jadel
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Connecting electromagnet to pi

Tue Feb 26, 2019 2:50 pm

I want to control an electromagnet that I built with my rpi.
I built the magnet myself and want to run 12V into it, it has 2 ohm resistance so the current will be 6 A.
I have a 12V relay board that I connected to the pi. I also have a 10A, 45V diode that I will be using for flyback.
My main issue is that I'm not sure how to set up the circuit, which way does the diode polarity go and which terminals should I connect to the relay?
Thanks in advance

pcmanbob
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Re: Connecting electromagnet to pi

Tue Feb 26, 2019 3:34 pm

You would wire it like this

Image

relay shown in the off state.
We want information… information… information........................no information no help
The use of crystal balls & mind reading are not supported

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mahjongg
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Re: Connecting electromagnet to pi

Tue Feb 26, 2019 5:25 pm

Hmm, considering that I think you actually want to control the solenoid with a PI, the above picture doesn't help much, except showing the correct orientation of the diode. Also, normally you would switch the solenoid on the GND side, not the 24V side (high side switching).
Unlike your assumption the diode doesn't have to be a large current type, the full solenoid current doesn't run through it. Normally an 1N4007 diode is enough, for small relays (500mA coil current) even an 1N4148 will do.

Perhaps this diagram helps:
Image

Note, to keep the dissipation on the FET low, you need the FET to either be completely closed (no voltage between gate and source) or open (a large voltage between gate and source), that is why two transistors are used to drive the gate voltage, also with this circuit the FET is off when you turn off the RPI.

Here is more explanation: https://raspberrypi.stackexchange.com/q ... t-gets-hot

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Burngate
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Re: Connecting electromagnet to pi

Tue Feb 26, 2019 6:41 pm

mahjongg wrote:
Tue Feb 26, 2019 5:25 pm
... the diode doesn't have to be a large current type, the full solenoid current doesn't run through it...
Surely it does, at least momentarily when the FET is switched off - dI/dt isn't infinite, and the only path it has is through the diode.

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Re: Connecting electromagnet to pi

Wed Feb 27, 2019 2:30 am

jadel wrote:
Tue Feb 26, 2019 2:50 pm
I built the magnet myself ... to run 12V, 2Ω, 6A.

DC12V 0.25A 3W Electromagnet Can Lift/Hold 2.5kg

Let me see. If 12V, 0.25A, 3W can lift 2.5kg, and power is proportional to square of current, your 6A should be able to lift (6 * 6) / (0.25 * 0.25) * 2.5kg = 1,440kg! Are you sure you want to lift that heavy, or my calculation is dodgy? :mrgreen:

AliExpress Electro-Magnet ZYE1-P20/15 DC12V 3W 0.25A Hold/Lift 2.5Kg US$5
https://fr.aliexpress.com/item/ZYE1-P20 ... 80522.html
...
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Re: Connecting electromagnet to pi

Wed Feb 27, 2019 3:48 am

jadel wrote:
Tue Feb 26, 2019 2:50 pm
1. I want to control with Rpi electromagnet 12V, 2Ω, 6 A.
2. I have a 12V relay board that I connected to the pi.
3. I'm not sure how to set up the circuit.

Cheap 5V Relay can control DC 6/12/24/36V 8A, AC 6/12/24/110/220, 8A

2. You don't need a 12V relay to switch a 12V load. You can use a cheapy 5V relay to switch DC6V to 36V, AC 6V to 220V up to 6A ("quite" safely) or 10A (might explode! :mrgreen: )

3. For newbies, I usually recommend a cheapy 5V relay with High level trigger for the following reason: Many of those Arduino Low level trigger relays won't work with Rpi, for the following reason: Those relays are designed to switch when Low, < 1V, and switch off when High, > 4V. Arduino's High is about 4.2V and can switch off without problem. However the "weaker" Rp's High is only about 3V, not strong enough to switch off the relay. In other words, those miserable Rpi newbies find their Low trigger relay always stay on, never off, thus go weeping in the dark, and me Arduino guys LOL ... :mrgreen:

I have been playing different relays in the past 6 months and I am more or less happy so far. One bad thing is that those relays cannot switch above 10A. For high currents, I am trying power MOSFETs which can easily go over 50A. Also relays are bulky to handle, ... You might like to refer to my power MOSFET switch schematic below. You can just replace the MOSFET by a relay, and everything "should" work.

WARNING - I don't guarantee that my suggested circuit won't explode - I have already fried one Rpi, two power supplies, and two MOSFETs! :mrgreen:

I am a relay/MOSFET newbie with only 6 months experience, and I always remind myself Murphy's Law:

"Any circuit that can explode, will explode."

