What do you think of your EEVblog Brymen BM235 Multimeter ?

[Hextejas]

A quick test:

Your signal was 1Khz exactly Ian ?

[Hextejas]

I like the meter a lot, except for one thing.
I cannot get the Hz function to work.
I rarely use it but I needed it today and luckily I have another meter that does it.
Has anyone else tried it and got it working ?

Can you give more information? Like what voltage and frequency were you trying to measure? Saying "it doesn't work" is hopeless. Obviously it does work.

Sorry Ian, I could have been more specific. I was trying to read 1.2khz @ 10v. I think that I had it set correctly, but will try again tomorrow. I followed the manual, set the freq/v to 6 and the meter showed 00.00.

It's not enough information.  My guess is you are biasing the signal above the meter's common and have not considered that the BM235 requires a zero cross.   If this is not what you are doing, please provide further details.

Ian, I have no idea what you are talking about with the "biasing the signal above the meter's common "
Sorry

[joeqsmith]

Place the BM235 into DC mode after you have applied the signal.  What value does it read?   

If for example, you have a 10Vp-p signal, or 5V peak and you have a 5V DC offset, the voltage would switch between 0 to 10 volts rather than -5 to +5.   The BM235 needs to see the signal go through 0 volts to read frequency.   In the last picture, I have places a series capacitor inline with the BM235 to block the DC.  As you can see, the meter reads the correct frequency where in the previous picture it reads zero.

[IanB]

It's not enough information.  My guess is you are biasing the signal above the meter's common and have not considered that the BM235 requires a zero cross.   If this is not what you are doing, please provide further details.

Ian, I have no idea what you are talking about with the "biasing the signal above the meter's common "
Sorry

It was Joe who said this, not me...

[IanB]

I was using the the LoZ  setting so as to be able to show the voltage under test. ~10.5

I do not understand. The LoZ setting is special and you do not have to use it for normal measurements. For these tests it will be better to use the AC volts setting as in my pictures.

Your signal was 1Khz exactly Ian ?

Yes, it was.

[malagas_on_fire]

Place the BM235 into DC mode after you have applied the signal.  What value does it read?   

If for example, you have a 10Vp-p signal, or 5V peak and you have a 5V DC offset, the voltage would switch between 0 to 10 volts rather than -5 to +5.   The BM235 needs to see the signal go through 0 volts to read frequency.   In the last picture, I have places a series capacitor inline with the BM235 to block the DC.  As you can see, the meter reads the correct frequency where in the previous picture it reads zero.

Thats correct
In the lines of joeqsmith try using a signal generator to add the 5V DC offset  to 10Vp signal at 10Hz 50Hz 100Hz 400hz and 1Khz . If you measure AC voltage and switch to frequency it will read correctly because the sinosoidal passes zero point. Remember in the manual that the true RMS is buit-in in the chip, not a dedicated chip, so frequency and / or behavior will be a bit compromised.

[joeqsmith]

Lets see.
I was showing the BM235 against the same input that shows correctly and as expected on the other meter.
I set the hz meter to 6v sensitivity as suggested. That supposedly is the most sensitive setting or so someone said.
I was using the the LoZ  setting so as to be able to show the voltage under test. ~10.5

The AMPROBE AM510, no matter if you had selected the AC volts mode and then select the Hz button or use the frequency mode as you have shown, does not require a zero cross.   

[Hextejas]

Place the BM235 into DC mode after you have applied the signal.  What value does it read?   

If for example, you have a 10Vp-p signal, or 5V peak and you have a 5V DC offset, the voltage would switch between 0 to 10 volts rather than -5 to +5.   The BM235 needs to see the signal go through 0 volts to read frequency.   In the last picture, I have places a series capacitor inline with the BM235 to block the DC.  As you can see, the meter reads the correct frequency where in the previous picture it reads zero.

Ok back at this and I am about to give up.
I put the Bm235 on Hz setting, it read 0, I switched to DC and it read -1.840.  Odd!
If its any consolation, it reads the line voltage correctly at 60hz

[Hextejas]

I give up

[IanB]

You have not told us the source of the signal you are measuring. What is generating it?

You are saying that if you set the meter to AC V you see a reading of ~10.5 V? And if you set it to DC V you see a reading of -1.84 V? This indicates you have an AC voltage of about 10.5 V RMS with a DC bias of -1.84 V. This should show the frequency correctly. You have definitely stopped using LoZ mode?

[IanB]

Please try the following steps:

1. Turn the selection knob to "VFD V" as per my previously uploaded photo
2. Press the "SELECT" button until the display shows "AUTO ~" (AC). What does it read?
3. Press the "SELECT" button until the display shows "AUTO" (DC). What does it read?
4. Press "SELECT" back to "AUTO ~". Press the "Hz" button. What do you see?

[Hextejas]

Please try the following steps:

1. Turn the selection knob to "VFD V" as per my previously uploaded photo
2. Press the "SELECT" button until the display shows "AUTO ~" (AC). What does it read?
3. Press the "SELECT" button until the display shows "AUTO" (DC). What does it read?
4. Press "SELECT" back to "AUTO ~". Press the "Hz" button. What do you see?

ok once again.
1. Selection knob to VFD V
2. select button shows Auto ~ .549 v
3. Select button again shows Auto ~ 000.1v
4. Select button shows Auto DC 10.49
5. Select button shows Auto ~ .549
6. Hz shows 6v momentarily, then Auto 00.00 Hz,

I got 1 more ac in there than you have.

