Broken rectifier? Outputs both DC and AC

Thread Starter

feikescholtens

Joined Nov 15, 2019
3
So I want to make a DC power source from my moped so I can charge my phone, but my rectifier from the coil puts out DC and AC on my multimeter. Am I doing something wrong or is it broken?
 

MrChips

Joined Oct 2, 2009
22,550
Welcome to AAC!

You have your meter set to AC range and it still shows voltage even though you think that the supply is DC only. Correct?

The AC scale is meant for measuring LINE voltage such as 120VAC @ 60Hz or 230VAC @ 50Hz depending on your country. You cannot use the AC range when measuring DC voltages. There is nothing wrong with your meter. That is simply how the meter is designed when measuring AC.
 

Doode

Joined Nov 15, 2019
8
Which ped you got? Might want to check out a moped specific forum for flywheel output and adding accessories. You can pick up a cheap lawnmower regulator rectifier for like $8 and wire it to convert AC to DC
 

dl324

Joined Mar 30, 2015
12,258
no I don't but I have this voltage regulator which converts it into 12V. On the output I connected this car 12V- 5V usb adapter thing. When I connect my phone to it, it shows the charging symbol and then it immediately disappears.
I doubt that there are many switching regulators that could generate a stable 12V under the conditions you have (as I understand them).

With a single rectifier, this is what you'd have:
1574381749452.png
The input drops to 0V. Without a filter cap it's not going to work very well.
 

Thread Starter

feikescholtens

Joined Nov 15, 2019
3
I doubt that there are many switching regulators that could generate a stable 12V under the conditions you have (as I understand them).

With a single rectifier, this is what you'd have:
View attachment 192132
The input drops to 0V. Without a filter cap it's not going to work very well.
I've got a rectifier with this: 26mb20 6b
written on it.

When measuring the voltage from it's output I can measure 14-16 V AC AND 6-8 V DC, while on the input is only 7-8 V AC. Is the rectifier broken or is it meant to do this?
 

shortbus

Joined Sep 30, 2009
8,197
The AC scale is meant for measuring LINE voltage such as 120VAC @ 60Hz or 230VAC @ 50Hz depending on your country.
So you mean that I can't measure the output of a step down transformer? I don't believe any of my meters stop working at measuring low voltage, non mains voltages. His moped puts out AC if it is like most, it uses a flywheel, PM alternator. The rectifier puts out a very bad half wave DC with a high AC bias, basically both AC and DC at the same time, so he is probably telling you exactly what he is seeing on the meter.
 

MrChips

Joined Oct 2, 2009
22,550
So you mean that I can't measure the output of a step down transformer? I don't believe any of my meters stop working at measuring low voltage, non mains voltages.
Read my text carefully. I said "LINE voltage such as..."
His moped puts out AC if it is like most, it uses a flywheel, PM alternator. The rectifier puts out a very bad half wave DC with a high AC bias, basically both AC and DC at the same time, so he is probably telling you exactly what he is seeing on the meter.
He is telling exactly what he is seeing on the meter.
That doesn't mean that the meter reading is correct.

All meters are not designed equally.
Is the meter AC coupled?
Does the meter compute RMS correctly?
Does the meter work over a wide frequency range?
 

shortbus

Joined Sep 30, 2009
8,197
Read my text carefully. I said "LINE voltage such as..."
Exactly, and a step down transformer is no longer at "line voltage". Or for that matter a step up transformer.

Even my "free" Harbor Freight meters measure both AC and DC as long as you pick the correct scale.
 

Wolframore

Joined Jan 21, 2019
2,195
You guys are both correct. The harbor freight meter has an AC setting, I believe its not true RMS. also the AC out of a a single coil measured with a multimeter will not be correct... it will show that there’s something there but the measurement is not correct. Imagine a square wave pulse at +/-10v, the RMS meter would say that’s 7.07V but it would rectify to 10v dc minus some diode drops. Only the oscilliscope will shows the correct output. We have no idea what the pulse is off the coil. A lumpy pulse would need quite a capacitor to try to filter to a usable DC. Thats why alternators we’re invented.
 

Wolframore

Joined Jan 21, 2019
2,195
Does the moped have a battery? Many are just kick start... the coil provides the juice for ignition... no clue if this one does... if it does, why are we not just pulling power off the battery?
 
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