...
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Re: Connecting electromagnet to pi

Wed Feb 27, 2019 3:48 am

pcmanbob wrote:
Tue Feb 26, 2019 3:34 pm
You would wire it like this

Image

relay shown in the off state.
There is no Raspberry Pi in your schematic though.
55:55:44:44:4C
52:4C:52:42:41

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Re: Connecting electromagnet to pi

Wed Feb 27, 2019 4:37 am

jadel wrote:
Tue Feb 26, 2019 2:50 pm
I also have a 10A, 45V diode that I will be using for flyback.

Flyback Diode Selection

Your diode spec of 10A 45V sounds like a power rectifier which usually switches at low 50/60Hz mains frequency. For flying back you must use a high speed switching guy. See my references below:

Flyback Diode Selection Notes
https://www.raspberrypi.org/forums/view ... y#p1417554
https://www.raspberrypi.org/forums/view ... y#p1417588
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Re: Connecting electromagnet to pi

Wed Feb 27, 2019 4:40 am

tlfong01 wrote:
Wed Feb 27, 2019 3:48 am
You might like to refer to my power MOSFET switch schematic below. You can just replace the MOSFET by a relay, and everything "should" work.

Duplicated Post - My Apologies

Should have posted the one following this one.
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Re: Connecting electromagnet to pi

Wed Feb 27, 2019 4:48 am

tlfong01 wrote:
Wed Feb 27, 2019 4:40 am
You might like to refer to my power MOSFET switch schematic below. You can just replace the MOSFET by a relay, and everything "should" work.

Controlling Solenoid By High Level Trigger 5V Relay KY019

Or check out my old post below:

Relay KY019 Switching Solenoid
https://www.raspberrypi.org/forums/view ... y#p1417740
...
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Re: Connecting electromagnet to pi

Wed Feb 27, 2019 9:08 am

tlfong01 wrote:
Wed Feb 27, 2019 2:30 am
jadel wrote:
Tue Feb 26, 2019 2:50 pm
I built the magnet myself ... to run 12V, 2Ω, 6A.
DC12V 0.25A 3W Electromagnet Can Lift/Hold 2.5kg

Let me see. If 12V, 0.25A, 3W can lift 2.5kg, and power is proportional to square of current, your 6A should be able to lift (6 * 6) / (0.25 * 0.25) * 2.5kg = 1,440kg! Are you sure you want to lift that heavy, or my calculation is dodgy?
Yes, your calculation is dodgy.
What it "can lift" is nothing to do with power.

The power used to maintain the current will depend on what the wire is made of - copper, probably in this case, hence the 2Ω quoted, and hence 72W for 6A.
But if he managed to get some superconducting wire, once the field is established no more energy would be required.

The magnetic field is proportional to the current, and the force is proportional to the field, so the force is proportional to current - not its square.

The force will also depend on the material it's pulling on, as well as the material of the core of the coil, and the size and shape and number of turns of the coil. Not easy to calculate.

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Re: Connecting electromagnet to pi

Wed Feb 27, 2019 9:39 am

Burngate wrote:
Tue Feb 26, 2019 6:41 pm
mahjongg wrote:
Tue Feb 26, 2019 5:25 pm
... the diode doesn't have to be a large current type, the full solenoid current doesn't run through it...
Surely it does, at least momentarily when the FET is switched off - dI/dt isn't infinite, and the only path it has is through the diode.
That might be true, but the pulse duration is so short that such a large current diode is unnecessary in practice.

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Re: Connecting electromagnet to pi

Wed Feb 27, 2019 10:13 am

How short is "short"?
We don't know the inductance of the coil, so we don't know the time constant L/R

What is the main cause of the death of a diode?
Assuming it's the maximum temperature that kills it, we would need to know its thermal capacity as well as how fast it can lose heat to the environment.

Using a diode that can carry the full current of the coil seems to be preferable to using a small one and hoping.

jadel
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Re: Connecting electromagnet to pi

Wed Feb 27, 2019 11:08 am

mahjongg wrote:
Tue Feb 26, 2019 5:25 pm

Note, to keep the dissipation on the FET low, you need the FET to either be completely closed (no voltage between gate and source) or open (a large voltage between gate and source), that is why two transistors are used to drive the gate voltage, also with this circuit the FET is off when you turn off the RPI.
Thanks for the information, but from what I gather the FET is a way to control the current flowing through a device. Correct me if I'm wrong but can't a relay switch do the same thing with less complicated components? So what would be the advantage of using the FET system?

