The isolation of the transformer assures that the return connection of the DUT is separate from the mains. Thus any point in the isolated circuit may be picked as the scope common point. The purpose of the isolation is not to eliminate the shock hazard from the DUT circuit, it is to remove the shock hazard from the mains. I should have made that clearer, it seems.
In any non-isolated circuit, every point may be a shock hazard because they are all referenced to the earth ground connection of the mains. THAT is the big shock hazard and noise source that an isolation transformer is reducing.
There is no protection against dumb moves. The isolation allows a degree of safety against a specific hazard only.

For complete safety stick to software simulations on a battery powered computer.

 

The isolation of the transformer assures that the return connection of the DUT is separate from the mains. Thus any point in the isolated circuit may be picked as the scope common point. The purpose of the isolation is not to eliminate the shock hazard from the DUT circuit, it is to remove the shock hazard from the mains. I should have made that clearer, it seems.

This first part all matches my understanding of things. Sounds good so far.

In any non-isolated circuit, every point may be a shock hazard because they are all referenced to the earth ground connection of the mains. THAT is the big shock hazard and noise source that an isolation transformer is reducing.

This is the part that feels a little misleading to me. It's true that no individual point in the isolated circuit represents a shock hazard... until you start testing it. However, once you connect your ground clip to whatever portion of the circuit you choose, now you've ground referenced it, meaning you've ground referenced the whole circuit. Any part of the circuit that has dangerous voltages relative to your chosen reference point now also has dangerous voltage relative to actual, regular, non-isolated earth ground.

The increased safety is very temporary. You're protected against the danger of clipping your oscilloscope common somewhere that you shouldn't... but once you clip it anywhere, you've lost isolation and made the circuit just as dangerous as it would've been without the isolation transformer.

There is no protection against dumb moves. The isolation allows a degree of safety against a specific hazard only.

I definitely agree that there's no protection against dumb moves. What bugs me about many of the isolation discussions is that the implication seems to be that no isolation equals danger (only true if you clip your ground to the wrong spot) and that isolation equals easy, convenient safety (not true at all since isolation is lost when you connect your probe ground.)

It seems to me that isolation transformers do two things really well:

1. They reduce noise in your measurements quite a bit by allowing you to put your ground reference clip anywhere you want.
2. They protect the oscilloscope itself from almost certain destruction when the ground clip is connected to anything other than ground.

It doesn't seem to me like the isolation transformer makes it much safer for the human, just for the scope.

 

This first part all matches my understanding of things. Sounds good so far.

This is the part that feels a little misleading to me. It's true that no individual point in the isolated circuit represents a shock hazard... until you start testing it. However, once you connect your ground clip to whatever portion of the circuit you choose, now you've ground referenced it, meaning you've ground referenced the whole circuit. Any part of the circuit that has dangerous voltages relative to your chosen reference point now also has dangerous voltage relative to actual, regular, non-isolated earth ground.

The increased safety is very temporary. You're protected against the danger of clipping your oscilloscope common somewhere that you shouldn't... but once you clip it anywhere, you've lost isolation and made the circuit just as dangerous as it would've been without the isolation transformer.

I definitely agree that there's no protection against dumb moves. What bugs me about many of the isolation discussions is that the implication seems to be that no isolation equals danger (only true if you clip your ground to the wrong spot) and that isolation equals easy, convenient safety (not true at all since isolation is lost when you connect your probe ground.)

It seems to me that isolation transformers do two things really well:

1. They reduce noise in your measurements quite a bit by allowing you to put your ground reference clip anywhere you want.
2. They protect the oscilloscope itself from almost certain destruction when the ground clip is connected to anything other than ground.

It doesn't seem to me like the isolation transformer makes it much safer for the human, just for the scope.

I consider safety for the scope to be quite important indeed. And working with line voltage devices it is not that difficult to avoid touching anything. And with the scope already grounded, touching the scope is safe. So it is still important for the person using the equipment to exercise a bit of caution and pay attention to what they are doing.
At some point every individual must assume responsibility for their own safety!!! To presume that any amount of rules and restrictions by others will assure safety is very unwise. THUS it is the sole responsibility of those using any equipment to understand how to do it safely. The whole concept that others are responsible for my safety is foolish.

