Safe to hook osciloscope probe directly to this bridge rectifier?

[wmundstock]

Hi All.
So I am studying some basics of power supplies. Built the simplest circuit possible. Used a transformer to convert AC 127V to AC12V. Then added a full wave rectifier with 4 diodes. One resistor to be the load.
I verified everything and the differential probe shows the full rectified wave.

Question is: Can I plug the standard probes to the output of the transformer and the output of the rectifier safely without ground loop issues?
In other words, will it blow? I think it should be safe as the transformer will isolate the main's ground from the circuit.

Thanks.
Walter

[adinsen]

The transformer will indeed isolate the circuit from the mains so you are safe and will not create a ground loop connecting the oscilloscope probe to your recitifier to see the rectified signal, even when the scope is grounded.

[wmundstock]

The transformer will indeed isolate the circuit from the mains so you are safe and will not create a ground loop connecting the oscilloscope probe to your recitifier to see the rectified signal, even when the scope is grounded.

Thank you very much!

[Ian.M]

Provided the secondary isn't already grounded, yes that's right.

Note you can connect scope ground to *either* one of the transformer secondary wires or one of the bridge rectifier DC output pins (normally the negative one), *NOT* *BOTH*.   If you need to look at the AC in at the same time as the DC out, use your differential probe for the AC, across both secondary wires.

An appropriately rated fuse in series with the secondary will save your ass if you have a brainfart and connect two ground clips to different nodes.  You wont melt your probes or the ground tracks in the scope, but the bridge rectifier diodes might suffer.   N.B. a fuse wont help much with mains live bridge rectifier circuits (though its still needed for fire safety), as the very high transient currents from a direct mains supply can damage your scope before the fuse blows.

[wmundstock]

Thank you!
Since I am learning, let me ask a follow up question.

Note you can connect scope ground to *either* one of the transformer secondary wires or one of the bridge rectifier DC output pins (normally the negative one), *NOT* *BOTH*.   

Got it, I cannot provide 2 different ground references to the scope as that could create potential difference between the two ground wires, messing it up. Is that it?

[Ian.M]

Yes.  Connect two grounds to input and output of the bridge, and you've effectively shorted one of the diodes in the bridge.  At best you'll over-stress the adjacent diode in the opposite arm of the bridge, at worst, if the secondary isn't fused, your probes and scope can get a bit melty!

[BeBuLamar]

So if you try to have 1 channel showing the AC voltage and the other channel showing the rectified (pulsed DC) voltage you can't. You can only display one at a time.

[Ian.M]

Unless like the O.P, you've also got a differential probe.

[tggzzz]

Got it, I cannot provide 2 different ground references to the scope as that could create potential difference between the two ground wires, messing it up. Is that it?

Almost all scopes have all probe shields connected together and also to protective mains earth. If you have one of the few scopes with isolated inputs, then you will already know it.

Never disconnect the protective main earth or run an ordinary scope from a battery (i.e. never float the scope) If you do then the scope case will be at the same potential as whatever you are probing (e.g. 240V ), and in some cases you will put a significant capacitive load on whatever you are testing.

BTW, the input transformer should be an isolating transformer, not an autotransformer like a variac. I expect that is the case.

[Doctorandus_P]

BTW, the input transformer should be an isolating transformer, not an autotransformer like a variac. I expect that is the case.

Not only that. It should be a double isolated transformer. These transformers have an extra reinforced isolation barrier between the primary and the secondary winding.

Also for beginners, The probe tip has a high impedance, and your scope is designed to withstand a reasonable voltage on it's input (My Siglent SDS 1104X-E says 400Vpk on it's front panel, and that is normal for a scope). But GND can be troublesome. For low frequency measurments, it's easiest to just connect one of the GND leads of the scope to the GND of your circuit. When you start getting into higher frequencies (10kHz and up) then each probe should have it's own  GND lead. I wonder if it's a good idea to put a fuse in your scope GND lead. This prevents your probe leads from melting if for example one of the GND leads is connected to "GND" and the other gnd lead is connected accidentally to a hard voltage (Power supply output).

If you want to measure signals in the MHz range, then the crocodile GND leads on your scope are useless. You have to use better grounding techniques, such as the spring clips that connect directly to the tip of your probe. (They should be in the kit with the probes).

