Sam Brown

[Sam Brown]

A Variable Frequency Drive (VFD) is a system not a type of motor. A VFD consists of a computer board, which sends pulse width modulated signals to an inverter board, which in turn interprets those signals and sends power to a BLDC motor.

I recommend you rewatch the videos if this still isn’t clear. After doing that, let me know if you have any more questions.

[Sam Brown]

Thanks for the answer. I still don’t understand exactly why he wanted to see the exact resistance on those 2 loads? Wouldn’t doing the continuity test sufficient enough?

Normally, yes, a continuity test would be sufficient. Scott wanted to take a resistance measurement for pedagogical purposes — he wanted to show the resistor and the heater’s resistances adding up.

[Sam Brown]

Can you use resistance check to test for continuity OR to test if loads in a circuit are working?

Yes, you can use a resistance measurement to check for continuity — a continuity measurement is just a dumbed-down resistance measurement. You cannot, however, test if a load is working with a resistance measurement. You can see if it is electrically within specifications, but there could be other things wrong with it.

When do i have to use resistance test?

When you’re checking a component’s resistance against specifications. If a particular heater is supposed to have a resistance of 10 ohms, you can do a resistance measurement to confirm that.

Is continuity test only used to find breakage within circuit lines?

Good question! Yes, that is essentially all you would use a continuity test for.

All of these questions get talked about in detail in the last video of Module 5, unit 4. If you haven’t watched it already, go ahead and do so! It will help clear things up a lot.

[Sam Brown]

Why wound’t he use use Continuity testing for this case instead of resistance testing?

Because he was interested in seeing the exact resistance of those two loads — continuity would only tell if the circuit was open or closed.

[Sam Brown]

For that particular test, Scott was interested in looking at the combined resistance of those two loads, which a voltage measurement wouldn’t have shown. Also, since one of his hands was stuck way back in the machine, it would have been very difficult to do that measurement without shocking himself if the machine were plugged in.

[Sam Brown]

” Cam number 1 needs to be closed in order to power the entire system. So I decide to check the cam and see it is closed, I measure O volts, this is telling me that it is closed and I am just reading one side of a closed switch to the other.”

doesn’t this question assume the rest of the circuit is in fact a circuit? wouldn’t an open neutral or bad L1 have the same result?

Excellent observation! You’re right, of course. Performing that voltage measurement would only be conclusive if you have confirmed that the circuit has a valid voltage supply and path to neutral. If either of those things are missing, you will read 0 VAC on your loading meter, just as you would if you had a closed switch with a valid power supply.

This illustrates the kind of thinking we have to do as technicians. Whenever we make a test, we have to think carefully about what that test can and cannot tell us. In the situation where you have previously confirmed that your circuit has a valid power supply, then reading 0 volts across the timer cam definitively proves that it is closed. But if you haven’t checked your power supply first, then that test doesn’t necessarily tell you anything.

Just like you have to use the right tool for the job, you have to use the correct test for your current situation. Make sense?

[Sam Brown]

Great markups! Looks like you’ve got it totally figured out — no errors that I can see. Very well done!

[Sam Brown]

Good catch! I believe that the Samurai just misspoke there — he meant to say “closed”. Well done staying on your toes! 😉

[Sam Brown]

Do you see how all those switches are drawn in bold, thick lines? That’s a common convention on schematics to show that all those switches are internal timer contacts. That’s why it doesn’t explicitly call them out as such.

We’re not interested in measuring the amperage on the black wire that brings L1 into the timer, so that’s why we don’t measure at C. That amperage would be the total current of L1, while what we’re interested in is the amperage in the heater circuit specifically.

The red wire coming off of A is an easy place to put our clamp that measures only the current passing through the heater circuit.

Make sense?

[Sam Brown]

It’s not that your meter can’t be trusted — you just have to be aware of a few things: the current setting your meter is on, the purpose of that setting, and the specifications of your meter.

When you’re doing resistance or continuity settings, your meter has a certain threshold for ohms beyond which it will simply report infinite resistance. To find out what this limit is for your meter, you have to look at its specs.

For example, if a particular meter only measures ohms up to 20,000 ohms, and you measure across a component with 20,100 ohms of resistance, then your meter will report it as open/infinite resistance.

We brought up this point in the unit for two reasons: a) you should be aware of what your meter’s specs are, and b) resistance measurements are not always conclusive. This does not mean that your meter is unreliable or that you can’t trust it — it just means that you need to be aware of your tool’s limitations and think carefully about what your measurements actually mean.

[Sam Brown]

Why exclude that 32 ohm resistor in the final equation is what I’m getting at?

Because in that particular equation, the Samurai is calculating the power output of just the loose connection, and so he only factors in the resistance of the loose connection. He did the same thing in the equation right above that one, but that time for the heating element. When calculating a particular load’s power output, you factor in the total circuit current, but only that particular load’s resistance.

If you want to calculate the power output of the entire circuit, then you would add together the resistances of all the loads present.

Make sense?

[Sam Brown]

The resistor and the controls are in parallel, but not with the heater, right?

That’s correct.

But going forward with this, is it still correct though to just add both the resistance values of the heater and resistor like they’re in series since in the earlier module/unit we learned about equivalent resistance in parallel circuits?

The heater and the resistor are in series. Remember that series-parallel circuits are a configuration that can exist. In this instance, the heater is in series with every other component in the circuit, while some of the components are in parallel with others. Your own diagram that you posted shows this quite well.

That the heater and the resistor are in series is in fact proven by the measurement that the Samurai made in the video — if they were not, how would he have measured the expected amount of ohms by reading across them?

Keep in mind too that when you make a resistance measurement, you are effectively isolating a part of the circuit. For the purposes of that test, all that matters is the section of the circuit that is between your leads.

Let me know if it’s still unclear.

[Sam Brown]

It might look that way because of the way the schematic is drawn, but don’t let that fool you. Pull up the image again and follow the lines from 1M to OR.

You see how you have to pass through both the heater and the resistor in order to get from one point to the other? That’s the path that the electrons have to follow, which means that those two components are in series — at least when the TM to OR contacts are closed on that switch by the timer motor.

Let me know if any of this still doesn’t make sense.

[Sam Brown]

Here’s the exact wording of question 4:

In a triac, the working voltage will be ____, and the control voltage will be ____.

I think you might be mixing it up with question 3, which states:

In an electromechanical relay, the working voltage will be ____, and the control voltage will be ____.

Those two questions are referring to separate pieces of technology. Triacs and relays work very differently, so it’s important to distinguish between them. Try reviewing the material in the unit to make sure that distinction is clear to you.

Once you’ve done that, write back and let me know if you’re still confused about anything.

[Sam Brown]

Hi Nate,

Remember the conditions that are set up in question 4 (question 5 is just a continuation of that same scenario). In question 4, it’s already stated that you have a good power supply. What you really need to do is make sure that the component in question is putting out the correct voltage.

Where would you put your meter’s probes to check the output of this component? (Make sure you read all the text on the schematics — text on schematics is always important.)

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