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# Teach me on the magic of moving electrons aka. how to figure out a mysterious power brick?

Registered User regular
edited January 2022
Full disclosure - I have bits of knowledge on electronics, but not a deep one and I need help to avoid messing up

I find my self trying to work out what comes out of a power brick that powers a old but fancy turntable which is acting out.
Essentially the turntable won't maintain constant rpm, instead of the 33.3 needed I may get 32 or even 31, then after using the trimming screw on the turntable it might play nice or maybe a few minutes later the can have dropped some or risen to perhaps 34+. It is driving me nuts, so I have decided to try and sort it out.

If it can be avoided I do not wanna take the unit apart and with it using a external power brick I figure I should start there. Only the power brick is totally unmarked one and from web research on the turntable its motor can be a AC one or a DC one, so how do I figure out what I have.

The brick:

The cable from the brick to the turntable has at one time been split or something, so it is really one cable from the brick to a cable connector and then another cable from the connector going into the turntable - the cables both only have two leads so no ground.
The connector has enabled me to measure voltage using a multimeter, where I tried measuring both DC and AC.

My results:
14.62 Volt DC idle motor, 14.52-14.57 Volt when the plate is rotating.
31.4 Volt AC idle motor, 31.3-31.5 Volt when the plate is rotating.

Now I do not understand how I can measure both DC and AC????

I am hoping it is DC that is driving the motor, because then I have a power supply I could use as a replacement to the brick as it will tell if it is the brick that is the cause of the rpm issues. The PSU I have is one used for electronics work, only I have a likely silly question on that.

My PSU:

I will of course need to set the PSU at the correct voltage, however am I correct in understanding that regarding amps I just need to make sure I do not set a limit to low as really the turn table will just get what it needs i.e. it is not like the PSU can somehow push to many amps???

Bones heal, glory is forever.
BlindZenDriver on

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ＭＥＭＥＴＩＣＨＡＲＩＺＡＲＤ interior crocodile alligator ⇔ ǝɹʇɐǝɥʇ ǝᴉʌoɯ ʇǝloɹʌǝɥɔ ɐ ǝʌᴉɹp ᴉRegistered User regular
edited January 2022
Now I do not understand how I can measure both DC and AC????

You can't. Most inexpensive multimeters are only built to measure DC voltage. When you tell it to measure AC instead, what it is actually doing is applying a conversion equation called a Root Mean Square equation to extrapolate the AC voltage.

You can easily replicate this behavior:

Measure the voltage across two poles of a battery
Your multimeter will probably report about 2.2x the actual voltage of the battery. (EG, a 5V battery will report roughly 11 volts.)

There are a couple of ways to deal with this:

A) Get a mains voltage tester or an AC-only voltage tester. These will give you a more accurate reading for AC.

Assume the turntable uses DC. Set your power supply to the DC voltage and use that to power the turntable. See if the turntable behaves.
I will of course need to set the PSU at the correct voltage, however am I correct in understanding that regarding amps I just need to make sure I do not set a limit to low as really the turn table will just get what it needs i.e. it is not like the PSU can somehow push to many amps???

Generally, yes, this is correct. A device on DC power will only pull the amps it needs.

However, there's a big caveat. If the turntable has a component giving too much resistance, or if there's a short, then the turntable might pull more current than normal. This can cause components elsewhere in the circuit to get overloaded, or it can cause heat buildup or even be a fire hazard. Since the turntable has some unknown problem right now, it's safest to limit it to the minimal amperage it needs to operate.

Feral on
every person who doesn't like an acquired taste always seems to think everyone who likes it is faking it. it should be an official fallacy.

the "no true scotch man" fallacy.
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Registered User regular
Thank you very much for sharing you knowledge.
Feral wrote: »
Now I do not understand how I can measure both DC and AC????

You can't. Most inexpensive multimeters are only built to measure DC voltage. When you tell it to measure AC instead, what it is actually doing is applying a conversion equation called a Root Mean Square equation to extrapolate the AC voltage.

You can easily replicate this behavior:

Measure the voltage across two poles of a battery
Your multimeter will probably report about 2.2x the actual voltage of the battery. (EG, a 5V battery will report roughly 11 volts.)

Indeed. Seems I have a magic 9V battery capable of also delivering AC at more the double the voltage
Feral wrote: »
There are a couple of ways to deal with this:

A) Get a mains voltage tester or an AC-only voltage tester. These will give you a more accurate reading for AC.

Assume the turntable uses DC. Set your power supply to the DC voltage and use that to power the turntable. See if the turntable behaves.

I am leaning towards option B. Would I be right in assuming that if the power brick of mine was actually delivering AC then I would not be measuring DC from it?
Feral wrote: »
I will of course need to set the PSU at the correct voltage, however am I correct in understanding that regarding amps I just need to make sure I do not set a limit to low as really the turn table will just get what it needs i.e. it is not like the PSU can somehow push to many amps???

Generally, yes, this is correct. A device on DC power will only pull the amps it needs.

However, there's a big caveat. If the turntable has a component giving too much resistance, or if there's a short, then the turntable might pull more current than normal. This can cause components elsewhere in the circuit to get overloaded, or it can cause heat buildup or even be a fire hazard. Since the turntable has some unknown problem right now, it's safest to limit it to the minimal amperage it needs to operate.

More great info. I feel silly for not considering that.