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Re: Connecting electromagnet to pi

Wed Feb 27, 2019 1:06 pm

Burngate wrote:
Wed Feb 27, 2019 10:13 am
Using a diode that can carry the full current of the coil seems to be preferable to using a small one and hoping.
Using a diode that can switch fast enough to take the spike would be preferable to frying everything.

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Re: Connecting electromagnet to pi

Wed Feb 27, 2019 1:23 pm

rpdom wrote:
Wed Feb 27, 2019 1:06 pm
Burngate wrote:
Wed Feb 27, 2019 10:13 am
Using a diode that can carry the full current of the coil seems to be preferable to using a small one and hoping.
Using a diode that can switch fast enough to take the spike would be preferable to frying everything.
So does that mean the ones I have wouldn't work, or would they be safe enough to keep?

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tlfong01
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Re: Connecting electromagnet to pi

Wed Feb 27, 2019 1:44 pm

Burngate wrote:
Wed Feb 27, 2019 9:08 am
tlfong01 wrote:
Wed Feb 27, 2019 2:30 am
DC12V 0.25A 3W Electromagnet Can Lift/Hold 2.5kg
If 12V, 0.25A, 3W can lift 2.5kg, and power is proportional to square of current, your 6A should be able to lift (6 * 6) / (0.25 * 0.25) * 2.5kg = 1,440kg!
Is my calculation is dodgy?
1. Yes, your calculation is dodgy.
2. What it "can lift" is nothing to do with power.
3. The power used to maintain the current will depend on what the wire is made of - copper, probably in this case, hence the 2Ω quoted, and hence 72W for 6A.
4. But if he managed to get some superconducting wire, once the field is established no more energy would be required.
5. The magnetic field is proportional to the current, and the force is proportional to the field, so the force is proportional to current - not its square.
6. The force will also depend on the material it's pulling on, as well as the material of the core of the coil, and the size and shape and number of turns of the coil.
7. Not easy to calculate.

If an electromagnet of 12V, 0.25A, 3W can lift 2.5kg, What can 12V, 2A lift?

When I read you comments the first time, I was a bit discouraged because I could not catch what you said. So I thought perhaps I mixed up electrical power with mechanical power, which is force * distance (not too sure :mrgreen: ). So I thought I needed to first google basic physics readings and then come back to read your comments the second time.

But I was too lazy to google a basic physics course on mechanical power etc. So I read your post a second time, slowly. Then I found you points though long winded, but actually structured and concise.

So I am reading it a third time, to clarify my mind. :mrgreen:

Your points can be grouped into three parts.

Part 1 (Points 1 to 4) explains that the [electrical] power is used to maintain the current, not [mechanical force or power] to hold or lift something. At first reading I thought the superconductor thing is not relevant, but then I found it very good to confirm or reinforce if I really understand the idea that mechanical lifting capability/power has nothing to do with this electrical power.

Part 2 (Points 5, 6) says lifting power is proportional to magnetic field which in turn is proportional to current. Therefor power is proportional to current, NOT square.

Part 3 (Point 7) The lifting force is not easy to calculate. This is the part I found interesting. I am thinking of googling more readings and try to begin with calculating the inductance of solenoid or electromagnet which is useful for selecting a flyback diode.

PS - I found your post easy to follow, because you speaks middle school physics English. Too many guys here speaks hard to follow geek English! :mrgreen:

Inductance References

Inductance of a Coil - Electronics Tutorials
https://www.electronics-tutorials.ws/in ... tance.html

How to Measure Inductance - WikiHow
https://www.wikihow.com/Measure-Inductance

How to measure inductance - Rose-Hulman Online 2016mar23
https://www.youtube.com/watch?v=ajycdvKyCEw

ICL8038 Mid Low Frequency (450kHz) Sinusoidal and Triangular Signal Generator - ¥16
https://item.taobao.com/item.htm?spm=a1 ... 8114234655
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Re: Connecting electromagnet to pi

Wed Feb 27, 2019 2:11 pm

tlfong01 wrote:
Wed Feb 27, 2019 1:44 pm
ICL8038 Mid Low Frequency (450kHz) Sinusoidal and Triangular Signal Generator - ¥16
https://item.taobao.com/item.htm?spm=a1 ... 8114234655

Sine/Triangle/Rectangular Wave Sig Gen

Now I am trying to use the cheapy US$3 sig gen to find the inductance of solenoids, solenoid valves, and electromagnets.