 

I consider safety for the scope to be quite important indeed. And working with line voltage devices it is not that difficult to avoid touching anything. And with the scope already grounded, touching the scope is safe. So it is still important for the person using the equipment to exercise a bit of caution and pay attention to what they are doing.
At some point every individual must assume responsibility for their own safety!!! To presume that any amount of rules and restrictions by others will assure safety is very unwise. THUS it is the sole responsibility of those using any equipment to understand how to do it safely. The whole concept that others are responsible for my safety is foolish.

Totally with you on the points above.

Thanks for having this discussion with me. Hope I didn't seem too combative or argumentative along the way.

I've been pretty comfortable with my understanding of these issues for a little while now, but you can never be too careful making assumptions about mains power safety. I really did want to talk through the details and make sure I wasn't overlooking any critical points. I feel a little better now, and feel like I understand your point of view on this fairly well now. Plenty of common ground there. Cheers!

 

Totally with you on the points above.

Thanks for having this discussion with me. Hope I didn't seem too combative or argumentative along the way.

I've been pretty comfortable with my understanding of these issues for a little while now, but you can never be too careful making assumptions about mains power safety. I really did want to talk through the details and make sure I wasn't overlooking any critical points. I feel a little better now, and feel like I understand your point of view on this fairly well now. Plenty of common ground there. Cheers!

Well, I have been shocked several times, but because of avoiding ground connections they were hand shocks, not body shocks, and so not potentially lethal. The whole safety thing is a combination of attention focus plus knowledge and understanding. (=insight??) Understanding and knowing which wires not to garb is a great asset.

 

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.)

Thank you for the explanation. I wanted to make sure that I understand everything before going at it.

 

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".
View attachment 164126
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.

This is really great Danko! I have been measuring LV and HV for a long time, but the application has always been straight forward and simple.

This heating pad opens a new area of learning for me. If i can understand how it works i will feel a sense of accomplishment and would not hesitate dealing with complex AC motherboards.

I will give it a try over the weekend and report back with any questions that may come up.

Thanks

 

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".
View attachment 164126
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.

Hi Danko,
I have connected the oscilloscope, the output that i get make me question my setup and the output. I am attaching the oscilloscope screenshots, the output if very confusing to me. Maybe you can shed some light.

 

Hi Danko,
I have connected the oscilloscope, the output that i get make me question my setup and the output. I am attaching the oscilloscope screenshots, the output if very confusing to me. Maybe you can shed some light.

Where you placed probes Ch1 and Ch2? On output?
All pictures are from output?
Then your "A minus B" setup is not right.
EDIT:
"A minus B" setting:
1. VERT MODE:
"ADD" - ON.
2. CH2:
"INVERT" - ON.
3. Coupling switches:
both in "DC" or both in "AC".
4: VOLTS/DIV switches both in same position,
for example both in 5V/DIV or both in 1V/DIV etc.
VAR controls on switches should be OFF.
5. Both probes should be X10 or switched to X10 position.
6. Ground clips should be removed from both probes.
-------
Measurement:
One probe connected to one wire of circuit's OUT,
Other probe connected to second wire of circuit's OUT.
-------
Also try to see with sweep 0.5s/div.

 

Is this the voltage across the heating element? Or is it connected someplace else? Without knowing the connection point it is hard to understand.

 

Hi Danko,
I have connected the oscilloscope, the output that i get make me question my setup and the output. I am attaching the oscilloscope screenshots, the output if very confusing to me. Maybe you can shed some light.
View attachment 164426

First, let me double check. You are doing the A-B style measurement with two separate probes in two separate channels, as described by Danko, and you're not connecting the probe grounds to any part of the circuit, right?

Referring to your original circuit image below, I'm guessing you're probing at ACN and OUT2. If I'm right, try moving the ACN probe to OUT1.



Also, perhaps one of the more experienced members can confirm or deny this, but I think that in A-B mode you need to keep your volts per division setting high enough to accommodate the voltage each individual input is seeing, not zoom in on the difference - in other words, if it takes 50V/division to see the full mains voltage, you'll need to leave it at 50V/division during all measurements. This seems to be the case with the digital scope I use most often, but I'm not totally sure if analog scopes behave the same way.