One thing that I noticed that might make a difference is that the DC offset on the signal generator was completely clockwise.
I have no idea what it does

[IanB]

1. Selection knob to VFD V
2. select button shows Auto ~ .549 v
3. Select button again shows Auto ~ 000.1v
4. Select button shows Auto DC 10.49
5. Select button shows Auto ~ .549
6. Hz shows 6v momentarily, then Auto 00.00 Hz,

I got 1 more ac in there than you have.

One of the AC readings will also say "VFD" (probably at step 3). Ignore that one.

However, it is now very clear what is happening:

1. You have a DC voltage of 10.49 V
2. You have an AC voltage of 0.549 V superimposed on top of this DC voltage.

As we have been telling you, the meter cannot detect frequency when the AC voltage does not cross zero. In this case half a volt varying around 10.5 volts is very far away from zero.

One thing that I noticed that might make a difference is that the DC offset on the signal generator was completely clockwise.
I have no idea what it does

This is your problem, of course. The DC offset control applies a DC offset to the signal (a 10.5 volt offset in this case). Set the DC offset to zero and then see what happens.

[Hextejas]

Ian, thank you a lot for your help.
The 1kz signal is coming from a HP 3311a function generator.
Is that 1khz signal seen as an AC voltage ? That's what I think that I was expecting but I am beginning to see that it is not.
I don't know what it is other than maybe fluctuating  DC.
I need to read more about this .

[malagas_on_fire]

Try adjust the DC offset knob to minus side untill you get a reading thus the zero cross control peeks the desired frequency.

Anyway here is a link for the documentation regarding the signal generator:

http://ohm.bu.edu/~pbohn/REPAIR__Advanced_Lab/Mossbauer/Data_Sheets/Generator.pdf

Don't be scared of the Repair It's just for a quick checkout

[joeqsmith]

Have you installed a DC blocking capacitor like I showed?  The value is not real critical. 

[IanB]

Ian, thank you a lot for your help.
The 1kz signal is coming from a HP 3311a function generator.
Is that 1khz signal seen as an AC voltage ? That's what I think that I was expecting but I am beginning to see that it is not.
I don't know what it is other than maybe fluctuating  DC.
I need to read more about this .

Generally speaking, a periodic signal has two parts, an "AC" part and a "DC" part. The "AC" part is the wobble, and the "DC" part is the center or baseline of that wobble. So a simple signal may oscillate around zero volts, going from say, -1 to +1. This is what the mains waveform looks like. But, it doesn't have to. Instead of oscillating around zero, it could oscillate around, say, 10 volts, going from +9 to +11, with +10 in the middle.

This signal would be described as the sum of two components: a DC component of 10 volts, and an AC component with an amplitude of 1 volt.

If you have a signal like this, you can block the DC component by putting a capacitor in the circuit, as Joe did in the picture below. This lets just the AC part through, and then the BM235 will be able to measure the frequency:

[Brumby]

Pictures can help here - so let's look at DC offset .....


Waveform B is your typical AC that everyone knows and understands.  It swings symmetrically above and below zero volts, crossing that zero level twice in each cycle.

Waveform A is an example of AC that swings between +1V and +3V - and here is where some confusion can arise.  It may appear as being a "varying DC" because the voltage never gets near zero volts, let alone go negative - but that is just how it looks.

When it comes to analysing such a signal and doing math, it is much easier to think of it as having two components: The pure AC part (which in this example is 1V peak or 0.707V RMS) and the amount this has been shifted from the zero volt axis, which is a DC value (which, in this case is +2V ).

What is most significant is that this two component approach works for practical efforts with circuits and makes life much, much simpler.

Your function generator has this ability to add this "DC offset" - in whatever amount you need - to the AC signal it produces.  This allows you to feed it to a circuit that uses an AC signal that does not go below 0V - such as an audio amplifier which is powered by a single, positive supply.

[YU2]

I am in Germany, which small size DMM for about 100€ is the best buy? Is it still bm257s?

[Valver]

I need to read the ripple current on the filter caps of a 450v DC power supply. Unfortunately this meter cannot do this, although my 25 year old cheap Radio Shack Micronta Smart2 has no problems. UNI-T 61E can't do this either. Can someone  suggest an affordable multimeter that is truly AC coupled on the AC voltage range? (No, I don't want to use a series capacitor). Thanks for any suggestions!

[malagas_on_fire]

Well here is a video how the zero cross is working with a cheapo DDS 1Vpp 1Khz signal that has the DC offset being Adjusted up and down.

Watch what happends to frequency and voltage reading when offset is being changed and the threshold point where it starts fail in lower DC offset.

https://youtu.be/3TF5SVDtduM

[Photolunatic]

How low in the Ohm range will BM235 go? I checked many spec sheets, user manual and they all state the accuracy level for 600Ω / 60MΩ but I can't see the lowest measurable resistance.
Mine does not work in the milliohms range so I suppose 1 Ohm is the lowest.
I try to measure the internal resistance of the 18650 batteries and it shows nothing. A bummer!

Anybody knows or could point me to this specs?
Thanks.

[Fungus]

How low in the Ohm range will BM235 go? I checked many spec sheets, user manual and they all state the accuracy level for 600Ω / 60MΩ but I can't see the lowest measurable resistance.

The math is easy: It's a 6000 count meter and it has a 600 Ohm range. How many digits after the decimal point?

Mine does not work in the milliohms range so I suppose 1 Ohm is the lowest.

No meter is going to measure milliohms directly. You need special techniques for that.

I try to measure the internal resistance of the 18650 batteries and it shows nothing. A bummer!

And no meter will measure the internal resistance of a battery. Meters measure resistance by passing a known current through the thing you're measuring then using Ohms law. You can't measure the resistance of anything that's powered on or that can supply a voltage (eg. a battery).