Hi, I just built a bridge rectifier that before the diodes has a transformer that transforms 220 volts in 10 volts AC.
I plugged it in then with with my multimeter I measured the voltages and other things, then I decided to measure the amperage from the exit of the transformer and it ended in a big spark, the home circuit breaker off, the multimiter fuse blew up and the tip of one measuring rod melted.
I think that happened because the rectifier wasn't plugged into anything asking for current.
How can I measure its amperage and the one of other devices
safely?

 

As you've discovered, that is NOT the way to measure current. On the current-measuring range your meter acts as an almost-short-circuit!
You need to connect the meter in series with a sensible load; one that draws less current than the transformer, rectifiers and meter are rated for.
Always do research to check the voltage/current/power ratings of electrical/electronic things before using them.

 

You need to connect the meter in series ....

Just adding a bit of emphasis on the key point.

Don't feel bad. The reason we all know what you did wrong is because we've also done it. Some may have done it more than once.

 

You can't.
An ammeter looks like a near short circuit, so connecting the ammeter directly to the transformer caused a very high current with the resulting sparks and breaker opening you observed.
It had nothing to do with whether the rectifier had a load or not.
The only thing you can measure with an ammeter is whatever the load current is when a load is connected.
This is done by connecting the ammeter in series with the load, not across the transformer.

If you want to know the maximum current the transformer can deliver safely, you need to look at the transformer rating.
You certainly can't determine that by shorting the output with an ammeter.

 

I think that happened because the rectifier wasn't plugged into anything asking for current.

No. With no load, there is virtually no current in the rectifier and nothing to measure. With a load, the current in the rectifier will be identical to the load in the rectifier.

What you actually "measured" was the current through the shunt in your meter. That was huge, and blew your fuse and circuit breaker. It's quite likely that the shunt in your meter – which is just a section of copper wire in cheap meters – may have been damaged also before these protection devices could do their jobs. No way to tell except to measure (properly, this time) the current from a known voltage through a known load. If you get an answer that's within 10% or so of the expected value, your meter's current shunt is probably OK.

 

I think my multimeter is OK because I opened it and the only thing to be damaged and burnt was the 20 amp fuse and it measure all but the amperage from the port for high amps.

 

Actually I tried to measure the current from the input of the transformer which is worse because that's 220 volts

 

Actually I tried to measure the current from the input of the transformer which is worse because that's 220 volts

Yup, I can't hardly imagine a worse abuse of your poor meter!

 

Yikes! You're lucky to be alive. Keep well away from mains voltage, at least until you have a lot more experience under your belt.

 

Yikes! You're lucky to be alive. Keep well away from mains voltage, at least until you have a lot more experience under your belt.

I will after today's experience... Thank all you a lot

 

Unfortunately a little late with a cartoon but...


When measuring current the meter is placed in series with the load, you need a load out there and wnen measuring voltage we measure across the load. You are not the first and certainly will not be the last to experience what you just discovered. Don't worry about it and consider it a lesson learned.

Ron

 

Unfortunately a little late with a cartoon but...
View attachment 133734

When measuring current the meter is placed in series with the load, you need a load out there and wnen measuring voltage we measure across the load. You are not the first and certainly will not be the last to experience what you just discovered. Don't worry about it and consider it a lesson learned.

Ron

Thank you,although I have two questions:
Why are there 2 meters in the cartoon?
Do I have to connect the positive to the red wire in the plug for amps and the black in COM and on the other wire?

 

The little cartoon shows two meters because one is configured for Volts V and the other Amps A.

Do I have to connect the positive to the red wire in the plug for amps and the black in COM and on the other wire?

You would configure an Amp meter to measure current in series with the load. In the case of DC the + lead to + of the source and the - lead as shown. Yes, most amp meters would be connected as you stated but this is a matter of the meter. Therefore I suggest you follow any instructions in the meter's user manual.

Make sure the power supply (source) has a load. Otherwise you are using the meter to place close to a short across the source output which is apparently what happened. The load can be as simple as a resistor or lamp.

Ron

 

This is nearly a duplicate of a thread from last week
https://forum.allaboutcircuits.com/...f-batteries-they-get-hot.139308/#post-1169988
https://forum.allaboutcircuits.com/...f-batteries-they-get-hot.139308/#post-1169988

 

As you've discovered, that is NOT the way to measure current. On the current-measuring range your meter acts as an almost-short-circuit!
You need to connect the meter in series with a sensible load; one that draws less current than the transformer, rectifiers and meter are rated for.
Always do research to check the voltage/current/power ratings of electrical/electronic things before using them.

Despite that agree with the colleagues that we have all done it, this is dangerous! Be careful and if you dont know what you are doing (how to measure voltage and current) please ask first. Also post a schematic.

The idea like mentioned before is this: if you put the meter as a load, the transformer may supply the maximum or peak current through the meter, because technically the meter acts as the load. If you put a resistor as the load and after the resistor the meter to the second lead of the transformer or ground you will have no problem. But keep in mind that the resistor needs to be with a high value, or you might burn the meter and yourself again! First you connect all the components, second you should have a defense (preferably a good one that will limit the current to the transformer and will fry instead of your house wiring or you). Than you connect and you test the circuit after you have put the meter stabely to the second lead of the resistor and to the second lead of the transformer or ground.

 

By a defense I ment you buy your own defence and put it right before the transformer or whatever you are building (sorry you are not good at building so its better to buy this part).

 

That is one way to check a power supply's output current capability.

I've performed this test on many hundreds of power supplies. The current meter was rated for the expected maximum current and there was a fuse for those power supplies with defective current limit.

 

That is one way to check a power supply's output current capability.

I've performed this test on many hundreds of power supplies. The current meter was rated for the expected maximum current and there was a fuse for those power supplies with defective current limit.

If this is towards me, I think the author is not that experienced or he could connect and "Am meter" to the leads, this is the easiest and cheapest way not to kill yourself ! Be careful cheap things tend to have a lower percentage of working efficiency, meaning the chances that you dont die are higher!

 

No, it is not towards anybody. It is a statement of that measuring a power supply's short circuit current is an important part of design validation and quality control.

 

No, it is not towards anybody. It is a statement of that measuring a power supply's short circuit current is an important part of design validation and quality control.

Yes, but there’s “measuring” with appropriate equipment and preparation and then there’s finding out the hard way that your power supply can overpower your meter.