A.H. is right! AND, by looking carefully at that neon bulb, you may see that at some times only one of the electrodes lights, That is because of only half a sine wave being sent through.
I am not familiar with TEK scope models, but I know that they had a battery one where I worked long ago and it got used for those line-powered circuit checks. So a plastic-case battery operated scope is another option, probably safer too.

 

It is certainly possible to provide varying AVERAGE power by any of a number of methods, including cycle counting switching, waveform chopping, as well as just plain off-on controlling. The heater provides an integration function so that in any minute the power can be from zero to the maximum. The control logic can get a bit complex but the hardware can be quite simple. So it is quite easy to control the heat in a slow responding heater system without varying current or voltage, except by on/off, which woukld not be apparent on many meter types.

Right, I'm well aware of all that. I was responding to an earlier post which suggested that the circuit might be delivering constant voltage, with varying current, which is impossible (assuming a fixed load.)

As for the various methods of heater control, it seems likely that the circuit in question here uses phase angle, or maybe burst fire, control. Either one would be easily recognized with an oscilloscope, but difficult to identify otherwise.

As others have pointed out, the average and RMS values for voltage, current, and wattage (all inextricably linked to one another) could be varied with the methods above. A true RMS meter would show those voltage and current changes, where a simpler meter might not.

In case phase angle and burst fire are new concepts for the thread starter, I've attached a nice introductory paper from Crydom that explains them well.

 

That is why I am posting this, because the meter reading contradicts my expectation and ohms law. I posted the board hopping that someone could provide an explanation to the contradiction. I have used 3 different meters and I get same result.

Check out the pdf attached to my last post. It should make things clear.

The distinction is instantaneous voltage/current vs average or RMS voltage/current over a reasonably short time span.

Your circuit is most likely passing essentially unaltered AC voltage and its corresponding AC current when "on" and not passing any of either when "off." However, it's switching on and off so fast that a meter doesn't see the switching cycles. Depending on meter construction, the displayed values may be calculated from either peak or average voltage levels. Since the peak levels don't change with burst fire and only change if you go below 50% power with phase angle control, you could easily read full voltage and current over a wide range of average/RMS power levels. An average responding meter should show some change in voltage and current, but the results would still be inaccurate. A true RMS meter would give the most accurate results.

 

P.S. Here's some background on the difference between peak, average, and RMS measurements.

https://www.allaboutcircuits.com/textbook/alternating-current/chpt-1/measurements-ac-magnitude/

 

Hello,

The BT131 is a small triac.
See the datasheet for its connections.

Bertus

I suspect the BT131 is for the Auto-Off feature that the heater has. After turning the pad ON it will work for 40min and then turn itself OFF. I am not sure how thyristor work.

 

As a simple average power indicator you could connect a neon lamp in series with a 120k resistor across the heating pad. This will be a lot cheaper than than the 'scope/transformer combo.

Just out of curiosity, why a neon lamp?

 

P.S. Here's some background on the difference between peak, average, and RMS measurements.

https://www.allaboutcircuits.com/textbook/alternating-current/chpt-1/measurements-ac-magnitude/

Thank you for the link. I now understand what is taking place in theory, but would really like to see it (with equipment). I am a visual learner.

 

Just out of curiosity, why a neon lamp?

It needs very little power to work hence the 120k resistor in series.
You could use an incandescent lamp but it should be low power so as not to overload your controller.
Compact fluorescent or standard LED lamps would probably not work for this application as their electronic circuitry will get in the way.

 

Thank you for the link. I now understand what is taking place in theory, but would really like to see it (with equipment). I am a visual learner.

Totally understand - I never totally trust any theoretical conclusions until I can see the proof with my own eyes (although I do really enjoy hypothesizing!) I'd scope the output if I were in your shoes!

I personally think people's insistence that oscilloscopes can only be used if the load is powered through an isolation transformer are a bit over the top.

The key concern, as I understand it, is that people sometimes clip the probe ground to something other than a ground. Since the probe ground is connected to the oscilloscope chassis ground, which in turn is grounded through its power cable, connecting any live voltage to the ground clip results in a short circuit to ground, which could be dangerous to the user and/or destroy the oscilloscope.

As bad as all that is, all you have to do to avoid it is never clip the ground clip to anything but ground. Seems pretty simple to me. Every other part of dealing with mains voltage has simple, but critical, rules like that, so I'm not sure why that's considered unreasonable and unsafe when it comes to oscilloscopes, or why isolation transformers are touted as the only correct method.

