiodes for linear operation.https://www.vishay.com/docs/83622/il300.pdfhttp://www.ixysic.com/home/pdfs.nsf/www/AN-107.pdf/$file/AN-107.pdfhttps://www.edn.com/design/test-and-measurement/4344950/Optocoupler-simplifies-power-line-monitoringA linear optocoupler is just one way to do it. A transformer may be better for your application.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: December 30, 2017, 04:14:59 am
content: Zettler has 0.35 VA line xfmrshttp://www.zettlermagnetics.com/pdfs/EE20_35.pdf

author: David Hess
author_rank: Super Contributor
timestamp: December 30, 2017, 04:59:34 am
content: When you say very low current isolated voltage detection, what exactly do you mean? Low supply current? Low current drawn from the 120 volt 400 Hz AC line? Besides being too large, did your previous solution draw too much current?A transformer rated for higher primary side voltage at 400 Hz will have higher inductance and a lower magnetization current. Two transformers could be used with their primaries in series and secondaries in parallel to boost the primary side voltage rating.Another way I would at least consider is using high voltage capacitive coupling. If the input resistance of the voltage divider is high enough, then a very low value of capacitance may be used while still having a low frequency cutoff significantly below the AC line frequency. The capacitor should be voltage derated and a safety type. Input protection after the voltage divider should be used. This is all standard high input impedance amplifier stuff. Two capacitors and two voltage dividers may be necessary to make the connection to neutral as well and then a difference amplifier used. In the extreme case, this duplicates how a high impedance oscilloscope probe and input works.

author: envisionelec
author_rank: Frequent Contributor
timestamp: December 30, 2017, 11:26:59 am
content: Quote from: f5r5e5d on December 30, 2017, 04:14:59 amBingo! I don’t know how I missed this but it’s perfect! Cheap, too!

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 30, 2017, 12:15:32 pm
content: You could also have a line-powered circuit that measured the voltage directly and sent the voltage value as short data bursts ( to minimise power draw from line side) into an optoisolator. Woudl be smaller than a transformer solution, line side powered by a small capacitive or resistive dropper.

author: envisionelec
author_rank: Frequent Contributor
timestamp: December 30, 2017, 03:57:11 pm
content: I was intentionally a bit vague on the application as it’s a proprietary classification. The measurement current isn’t important and is only an incidental to driving a buffer into an ADC. The source is capable of generating over 100kW but must be monitored for irregularities. I’m replacing a trio of fairly large transformers and wanted to find a small and cheap solution. The Zettler is about 2USD which is great.

author: MasterT
author_rank: Frequent Contributor
timestamp: December 30, 2017, 04:28:24 pm
content: There are Isolation amplifier IC, for example:https://www.digikey.ca/product-detail/en/silicon-labs/SI8920BC-IP/336-3329-5-ND/5358956

author: Kleinstein
author_rank: Super Contributor
timestamp: December 30, 2017, 04:39:54 pm
content: Those very small transformers ( < 1 VA) are rather far from ideal. The magnetization current is rather high and can give a significant distortion to the waveform. The higher frequency can help as this reduces the needed magnetization. If can also help to still use the 230 V winding as a primary to reduce the field even further. If linear transfer is more important than load current, one might consider a series resistor and a current transformer (could be common mode choke used as a 1:1 transformer). Compared to a poor quality 1VA transformer the load current might still be lower.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 30, 2017, 04:45:37 pm
content: Quote from: Kleinstein on December 30, 2017, 04:39:54 pmThe high winding resistance means they tend to run quite warm, even with no load.

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 30, 2017, 06:43:28 pm
content: As some have stated, small transformers are not ideal. EMI may also be a concern depending on the environment in your device and the noise in your 120V signal. You could consider isolated analog amplifiers also, as MasterT suggested, which may fit the bill, but you will need to add a (decently-protected) regulated voltage to power them, generated from your 120V voltage. In both cases, you will essentially bind yourself to a specific vendor, since those components rarely have direct equivalents.One simple solution I would suggest in your case is capacitive isolation (see schematics attached). Use X2 capacitors or such.This essentially forms a capacitive divider with a gain of 1/200, and the output voltage is centered around half your isolated power supply voltage (which I assumed here to be 3.3V, but it doesn't matter). It also forms an high-pass filter. With the chosen values, 400 Hz is well beyond its Fc, so its gain is very close to: 1/(1/C1 + 1/C2)/C3.You could add some MOV on the 120V side for added protection.The downside of this solution is that the precision of the gain depends on the tolerance of the 3 capacitors, which will be each at +/5% for the best to +/-20% for regular caps. May not be a real problem in your application.But it's definitely low cost, low power, low EMI and independant from any specific vendor.

author: Zero999
author_rank: Super Contributor
timestamp: December 30, 2017, 08:03:48 pm
content: My experience of small mains transformers for this type of application has been quite positive. I remember using one to generate a 40VAC RMS signal, at a few hundred Hz by driving one with an audio amplifier and signal generator. The distortion couldn't be seen on an oscilloscope, which I know isn't excellent, but it was good enough.You'll need to calibrate the transformer, just looking at the maximum unloaded voltage of regulation specifications and calculating the secondary voltage won't do. Make sure you use a decent multimeter with an RMS function which can work up to 400Hz or more.

author: flowib
author_rank: Contributor
timestamp: December 31, 2017, 08:05:28 pm
content: If you decide to go with insulation amplifiers, the SN6501 can be used to provide the supply needed to operate the insulation amp.

author: Kalvin
author_rank: Super Contributor
timestamp: January 03, 2018, 03:38:05 pm
content: Quote from: mikeselectricstuff on December 30, 2017, 12:15:32 pmI had similar thought, but you were fasterMy idea is to use a small rectified (50uA or less) current to charge a capacitor which will supply power to a small microcontroller. The microcontroller may wake every 0.5 seconds to measure the actual mains voltage. After measurement the microcontroller will print the result in a numerical format using a simple UART interface and optocoupler. The capacitor should be large enough to be able to provide needed energy for the microcontroller and the optocoupler. Adding a zener will limit the maximum voltage.When the microcontroller is not doing anything the microcontroller will stay in a low-power sleep-mode consuming only 1 uA - 3 uA while the internal low-power RC oscillator is running and waking the device every 0.5 seconds.if we have 100 V mains voltage we need approx. 2 MOhm resistor for 50 uA, dissipating 5 mW. On 300V mains voltage the 2 MOhm resistor will provide 150 uA and the power dissipation is 45 mW. If the microcontroller needs 10 mA for 2 ms (Baud rate 115200 bits per second, approx 10 characters in one millisecond plus other activities) including the optocoupler current, and the desired maximum voltage droop during this 2 ms is 100 mV, we need a 200 uF capacitor which is quite reasonable. The recharging time for 100 mV with 50 uA is less than 0.5 seconds, so the device can provide voltage information twice per second.