Pump or just capacitor?

[Agile]

Unfortunately neither of the last two posts have understood what I was suggesting.

All I suggested was that by connecting a bridge rectifier to a DC current meter then it can be used to measure AC current.

Thats actually what meters use to measure AC current.

I dont have any programs to draw circuits but if you think of a bridge as having two AC inputs then they are placed in series with the item whose current is to be measured and then the + and - connections are used to connect to the meter.

Tony

 

[Agile]

Its so simple but here is a circuit:-



View media item 61076

 

[andytw]

Yes...that is simple. Or perhaps simplistic. One would hope the meter is set for DC volts otherwise it is likely to draw a lot of current.

Point remains that you can't measure the AC current drawn by an AC motor by using a bridge rectifier to make it DC current.


andytw

 

[Agile]

That circuit is most easily interpreted as for measuring voltage.

But the same connection to a bridge can equally be used to measure AC current on a DC meter.

Thats how multimeters do it.

Tony

 

[andytw]

Good multimeters (AVO, Fluke) that measure AC current use a current transformer that passes the AC current straight through the transformer primary without affecting it. The output of the secondary of the transformer is then rectified and passed on to the DC measuring device.

I would guess, but don't know for sure that cheap meters that measure AC current use a very very small resistor in series with the current being measured and then measure the AC voltage across the resistance by the same method above.

The message is that nothing in the AC current circuit gets rectified.

andytw

 

[Agile]

A current transformer is used to isolate the meter from voltages on the test circuit and increase the voltage produces so that the forward voltage drop within rectifiers is overcome.

But they increase the cost and weight!

 

[andytw]

Unfortunately neither of the last two posts have understood what I was suggesting.
.........

I dont have any programs to draw circuits but if you think of a bridge as having two AC inputs then they are placed in series with the item whose current is to be measured and then the + and - connections are used to connect to the meter.

Tony

Yep fully understand you...but do you understand me ?

Once you have done as suggested in your quote above, then you now only have current going around the circuit in one direction. You now have DC current and not AC current. If the item is a light bulb then fine, it will be a tad dimmer due to the tiny voltage drop across the diodes of the bridge rectifier, but if you are talking about an item that actually needs AC to make it run (in fairness this post is about CH pumps) and consequently show some impedance rather then just resistance then sure you'll get a reading but A) the pump will not be turning B) the current measured will in no way reflect what was expected or being tested for.

My Summary...For an AC motor to work it requires AC current flowing through it. Putting diodes in series with the motor will stop it working and its characteristics will change dramatically.

 

[DP]

Tony, your drawing in NOT measuring current BUT voltage at the AC source. The source may well be the L and N terminals on a socket being one example.

If I needed to measure current in an AC circuit, I would introduce a known small resistance in series with the load and measure the voltage across it to give me current flow through the resstance, hence the current flow in the circuit. This exercise would be a waster of time though, as measured current value is just that. One needs to have a refernce value to make sence of readings

 

[Agile]

Its not "my" drawing but one I came across to copy.

Sorry but I dont have time to make a video of using a bridge rectifier.

I agree that a small series resistance is easier to manage than a bridge rectifier.

That will give a reasonable accuracy of measurement too providing a smaller resistance like one ohm is chosen which will not affect the circuit under test.

 

[Dan Robinson]

Tony, , can't be expecting diy'ers to to fill in the blanks for you.

 

[DP]

Tony I do concede you are clever and knowledgeable chap and I have in the past picked gems of information from your multitude of posts.

In this post you are digging a hole that is getting deeper and information is merely sketchy instead of being precise and exact a quality you demand of others. What started as need to replace a capacitor in a pump that a 10 year old could carry out under instructions, has become a topic on how to use moving coil/ moving iron meters to measure current in circuit with total disregard to phase angles involved, shunt/ series resistances and availability of reference values against which measured readings could be compared and the need to push the boat this far

 

[Agile]

I only posted one line on how a DC only current meter could be used to measure AC currents.

Others have chosen to extend that aspect of the posting.

Using a rectifier bridge will enable an AC current to be read on a moving coil or a digital meter.

A moving iron meter will respond to AC or DC current.

Tony

 

[andytw]

Try taking head off and removing terminal cover and put it somewhere quite warm like an electric oven for an hour and see if it will dry it out.

But usually water in the windings is caused by a seal failure and will probably leak again when refitted.

A bridge rectifier will enable your test meter to measure AC current!

Tony

It's just that you included it in your reply to the OP, which really made it look like it was on-going advice for him and his pump, or anyone finding the thread in the future. I think we, others, rightly interpreted it as part of the on-going help. If it is, as you say, just a single throwaway comment then it should be posted elsewhere.

And, BTW, doing as you suggest will not measure AC current as the current flowing around the circuit will now be DC because the bridge rectifier you introduce has rectified the AC and turned it into DC for all components in the circuit......or HAVE I MENTIONED THAT ? Er..Hello !!!

I understand where you are coming from, but you are not right. Just kind of right'ish, (even that is generous).

 

[Agile]

The current flowing through the pump will continue to be AC but the current presented to the meter after the bridge rectifier will be DC.

Tony

 

[andytw]

I could just crawl off and not reply but I will reply.... I humbly concede. I had the pump in series with the meter..
I will now crawl off.