I shall measure what the brick supplies and make sure to set my power supply current limit threshold to not surpass that (I've had this fancy PSU for years and I got it for nothing - first time I shall use it for more than testing fans).

Bones heal, glory is forever.
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Registered User regular
edited January 2022
Thank you very much for sharing you knowledge.
Feral wrote: »
Now I do not understand how I can measure both DC and AC????

You can't. Most inexpensive multimeters are only built to measure DC voltage. When you tell it to measure AC instead, what it is actually doing is applying a conversion equation called a Root Mean Square equation to extrapolate the AC voltage.

You can easily replicate this behavior:

Measure the voltage across two poles of a battery
Your multimeter will probably report about 2.2x the actual voltage of the battery. (EG, a 5V battery will report roughly 11 volts.)

Indeed. Seems I have a magic 9V battery capable of also delivering AC at more the double the voltage
Feral wrote: »
There are a couple of ways to deal with this:

A) Get a mains voltage tester or an AC-only voltage tester. These will give you a more accurate reading for AC.

Assume the turntable uses DC. Set your power supply to the DC voltage and use that to power the turntable. See if the turntable behaves.

I am leaning towards option B. Would I be right in assuming that if the power brick of mine was actually delivering AC then I would not be measuring DC from it?
Feral wrote: »
I will of course need to set the PSU at the correct voltage, however am I correct in understanding that regarding amps I just need to make sure I do not set a limit to low as really the turn table will just get what it needs i.e. it is not like the PSU can somehow push to many amps???

Generally, yes, this is correct. A device on DC power will only pull the amps it needs.

However, there's a big caveat. If the turntable has a component giving too much resistance, or if there's a short, then the turntable might pull more current than normal. This can cause components elsewhere in the circuit to get overloaded, or it can cause heat buildup or even be a fire hazard. Since the turntable has some unknown problem right now, it's safest to limit it to the minimal amperage it needs to operate.

More great info. I feel silly for not considering that.

I shall measure what the brick supplies and make sure to set my power supply current limit threshold to not surpass that (I've had this fancy PSU for years and I got it for nothing - first time I shall use it for more than testing fans).

Hook the leads of the multimeter "backwards", positive to negative and negative to positive. If you do it with a DC source like a battery, a cheap multimeter that is not "true RMS" for AC voltages like yours will read 0. A "true RMS" multimeter would* read 0 ACV for a DC source regardless of polarity. If the power supply was actually outputing AC, the meter would read the same AC voltage regardless of which way you have the leads attached.

This is because cheaper multimeters are built assuming you are trying to measure a simple AC full sin wave signal. The simplest/cheapest circuit to do that is to put the AC voltage through a half-wave rectifier, this has a diode that only allows positive voltages to pass, measure the resulting DC voltage and mulitply it by a correction factor. The rectifier cuts off the bottom half of the sin wave of an AC signal. Mathmatically, the average value from a truncated signal like that is Vavg = Vpeak / PI. But meters display AC Volts as the RMS voltage, not peak or peak-to-peak. RMS voltage is Vrms = Vpeak / sqrt(2). So if you combine those you get Vrms = Vavg * PI / sqrt(2), or Vrms = Vavg * 2.2, the correction factor Feral mentioned.

But if you apply a DC voltage to that rectifier in the meter, the diode just lets it pass straight through, and in AC mode, it believes that voltage is the Vavg of an AC signal. So Vavg = Vdc. Therefore, if you apply a DC voltage, the equation above becomes Vrms = Vdc * 2.2. Which is why you measure roughly double the AC voltage from a DC signal on a cheap multimeter.

But because the diode cuts off negative voltages, a negative DC voltage from flipping the leads around, results in Vdc = 0 and so the meter reads Vrms = 0.

*You can have signals with both an AD and DC component. For example a 40V peak-to-peak AC sin wave with a +10V DC offset would be a sin wave from -10V bottom peak to +30V top peak. Much nicer to identify with an oscilloscope though.

SiliconStew on
Just remember that half the people you meet are below average intelligence.
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Registered User regular
edited January 2022
Hook the leads of the multimeter "backwards", positive to negative and negative to positive. If you do it with a DC source like a battery, a cheap multimeter that is not "true RMS" for AC voltages like yours will read 0. A "true RMS" multimeter would* read 0 ACV for a DC source regardless of polarity. If the power supply was actually outputing AC, the meter would read the same AC voltage regardless of which way you have the leads attached.
<SNIP>

I am learning a lot here. Thank you.

I did the "backwards" thing and did get the 0 A. Nice to be certain of what I have.

Also I did measure the current the brick supplies to the turntable. It came to 0.06 A at speed and up to 0.2 A for a short second while accelerating up to speed. Now these figures seemed sort of low to me, but doing some searches on motors for turntables to see what they tend to be put me at ease.

BlindZenDriver on
Bones heal, glory is forever.
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Registered User regular
edited February 2022
Just a update on what came of me using what I learned here.

I have now replaced the mysterious black power brick and is instead utilizing my lab style PSU. Doing so hasn't completely fixed my turntable issue, but it seems to be more stable speed wise only it will not hit the 33.1/3 at every start (I am using a tachometer to check). Likely I will need to do more, but for now I will just be seeing how it goes.

Once again - thank you @Feral and @SiliconStew and really just the PA community at large.

BlindZenDriver on
Bones heal, glory is forever.