Measuring Current Through Bridge Rectifier Circuit - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: October 23, 2022, 01:44:22 pm
content: I have a silly question and feel I should know the answer, but I confused myself when I began thinking about it.If I have bridge rectifier circuit (assume 120VAC through a 10:1 transformer, so I'm getting 12VAC out), through a rectifier, and want to measure the current, do I set the meter to measure it on AC or DC?The part that confuses me: if I measure DC current and it's a single diode rectifier with a capacitor (or a poorly selected capacitor that has poor filtering), it will have much more fluctuation than a full wave rectifier (four diodes). Setting it on AC will give me an RMS average.

author: macboy
author_rank: Super Contributor
timestamp: October 23, 2022, 10:36:04 pm
content: Most multimeters on AC will use a DC blocking capacitor in line with the input, so will not correctly measure the RMS voltage of a DC voltage with ripple. It will instead measure the RMS value of only the ripple or noise which is superimposed on the DC (and that assumes a true RMS meter). If the meter explicitly has a AC+DC true RMS mode then it will give you the RMS value including DC. If set to DC you will get the average voltage, effectively filtering out the ripple or noise. This is probably closer to what you want.

author: bdunham7
author_rank: Super Contributor
timestamp: October 23, 2022, 10:45:33 pm
content: Well, which current do you want?If you have a complete circuit with a rectifier (bridge or otherwise), filter capacitor and a load, then the RMS AC current (as measured between the transformer and the bridge) and the DC current through the load (however measured) will be different numbers.

author: bostonman
author_rank: Super Contributor
timestamp: October 24, 2022, 03:17:23 am
content: The circuit is a DC motor fed by a bridge rectifier and a single capacitor.I wanted to check the current going through the motor and realized I didn't know whether to set the meter (a fluke 179) to DC or AC to measure the current.

author: bdunham7
author_rank: Super Contributor
timestamp: October 24, 2022, 03:26:33 am
content: Quote from: bostonman on October 24, 2022, 03:17:23 amOK, then you would connect the meter in one of the motor leads and use the DC current setting. You'll get an averaged result, but unless your filter cap is very small the difference between that and a TRMS AC+DC measurement will not be very much. And even if there were a difference, it isn't obvious which one you would want if you had the choice.

author: bostonman
author_rank: Super Contributor
timestamp: October 24, 2022, 03:43:30 am
content: I could be wrong, but anytime I've measured current, it's always been after a regulator. This is the first time I haven't had "true" DC voltage.In the case of a Weller WES51 soldering iron that I recently worked on, it had a single diode and capacitor, with horrible ripple.At the time I didn't think about how to measure the current (unless I took the measurement after the regulator), but now that I began thinking about measuring directly after the rectifier, I began thinking about a circuit with a large sawtooth wave, a full-wave that is much quieter, etc...Recently a co-worker needed to measure the current through a DC motor at work. I wanted to confirm his measurements and confused myself on whether to use DC or AC; hence why I asked the question.

author: bdunham7
author_rank: Super Contributor
timestamp: October 24, 2022, 03:52:10 am
content: Quote from: bostonman on October 24, 2022, 03:43:30 amIt is an issue worth thinking about, but you can't use AC in any case, at least not by itself. If you have TRMS AC+DC as a choice, then you have to decide why you are measuring the current in the first place.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 27, 2022, 04:40:37 pm
content: The RMS current in the bridge must be the same as the RMS current out. So you measure the AC. As pointed out the true RMS meter will always ignore the DC component.The DC current will drop to zero after a cycle or few if there is no load.

author: TimFox
author_rank: Super Contributor
timestamp: October 27, 2022, 05:34:19 pm
content: Be careful about measuring the DC current (mean value) for a waveform with large peak-to-average ratio.In extreme cases (actually a pulsed waveform that was positive-definite), I found large discrepancies between different digital ammeters due to the input stage saturating or compressing on the peaks.To check for this, change to a larger than required full-scale value and see if there is a discrepancy with the normal full-scale value measurement.The cure for that was to put a large capacitor across the ammeter input: capacitance values depend on the application.With an old-style d'Arsonval meter, the mechanical time constants sufficed to get a proper average value.Also, true RMS meters may not ignore the DC (mean) component, depending on their construction and setting, so verify this before assuming.