While we're at it, you do have the volts/division set to the same level on both channels for all these tests, right?

None of these ideas provides a clear answer to why you see no change on different heater settings, but maybe that will be clearer once we start seeing better scope images.

Of course, another possibility is that the device is malfunctioning and any on setting always delivers full heat. That would explain all settings looking the same on the scope. Are you confident that different settings are actually delivering different heat output?

 

First, let me double check. You are doing the A-B style measurement with two separate probes in two separate channels, as described by Danko, and you're not connecting the probe grounds to any part of the circuit, right?

Referring to your original circuit image below, I'm guessing you're probing at ACN and OUT2. If I'm right, try moving the ACN probe to OUT1.

View attachment 164442

Also, perhaps one of the more experienced members can confirm or deny this, but I think that in A-B mode you need to keep your volts per division setting high enough to accommodate the voltage each individual input is seeing, not zoom in on the difference - in other words, if it takes 50V/division to see the full mains voltage, you'll need to leave it at 50V/division during all measurements. This seems to be the case with the digital scope I use most often, but I'm not totally sure if analog scopes behave the same way.

While we're at it, you do have the volts/division set to the same level on both channels for all these tests, right?

None of these ideas provides a clear answer to why you see no change on different heater settings, but maybe that will be clearer once we start seeing better scope images.

Of course, another possibility is that the device is malfunctioning and any on setting always delivers full heat. That would explain all settings looking the same on the scope. Are you confident that different settings are actually delivering different heat output?

Where you placed probes Ch1 and Ch2? On output?
All pictures are from output?
Then your "A minus B" setup is not right.
EDIT:
"A minus B" setting:
1. VERT MODE:
"ADD" - ON.
2. CH2:
"INVERT" - ON.
3. Coupling switches:
both in "DC" or both in "AC".
4: VOLTS/DIV switches both in same position,
for example both in 5V/DIV or both in 1V/DIV etc.
VAR controls on switches should be OFF.
5. Both probes should be X10 or switched to X10 position.
6. Ground clips should be removed from both probes.
-------
Measurement:
One probe connected to one wire of circuit's OUT,
Other probe connected to second wire of circuit's OUT.
-------
Also try to see with sweep 0.5s/div.

Where you placed probes Ch1 and Ch2? On output?
All pictures are from output?
Then your "A minus B" setup is not right.
EDIT:
"A minus B" setting:
1. VERT MODE:
"ADD" - ON.
2. CH2:
"INVERT" - ON.
3. Coupling switches:
both in "DC" or both in "AC".
4: VOLTS/DIV switches both in same position,
for example both in 5V/DIV or both in 1V/DIV etc.
VAR controls on switches should be OFF.
5. Both probes should be X10 or switched to X10 position.
6. Ground clips should be removed from both probes.
-------
Measurement:
One probe connected to one wire of circuit's OUT,
Other probe connected to second wire of circuit's OUT.
-------
Also try to see with sweep 0.5s/div.

Thank you Danko,

I set up my oscilloscope per your outline. The only thing different is that i set the VOLTS/DIV to 50. I am using the 10X probes so my X10 MAG is off. I took the first test per MisterBill2 and ebeowulf17 to make sure that the oscilloscope was reading correctly. Below is what i end up with:

 

Thank you Danko,
I set up my oscilloscope per your outline. The only thing different is that i set the VOLTS/DIV to 50. I am using the 10X probes so my X10 MAG is off. I took the first test per MisterBill2 and ebeowulf17 to make sure that the oscilloscope was reading correctly. Below is what i end up with:

Excellent! Now test it with the slowest sweep 0.5s/div (TIME/DIV) and at different heating levels.
Of course, heating element should be connected to OUT.

 

Excellent! Now test it with the slowest sweep 0.5s/div (TIME/DIV) and at different heating levels.
Of course, heating element should be connected to OUT.

Danko, the last tow pictures are taken at 0.5ms, but when I switch to 0.5s I get a vertical line with no change when i switch the pad temperature.

