Measuring DC Ripple with a DMM? - Page 1
Forum section: electronics / beginners

author: DW1961
author_rank: Frequent Contributor
timestamp: November 12, 2021, 06:09:09 pm
content: An electrician technician friend of mine said I could measure ripple using a DMM. Of course, it would not give you specifics, such as frequency or peak to peak, but he said it would give you a reading on how much AC was getting through.Is there any other information I should keep in mind when looking at results (He didn't really have any specifics other than you can check DC for "dirty power" He's a field tech not a lab tech.)

author: bdunham7
author_rank: Super Contributor
timestamp: November 12, 2021, 06:14:05 pm
content: It depends. See earlier discussions on DC vs AC coupled AC voltage readings, I think it was in a Brymen meter thread. If the range you are using is AC coupled, you can just read the AC voltage directly as long as the DC voltage doesn't exceed the blocking capacitor rating. If it is DC coupled, there are issues. Since you typically want to measure ripple using a lower range (like mVAC), you would need to know the specific meter you intend to use and how its ranges operate.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 12, 2021, 07:11:17 pm
content: Quote from: bdunham7 on November 12, 2021, 06:14:05 pmI'm not sure I understand coupled, but this measurement will be from a switching power supply. As such, it does have capacitors and filters, so I assume it is not coupled?I'll be sure to look at the specs for mVs and hopefully find a meter that can do it. If it doesn't work, I'll just get out of range or nothing on screen?

author: bdunham7
author_rank: Super Contributor
timestamp: November 12, 2021, 07:20:01 pm
content: Quote from: DW1961 on November 12, 2021, 07:11:17 pmDC or AC coupled refers to the meter itself.

author: floobydust
author_rank: Super Contributor
timestamp: November 12, 2021, 07:22:53 pm
content: You have really know your multimeter, if the measurement will work. You can get a number- but it's bogus. Some electricians will think this is a figure of merit.Many low-cost DMM's do not have the AC frequency response, they cannot measure above say 10kHz or 1.5kHz and since SMPS ripple is much higher frequency 50-100kHz, it would not read that properly. Check your DMM is not too slow.Many DMM's are constructed such that on ACV, they are actually DC-coupled and they get saturated by any DC present. Example ACmV range on a bus with 100VDC, it saturates the multimeter and it will read silly or even crash. It can't look at 100V and try to measure mV at the same time. You would need to use a blocking capacitor in series with the multimeter to get around this problem.Just check your meter's specs.

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 12, 2021, 08:42:38 pm
content: Only one of my four DMMs can measure a ripple. It is old 890 model.

author: TimFox
author_rank: Super Contributor
timestamp: November 12, 2021, 08:43:01 pm
content: For safety, it is wise to use an external series capacitor and shunt resistor (C-R high-pass filter) ahead of an instrument to read the AC component riding on top of a large DC value. The time constant needs to be long enough to pass the lowest frequency you anticipate (e.g., 60 Hz). However, if your ripple is from a switching supply, it will be at a much higher frequency and you need to check that the AC voltmeter has sufficient frequency response. An oscilloscope is a better choice, but the external filter is still a good idea.Note that if you hot-connect the filter to a high DC voltage, the initial transient may be damaging to the voltmeter if it is set to a sensitive scale. When in doubt, go to manual ranging and watch for the initial spike, then slowly reduce the full-scale range.

author: David Hess
author_rank: Super Contributor
timestamp: November 13, 2021, 12:26:07 am
content: In AC mode a multimeter works fine for measuring ripple within its AC bandwidth. This will not be used for measuring switching regulator ripple, but it works fine for measuring ripple at the input of a regulator. The output should have ripple too low to measure unless something is broken.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 13, 2021, 01:49:07 am
content: Quote from: floobydust on November 12, 2021, 07:22:53 pmUNI-T 133A: Range for Hz is 60Hz - 1MHz

author: TimFox
author_rank: Super Contributor
timestamp: November 13, 2021, 04:42:09 am
content: That table seems poorly written: the 60 Hz to 1 MHz spec seems to be for a frequency counting mode. Note that in that mode, the resolution is specified in Hz.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 13, 2021, 04:51:35 am
content: Quote from: TimFox on November 13, 2021, 04:42:09 amThey are poorly written. What do you think they really mean?