ICL8038 Signal Generator Medium/Low Frequency 10hz-450khz Triangular/Rectangular/Sine Wave Generator Module 12v to 15v - US$2.59
https://fr.aliexpress.com/item/ICL8038- ... 0f8e7e4-4

The LC ICL8038 module is designed for common frequency ranges. It is adjustable from 5 gears and generates low-distortion triangular waves, square and sinusoidal waves from 10 hz to 450 khz. Sine wave can be used for test signals, control signals, carrier frequency signals, etc; square wave can be used for switching signals, trigger signals, etc .; Triangular wave is mainly used for digitizing signals.

Features:
Operating voltage: 12v ~ 15v
Output: Triangular wave, Square wave and sine wave.
Frequency range: 10 Hz ~ 450 kHz
Low sine wave distortion: 1%
Duty range cycle: 2% 98%
Low temperature drift: 50ppm / °
triangular wave output linearity: 0.1%
Operating Temperature: 0 ~ 70 °
Size: 54.5 * 37.8mm

Functions:
VCC, GND:
AC operating voltage : with bias DC output:
DC field : pure output signal
R11:
R12 frequency setting : Square cyclic wave adjustment
R13: sine wave linear adjustment
R14: amplitude adjustment
R15 : Square wave Linear adjustment
Jumper P2: select different waveforms (triangular, square or sine waves)
Jumper P3: select different frequency ranges
10 hz-450 hz
90 hz-1.5 khz
940 hz-15 khz
6 khz-120 khz
20 khz-450 khz

...
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Re: Connecting electromagnet to pi

Wed Feb 27, 2019 6:22 pm

tlfong01 wrote:
Wed Feb 27, 2019 1:44 pm
... Then I found you points though long winded,
Sorry!
but actually structured
I try hard - generally three or four tries before I'm happy
and concise.
Isn't that the opposite of concise? But I won't argue!
PS - I found your post easy to follow, because you speaks middle school physics English. Too many guys here speaks hard to follow geek English!
So much for my BSc (failed) education!

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Re: Connecting electromagnet to pi

Wed Feb 27, 2019 7:12 pm

rpdom wrote:
Wed Feb 27, 2019 1:06 pm
Burngate wrote:
Wed Feb 27, 2019 10:13 am
Using a diode that can carry the full current of the coil seems to be preferable to using a small one and hoping.
Using a diode that can switch fast enough to take the spike would be preferable to frying everything.
Fast enough? We're back to the time constant(s) of the coil - L/R - and everything else.

We don't know what jadel's "10A, 45V diode" is, but just out of interest, I looked at mahjongg's suggestion of 1N4007

According to the Diodes Incorporated datasheet, the junction capacitance is about 8 pF, but that'll be swamped by the self-capacitance of the coil windings and the rest of the circuit.
If the RC time constant were about 16 ps, to limit the spike to a reasonable size, you'd need something capable of switching at GHz speeds - not normally available except at low current.

And from figure 2, at 25C junction temperature, with a pulse width of 300 μs and 2% duty cycle, its instantaneous forward voltage carrying 6A will be about 0.9v

I don't think anything will fry

jadel
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Re: Connecting electromagnet to pi

Wed Feb 27, 2019 7:32 pm

Burngate wrote:
Wed Feb 27, 2019 7:12 pm
rpdom wrote:
Wed Feb 27, 2019 1:06 pm
Burngate wrote:
Wed Feb 27, 2019 10:13 am
Using a diode that can carry the full current of the coil seems to be preferable to using a small one and hoping.
Using a diode that can switch fast enough to take the spike would be preferable to frying everything.
We don't know what jadel's "10A, 45V diode" is,
It is a 10SQ045, here's the data sheet: http://www.senocn.com/data/PDF/10SQ030-10SQ100.pdf
Would it still work?

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tlfong01
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Re: Connecting electromagnet to pi

Thu Feb 28, 2019 2:08 am

jadel wrote:
Wed Feb 27, 2019 7:32 pm
Burngate wrote:
Wed Feb 27, 2019 7:12 pm
We don't know what jadel's "10A, 45V diode" is,
It is a 10SQ045: http://www.senocn.com/data/PDF/10SQ030-10SQ100.pdf
Would it still work?

10SQ045 vs 1N5822

Oh my goodness, your Schottky guy is 3 times stronger than mine. :mrgreen:

10SQ045 vs 1N5822
https://penzu.com/p/377d4751

TaoBao 10SQ045 - ¥3.5 for 10, ¥2.8 for 100
https://item.taobao.com/item.htm?spm=a2 ... t=6#detail
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Re: Connecting electromagnet to pi

Thu Feb 28, 2019 7:18 am

Burngate wrote:
Wed Feb 27, 2019 10:13 am
1. How short is "short"?
2. We don't know the inductance of the coil, so we don't know the time constant L/R
3. What is the main cause of the death of a diode?
Assuming it's the maximum temperature that kills it, we would need to know its thermal capacity as well as how fast it can lose heat to the environment.
4. Using a diode that can carry the full current of the coil seems to be preferable to using a small one and hoping.