[wmundstock]

Thanks for the recommendations.

Unfortunately, my home do not have a real earth connection. In fact most of the main's sockets only have 2 pins (neutral and phase/live) with 127v. In my bench they are interconnected with 3 wires, so the function generator and the scope do share the same ground but, again, they are not going to earth.

I don't typically mess with higher voltages when I am using the scope, and when I do I tend to use the high voltage differential probe.

In this particular case, I was testing the bridge rectifier using the function generator to generate the sine wave and I wanted to try out the real deal with a transformer to see the differences. With the function generator, it was impossible for me to see the full wave rectified, unless I use the diff probe. I would only see one half or another, I think that is because of the shared ground. Only way to see the full wave was using scope math channel. It as an interesting learning nevertheless.

[tggzzz]

BTW, the input transformer should be an isolating transformer, not an autotransformer like a variac. I expect that is the case.

Not only that. It should be a double isolated transformer. These transformers have an extra reinforced isolation barrier between the primary and the secondary winding.

Why is that necessary?

Also for beginners, The probe tip has a high impedance, and your scope is designed to withstand a reasonable voltage on it's input (My Siglent SDS 1104X-E says 400Vpk on it's front panel, and that is normal for a scope).

Only at audio frequencies. Check the probe's max AC voltage as a function of frequency. (And scope's).

[tggzzz]

Thanks for the recommendations.

Unfortunately, my home do not have a real earth connection. In fact most of the main's sockets only have 2 pins (neutral and phase/live) with 127v. In my bench they are interconnected with 3 wires, so the function generator and the scope do share the same ground but, again, they are not going to earth.

Consider an RCD/GFCI/ELCB, to ensure all current coming from one conductor goes to the other, and that none goes through you. 30mA is the max trip current you should accept; lower is better.

[Zero999]

Here's a schematic showing how a bridge rectifier works. Note the diodes conduct alternately, every time the phase conductor's polarity flips from positive to negative.


Here's what will happen if you connect an oscilloscope's ground leads to both the AC and DC sides to the bridge rectifier. One of the diodes will be bypassed and on either the positive, or negative half cycles, depending on which diode it is, a large current will flow through the other diode. This will cause the transformer, diode and possibly the oscilloscope's probe and internal connections to overheat, if this occurs for too long.

[wmundstock]

Consider an RCD/GFCI/ELCB, to ensure all current coming from one conductor goes to the other, and that none goes through you. 30mA is the max trip current you should accept; lower is better.

Thank you very much tggzzz! I will learn more about this and see if I can set this up.
If I get this right, its quite simple, and I could install one just for my bench and that will help protecting the equipment a great deal!

[wmundstock]

Here's a schematic showing how a bridge rectifier works. Note the diodes conduct alternately, every time the phase conductor's polarity flips from positive to negative.
(Attachment Link)

Here's what will happen if you connect an oscilloscope's ground leads to both the AC and DC sides to the bridge rectifier. One of the diodes will be bypassed and on either the positive, or negative half cycles, depending on which diode it is, a large current will flow through the other diode. This will cause the transformer, diode and possibly the oscilloscope's probe and internal connections to overheat, if this occurs for too long.
(Attachment Link)

Thank you very much for taking the time to walk me through this Zero999. The schematics helped a lot and it is very easy to see why it is not a good idea to hook the scope probes that way.

[tggzzz]

Consider an RCD/GFCI/ELCB, to ensure all current coming from one conductor goes to the other, and that none goes through you. 30mA is the max trip current you should accept; lower is better.

Thank you very much tggzzz! I will learn more about this and see if I can set this up.
If I get this right, its quite simple, and I could install one just for my bench and that will help protecting the equipment a great deal!

That's what I have done, even though my mains sockets have an earth connection.

Be aware that there is no substitute for thought and understanding. No one device makes something completely safe, and can lure someone into a false sense of security.

In addition, having two devices can remove safety .

Consider having a mains-powered UUT, an RCD and an (mains) isolating transformer. If you use the RCD connected to the isolating transformer connected to the UUT, then then the RCD will never trip. All the RCD "sees" is the isolating transformer; any current imbalance on the other side is "invisible" to it.