Of course, there are other concerns with mains. You need to verify that your probes and your scope inputs are all rated appropriately - not all scopes are made for these voltages, with or without isolation!

Still, assuming all the other parts are up to it, I have no issue scoping mains powered equipment, as long as you know where your grounds are.

 

Just out of curiosity, why a neon lamp?

A neon llamp is a very high impedance device that has a very non-linear effective resistance. And it does not conduct hardly at all below about 65 volts. With the 120K resistor it is good for showing when 120 volts is present. Plus, if the voltage is pulsing the light will also pulse, so that you can see what is happening. and, if only one polarity is present then only one electrode will illuminate. Besides that, an NE2 neon bulb is a lot cheaper than a scope.

 

I have an old Tektronix 475 oscilloscope that i use for DC projects and low voltage AC. I am not sure if i can use it with 120VAC. Can you recommend a low budget meter that i can use to see the RMS voltage.

How to see 120VAC:
Oct 26, 2018 #15
Oct 26, 2018 #16
Oct 26, 2018 #17
Oct 28, 2018 #18

https://forum.allaboutcircuits.com/...scope-a-bridge-rectifier.153406/#post-1318106

 

Totally understand - I never totally trust any theoretical conclusions until I can see the proof with my own eyes (although I do really enjoy hypothesizing!) I'd scope the output if I were in your shoes!

I personally think people's insistence that oscilloscopes can only be used if the load is powered through an isolation transformer are a bit over the top.

The key concern, as I understand it, is that people sometimes clip the probe ground to something other than a ground. Since the probe ground is connected to the oscilloscope chassis ground, which in turn is grounded through its power cable, connecting any live voltage to the ground clip results in a short circuit to ground, which could be dangerous to the user and/or destroy the oscilloscope.

As bad as all that is, all you have to do to avoid it is never clip the ground clip to anything but ground. Seems pretty simple to me. Every other part of dealing with mains voltage has simple, but critical, rules like that, so I'm not sure why that's considered unreasonable and unsafe when it comes to oscilloscopes, or why isolation transformers are touted as the only correct method.

Of course, there are other concerns with mains. You need to verify that your probes and your scope inputs are all rated appropriately - not all scopes are made for these voltages, with or without isolation!

Still, assuming all the other parts are up to it, I have no issue scoping mains powered equipment, as long as you know where your grounds are.

I have been reading a lot about isolation transformers and I realized that my heating pad only has two pins (no ground).
Am I to understand that the only time one will need an isolation transformer is when the AC device has a ground pin?

 

How to see 120VAC:
Oct 26, 2018 #15
Oct 26, 2018 #16
Oct 26, 2018 #17
Oct 28, 2018 #18

Great links Danko,

The pad that i have only has two pins to power, so my understanding that i will not need an isolation transformer in this case. I also like the trick with two channel oscilloscope, will see if i can do same with my oscilloscope.

 

I have been reading a lot about isolation transformers and I realized that my heating pad only has two pins (no ground).
Am I to understand that the only time one will need an isolation transformer is when the AC device has a ground pin?

No! The issue has to do with the ground clip on the scope probes, and what it does or doesn't get connected to.

Since the scope itself is grounded (at least in the vast majority of cases) and the ground clip on the probe connects directly to the scope ground, it's dangerous to connect the probe ground to anything other than a ground reference, or a completely isolated voltage. The voltages in your warmer are not intrinsically isolated. Mains power is ground-referenced, regardless of whether or not you've got a ground pin on your device's plug.

If you don't connect the probe ground clip to anything, you'll be fine using the tip or grabber of your probe on the blanket wiring.

*** DON'T CONNECT THE PROBE GROUND TO ANY PART OF YOUR MAINS POWERED CIRCUIT UNLESS YOU'RE USING AN ISOLATION TRANSFORMER!!! ***

But, if you keep your probe grounds out of harm's way, you should be fine (assuming your probes and scope are rated for these voltages, and probably also assuming you'll want the 10:1 attenuation setting if your probes are switchable.)

 

No! The issue has to do with the ground clip on the scope probes, and what it does or doesn't get connected to.

If you don't connect the probe ground clip to anything, you'll be fine using the tip or grabber of your probe on the blanket wiring.