Finding Inductance of OP's Electromagnet

2. The problem is that we don't know the inductance of OP's electromagnet. Since OP is DIYing the electromagnet, he might let us know the core material, number of turns etc, then we can calculate the inductance.

Once we have got the inductance, then we can calculate the energy stored in the coil, and select a flyback diode with the appropriate current limit, breakdown voltage, and thermal capacity etc, ...

Another lazy hobbyist's quick and dirty, worry no more solution is to use OP's suggested diode 10SQ030, which is the strongest I know (275A surge current! :mrgreen: ) and cheap (4 yuan only 3.5 yuan for 10, 2.8 yuan for 100!).
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Re: Connecting electromagnet to pi

Thu Feb 28, 2019 8:37 am

tlfong01 wrote:
Thu Feb 28, 2019 7:18 am
The problem is that we don't know the inductance of OP's electromagnet.
Once we have got the inductance, then we can calculate the energy stored in the coil, and select a flyback diode with the appropriate current limit, breakdown voltage, and thermal capacity etc, ...

Learning how to find the inductance of a electromagnetic thing

So first thing first is to learn how to find the inductance. I searched my junk box and found a couple of electromagnet things, including an electromagnet, a inductor coil, buzzers. Now I am ready to start, ... :mrgreen:

Inductance References

e Euler Identity - BBC In Our Time
https://www.bbc.co.uk/programmes/b04hz49f

j Imaginary Numbers - BBC In Our Time
https://www.bbc.co.uk/programmes/b00tt6b2

Complex Numbers and Phasors - Electronics Tutorials
https://www.electronics-tutorials.ws/ac ... mbers.html

Inductance of a Coil - Electronics Tutorials
https://www.electronics-tutorials.ws/in ... tance.html

Intro to AC Circuits using Phasors and RMS Voltage and Current - Doc Physics 472,080 views
https://www.youtube.com/watch?v=FrRmihdF52o
(Phasor/vector 4:03, rms 8:21)

How to Measure Inductance - WikiHow
https://www.wikihow.com/Measure-Inductance

ES203 Lec 6-2: Inductors and Capacitors - Rose-Hulman Online 1,079 views
https://www.youtube.com/watch?v=BHkMLGihRdY

ES203 Lec 7-2: Sinusoids, RMS, Phasors - Rose-Hulman Online 1,673 views
https://www.youtube.com/watch?v=9O9_I1LD7Rk
(phasor starts 14:04, exponential/rectangular (Cartesian)/polar (angle) form/notation 16:43)

ES203 Lec 7-3: Impedances, KVL and KCL - Rose-Hulman Online 1,977 views
https://www.youtube.com/watch?v=rcv7H8TXuXI

How to measure inductance - Rose-Hulman Online 8,195 views
https://www.youtube.com/watch?v=ajycdvKyCEw

Complex number calculator with steps
https://www.hackmath.net/en/calculator/complex-number

ICL8038 Mid Low Frequency (450kHz) Sinusoidal and Triangular Signal Generator - ¥16
https://item.taobao.com/item.htm?spm=a1 ... 8114234655

About Rose-Hulman? - Rose-Hulman Institute of Technology
https://www.rose-hulman.edu/academics/l ... index.html

This is Rose-Hulman - 8,027 views
https://www.youtube.com/watch?v=-EbZ-9g ... 5jtVF2WU2A (youtube)
https://www.youtube.com/watch?v=MlHN35f ... A&index=83 (youtube)

Rose-Hulman Institute of Technology - Wikipedia
Rose-Hulman Institute of Technology (abbreviated RHIT), formerly Rose Polytechnic Institute, is a small private college specializing in teaching engineering, mathematics and science in Terre Haute, Indiana.

Academics
The curricula at RHIT concentrate on engineering and the natural sciences. The school's primary focus is undergraduate education, though there is a small graduate program for master's degree students.

Rankings and reputation
As of 2018, the institute has been ranked #1 among engineering colleges that do not offer a doctorate degree by U.S. News & World Report for 20 consecutive years.
...
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Burngate
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Re: Connecting electromagnet to pi

Thu Feb 28, 2019 11:13 am

jadel wrote:
Wed Feb 27, 2019 7:32 pm
It is a 10SQ045, here's the data sheet: http://www.senocn.com/data/PDF/10SQ030-10SQ100.pdf
Looks good to me.
However, many people often disagree with me, in particular mahjongg and rpdom.
So watch this space.

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