 

Danko, the last tow pictures are taken at 0.5ms, but when I switch to 0.5s I get a vertical line with no change when i switch the pad temperature.

With sweep 0.5s/div:
Switch trigger to AUTO and switch trigger source to LINE position.
Then see at screen some tens seconds.
If will no changes, then circuit is not working properly (do not changes temperature).

 

With sweep 0.5s/div:
Switch trigger to AUTO and switch trigger source to LINE position.
Then see at screen some tens seconds.
If will no changes, then circuit is not working properly (do not changes temperature).

Just to clarify, this is to try to get a broad view and see if the board is doing burst fire as opposed to phase angle switching, right?

And, as you say, if no evidence of burst fire behavior can be found, it would seem that the board isn't working right. That brings me back to my earlier question for @BarryTron:

Can you tell if the device actually delivers different heat on different settings? Is it possible that all settings really are behaving the same?

 

With sweep 0.5s/div:
Switch trigger to AUTO and switch trigger source to LINE position.
Then see at screen some tens seconds.
If will no changes, then circuit is not working properly (do not changes temperature).

LOL, I was too hasty with the vertical line assumption, did not think too much of it at first. So I calibrated the settings per your instructions and when I saw the line I started to type my response with illustration. After some time I looked at the oscilloscope and what do you know: the line switched to a dot. I have included the picture of the line. So here is what I derived:

When any of the heat mode on the pad is selected (Warm, Low, Med, Hi) a warmup phase is initiated shown by a line on the oscilloscope. After the warmup time the voltage is then turned on and off per each setting:

Warm: 5min Warmup > 2sec ON and 7sec OFF iteration
Low : 3min Warmup > 3sec ON and 7 sec OFF iteration
Med : 3min Warmup > 4sec ON and 7 sec OFF iteration
HI : 5 min Warmup > 6sec ON and 3 sec OFF iteration

This is a great step forward for me, thank you everyone for all your help. But I do have more questions:

The time listed above indicates what I see with my eyes on the oscilloscope. Does that mean that because my TIME/DIV is set to 0.5 the actual time is half of what I observed on the oscilloscope? Also, am I to assume correctly that the it’s the programmed DC microcontroller that is executing above algorithm via clock speed utilizing the transistor as a relay for on/off switching?

 

Just to clarify, this is to try to get a broad view and see if the board is doing burst fire as opposed to phase angle switching, right?

And, as you say, if no evidence of burst fire behavior can be found, it would seem that the board isn't working right. That brings me back to my earlier question for @BarryTron:

Can you tell if the device actually delivers different heat on different settings? Is it possible that all settings really are behaving the same?

Ebeowulf17, yes the pad works per each setting. I posted the oscilloscope readout after Danko's setup.

 

The time listed above indicates what I see with my eyes on the oscilloscope. Does that mean that because my TIME/DIV is set to 0.5 the actual time is half of what I observed on the oscilloscope? Also, am I to assume correctly that the it’s the programmed DC microcontroller that is executing above algorithm via clock speed utilizing the transistor as a relay for on/off switching?

The time/division is telling you each of the ten divisions on the x-axis is a half second, meaning it should take 5 seconds for the trace to sweep from left to right, before restarting on the left. The screen should refresh every 5 seconds, but what you see appearing at any given moment is in real time, not compressed or altered.

Yes, there must be some sort of microcontroller handling all the timing/switching decisions. And the output switching is being done through a TRIAC - not a transistor per se, although they are related.

 

The time/division is telling you each of the ten divisions on the x-axis is a half second, meaning it should take 5 seconds for the trace to sweep from left to right, before restarting on the left. The screen should refresh every 5 seconds, but what you see appearing at any given moment is in real time, not compressed or altered.

ebeowulf17, i don't get a moving trace that sweep from left to right. I only get a vertical fixed line per my last photo post. Only when i switch my Time/Dev to 0.2 that I see the trace move from left to right?

Yes, there must be some sort of microcontroller handling all the timing/switching decisions. And the output switching is being done through a TRIAC - not a transistor per se, although they are related.

Do you know the number of the microcontroller. I have 14 pins on my microcontroller, any way for me to find out?

Thanks,