author: TimFox
author_rank: Super Contributor
timestamp: November 13, 2021, 02:57:53 pm
content: Quote from: DW1961 on November 13, 2021, 04:51:35 amDoes that device include a frequency counter function? That's how one would specify such a device (for "60 Hz to 1 MHz": accuracy and resolution).

author: floobydust
author_rank: Super Contributor
timestamp: November 13, 2021, 06:29:28 pm
content: That spec is for the frequency counter function. Uni-T does not give a AC measurement bandwidth spec. for the133A.The DMM ICSD7501by SDIC is rated 2kHz true-RMS. So the frequency response is only good for mains use and a bit more.Next, the Uni-T like most of their models, has no high voltage DC blocking capacitor. These things cost money. It likely would be a DC-coupled path, which means the multimeter front-end is saturated if on ACmV but a few volts DC is present.You can experiment with a signal generator connected to the multimeter, check AC freq response and how it reacts to DC offset on ACmV.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 13, 2021, 09:13:40 pm
content: Quote from: floobydust on November 13, 2021, 06:29:28 pmFlooby, thanks for that research and information. I don't have a signal generator, but if the meter isn't really set up to measure ripple, then kinda wasting time testing it. I think I might look at a couple of my other MMs and post those to see if they can do it.In the mean time, do you know of any MMS that can actually test ripple accurately? I keep having this urge to buy and learn how to use a scope.

author: floobydust
author_rank: Super Contributor
timestamp: November 13, 2021, 09:38:37 pm
content: I've only ever used an oscilloscope for measuring ripple on SMPS.If this is about indirectly testing electrolytic capacitors, I use an LCR meter and look at ESR and capacitance.The ripple waveform is not easy to assess because it has spikes, overshoot, ringing, there's stray inductance and you'd have a varying load. If it's high amplitude ripple due to too small/aged filter capacitors or a poor output filter, blown rectifier diode etc. the scope shows peak-peak or RMS and which is more telling? I guess I'm saying the ripple voltage "number" might only be useful if comparing the same devices in production or the field.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: November 20, 2021, 04:27:12 am
content: amplitude peak detector ?

author: Electro Fan
author_rank: Super Contributor
timestamp: November 20, 2021, 05:16:52 pm
content: Quote from: DW1961 on November 13, 2021, 09:13:40 pmAmazing that you made it over 600 posts on EEVblog and don’t have an oscilloscope. Tremendous discipline and will power in a place where par for oscilloscopes is rumored to be 19.I sense it’s only a matter of time.Does anyone here with 1000 or more posts not have an oscilloscope?It’s possible that a desire to measure ripple could be the thing that causes the tipping point.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 20, 2021, 07:40:58 pm
content: Quote from: floobydust on November 13, 2021, 09:38:37 pmAll of the computer power supply reviews use scopes to asses ripple. Are they chasing ghosts?

author: DW1961
author_rank: Frequent Contributor
timestamp: November 20, 2021, 07:42:43 pm
content: Quote from: Electro Fan on November 20, 2021, 05:16:52 pmLOL, yeah damn ripple. I'm not sure how much I would really use it is the problem (cost to use ratio).

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 21, 2021, 01:17:28 pm
content: I just picked up an old Simpson 260, and I actually read the instructions. (maybe I could go for a neurological check). It actually has a separate jack for "OUTPUT Voltage" to use:Quote from instructions:It is necessary to measure the AC component of an output voltage when both AC and DC levels exist. This occurs primarily in amplified circuits. The 260 has a .1 uF 400 volt capacitor in series with the OUTPUT jack. The capacitor blocks the DC component of the current in the test circuit, but allows the AC or desired component to pass on to the indicating instrument.Now, looking at their charts the unit only works with AC up to maybe 20 KHzI have not noticed this feature on other old Multimeters I have and ones that I have parted with.Apparently this AC/DC measurement problem is nothing newAs an aside, the old Simpson reads volts and ohms quite well compared with my calibrated HP 3456. Really good correlation in the middle of the meter movement. I did try this OUTPUT VOLT feature and I do believe it works. It is probably there to measure audio output from tube radios.