Picaxe Oscilloscope

[joe paul]

Hi Everyone,

I took the advice from Eclectic and just ordered the Picaxe oscilloscope OSC001
http://www.picaxestore.com/index.php/en_gb/osc001.html
(waiting for it to come to the US), but someone not familiar with the device told me that it might not be capable of handling voltages over 5 volts. I find it hard to believe that would be the case, however I gather that some precautions need to be taken. So, is it possible to safely view wave forms of approx. 24 volts AC?

Thanks!

Take care, Joe.

 

[AllyCat]

Hi Joe,

It's basically a DPScope SE without the box or BNC connectors. As such, it appears to be rated up to 20 volts dc, but the "screen" has 12 divisions at maximum 2v/div. So only good to 24 volts peak-peak, or about 8 volts rms ac.

With the DPScope you'd normally use a 1:1 / 1:10 swtichable probe (with BNC plug) but with the PCB scope I think you'd have to make an attenuator, e.g. with a 9 Mohm resistor and a small capacitor. And don't forget that its "earth" is the same as your PC's.

Cheers, Alan.

 

[joe paul]

Hi Alan,

Thanks for the info! I am now a little leery of using it with my one and only good laptop. Guess I need to get a machine I can afford to blow up!

Take care, Joe.

 

[AllyCat]

Hi Joe,

In some ways you're probably safer with the PCB Scope. With "proper" probes one is always tempted (by years of training and experience) to always connect the earth clips (to give clean/accurate waveforms). But with the PCB scope you'll usually use a "one wire" connection, which should be pretty safe (particularly with a series resistive attenuator).

Cheers, Alan.

 

[joe paul]

Hi Alan,

Thanks! What got me motivated to get the scope was the need to compare a chopped AC waveform from a commercial electric train controller with the waveform from a homemade circuit with a triac (both circuits have triacs). So I am using a power supply (wall wart) that is 18 VAC. After I get the board and install the software, I'll be asking a lot of questions.

Take care, Joe.

 

[techElder]

Alan, what in the heck do you mean by this? Makes little sense.

... With "proper" probes one is always tempted (by years of training and experience) to always connect the earth clips (to give clean/accurate waveforms). But with the PCB scope you'll usually use a "one wire" connection, which should be pretty safe (particularly with a series resistive attenuator).

 

[premelec]

@Tex - I expect he means that the V- will already be connected to the computer through the scope being attached to same... "safe" is something else...

 

[SteveDee]

Joe, take a look at the documentation (especially paras 6.4.1 & 6.4.2) which is quite clear on the input voltage limitations of this unit: http://www.picaxe.com/docs/osc001.pdf

This scope module is really designed for people working with low voltage circuits. I wouldn't use it with a x1/x10 to check high voltages (e.g. 24-240V) as a simple error (like flicking the probe attenuator back to x1) could end in disaster.

 

[AllyCat]

Hi Tex,

Alan, what in the heck do you mean by this? Makes little sense.

Which part doesn't make sense? Normally a "proper" 'scope probe is on the end of a metre or more of coaxial cable and, particularly at high frequencies, it is essential to connect its Earth clip to the "local" (e.g. circuit board) earth. Otherwise all the stray inductance and capacitance can play havok with the measured waveforms.

However, the PCB scope doesn't (shouldn't) use long probe cables, the bandwidth isn't particularly high and normally it will be mounted on a patchboard where the signals are to be measured (so already have a "common earth"). Short input wires shouldn't have much stray inductance or capacitance.

It's the DPScope (I have both the _SE and the _II) which needs some care because it does use proper BNC terminated probes (switchable 1:10 can be supplied) , but the earth is already connected to the "earth" of the Desktop or (plugged in) laptop PC which is being used for the display.

Cheers, Alan.

 

[hippy]

This is one of those cases where the distinction between "0V" and "earth" is extremely important.

 

[techElder]

Thanks for the additional explanation, Alan.

... the distinction between "0V" and "earth" is extremely important.

... especially to someone that is new to the equipment.

Frankly, the "PCB scope" should only be used on connections that the PICAXE could be connected to.

One should always have a common "0V" connection between the "PCB scope" and the project that is getting measured using short leads.

You'll save money by treating it like another ADC input to your PICAXE.

It isn't a replacement for a "real" oscope. You still need one of those. Eventually.

 

[joe paul]

Hi Folks,

O.K., I just tried to cancel the order. It would be of little use to me with electric train voltages. I wish I had known this when I mentioned before that I had concerns about damaging my computer with my toy train circuits and projects.

Thanks for the heads-up!

Take care, Joe.

 

[AllyCat]

Hi Joe,

Did you manage to cancel? If not, IMHO it should be possible to make a simple attenuator "probe" which would be "good enough" and safe enough (for you and your PC) for the particular application that you've described (i.e. low frequency and ~20 v rms ac).

Cheers, Alan.

 

[joe paul]

Hi Alan,

I haven't gotten an answer from the Picaxe store yet, or a shipping conformation, so I don't know the status of the order. But after looking into it better, even with the correct probe, I wouldn't feel good about using it and probably never would. "Safe enough" won't due. Computer damage is what I fear, not my personal safety. High frequency is also part of some of my circuits, like the old-fashioned HF AC lighting circuits (and track cleaning circuits like the Relco and GaugeMaster).

Thanks!

Take care, Joe.

P.S. It would be helpful if the same disclaimer and precautionary statements in the manual were clearly listed on the page for the oscilloscope on the Picaxe store's site. A lot of my time and the time of others have been devoted to this issue that could be cleared up in a couple sentences. J.

 

[Goeytex]

Having been spoiled over the years by TEK scopes. I cannot imagine using a PCB scope. I currently use a TEK THS730A that originally cost several thousand dollars. It still works very well.

But one of my friends just got an entry level Siglent SDS1102CML and I am amazed at what it can do for only $360.00

 

[BESQUEUT]

But one of my friends just got an entry level Siglent SDS1102CML and I am amazed at what it can do for only $360.00

Just bought a Rigol DS1054Z (upgraded to 100Mhz + trigers and decoders + 4 channels ...) for a little less money...