But, if you keep your probe grounds out of harm's way, you should be fine (assuming your probes and scope are rated for these voltages, and probably also assuming you'll want the 10:1 attenuation setting if your probes are switchable.)

Scary! while technically correct, I would not contemplate any scoping without an isolation transformer.
It's just too easy to make a mistake that could destroy expensive gear or give you a lethal shock.

Human nature is to get sloppy.

 

The pad that i have only has two pins to power, so my understanding that i will not need an isolation transformer in this case.

Look at outlet - two slots, "Neutral" and "Ground" are grounded,
therefore from your two power pins one is "Neutral" (grounded !!)
and second is "Hot" with voltage 120VAC in reference to "Ground" and/or "Neutral".

So, for measurement you should use A minus B technique, keeping two rules:
1. Use two X10 probes, or two universal probes, switched to X10 position.
2. Take out ground clips from both probes.
To set your oscilloscope switches for A minus B (Ch1 - Ch2) measurement,
you can use Tektronix 475 oscilloscope manual, page 2-4.
EDIT:
Only those, who survive after HV measurements,
become true electronicians.

 

The simplest SAFE method is to power the item being tested from a true isolation transformer. Of course, that transformer must have adequate wattage capacity for the load being tested. Leaving the scope frame grounded is usually the safer approach.
But before using a transformer for this purpose you need to check that the line side and the output side are actually isolated, meaning a very high resistance between the two, at least half a megohm as a minimum.

 

The simplest SAFE method is to power the item being tested from a true isolation transformer. Of course, that transformer must have adequate wattage capacity for the load being tested. Leaving the scope frame grounded is usually the safer approach.
But before using a transformer for this purpose you need to check that the line side and the output side are actually isolated, meaning a very high resistance between the two, at least half a megohm as a minimum.

What do you consider UNSAFE about the method Danko described above?

With the ground clips removed and X10 probes, how is this method more dangerous than probing the same high voltage circuit with a multi meter?

Don't get me wrong, caution is always in order, and I take safety very seriously. But, no-one tells people you can't use a multi meter on mains, and everyone seems to say you can't scope them without isolation. If there are more issues here than I'm aware of, I'd truly like to know.

 

What do you consider UNSAFE about the method Danko described above?

With the ground clips removed and X10 probes, how is this method more dangerous than probing the same high voltage circuit with a multi meter?

Don't get me wrong, caution is always in order, and I take safety very seriously. But, no-one tells people you can't use a multi meter on mains, and everyone seems to say you can't scope them without isolation. If there are more issues here than I'm aware of, I'd truly like to know.

The reason that it is safe to use a current standards multimeter to read the mains voltage, or even up to quite a few hundreds of volts, is that it was intentionally and very deliberately designed for that purpose. In fact, it was designed to protect the unskilled and careless individuals as well. An regular oscilloscope is designed for use observing much lower voltages, and the probes used for those lower voltages are not often built to handle higher voltages. Some of those probes may be adequate, some may not. The probe specification may include a maximum voltage that is quite a bit less than the mains voltage. There are higher voltage probes made for many oscilloscopes, but they are usually not the ones used for small signals and logic applications. Look at a scope accessories catalog and you will discover that the probes for higher voltages are quite different.

 

The reason that it is safe to use a current standards multimeter to read the mains voltage, or even up to quite a few hundreds of volts, is that it was intentionally and very deliberately designed for that purpose. In fact, it was designed to protect the unskilled and careless individuals as well. An regular oscilloscope is designed for use observing much lower voltages, and the probes used for those lower voltages are not often built to handle higher voltages. Some of those probes may be adequate, some may not. The probe specification may include a maximum voltage that is quite a bit less than the mains voltage. There are higher voltage probes made for many oscilloscopes, but they are usually not the ones used for small signals and logic applications. Look at a scope accessories catalog and you will discover that the probes for higher voltages are quite different.

If you use an isolation transformer to power the DUT, you're still exposing the probes, the scope, and potentially yourself to the same voltage levels. As soon as you clip your probe ground to something in the circuit to establish your reference point, you've broken the isolation - the DUT is no longer floating.

The difference is you get to define which point in the circuit is grounded based on your probe placement, which you wouldn't be able to do without the isolation transformer. But you still end up with the same voltage applied to the probes.

So yes, you do absolutely need to confirm that your scope and probes are suitable for the voltages you want to probe... but that's true with or without the isolation transformer. You haven't yet convinced me that the isolation transformer protects you from anything except errant ground clip placement.