Powering LEDs: Array of 168 LEDs, 120V AC, 25mA, 2.8V Forward Bias

In summary, you would need 4 resistors each in series with 42 LEDs. These resistors should be about 1.8 K and about 5 watts rating, to be safe.
  • #1
jgordo32
15
0
Hey, so I have a large array of LEDs. I think I have 168 of them.

I want to power them from my wall in the US (120V AC). The forward bias for each LED is 2.8V. The current rating is 25mA.

I plan to arrange them in chains of about 42 to get 117.6V drop over all the LEDs. That leaves me with 2.4V to drop over a resistor. So to get 25mA I would need a 96 ohm resistor in each chain (2.4V / 96Ohms = 25mA). The diodes should act as rectifiers by themself.

First question, does all of that make sense or am I missing anything?
Second question, will there be any points in my circuit that are super dangerous?

Thanks!
 
Engineering news on Phys.org
  • #2
Aside from the obvious safety implications, here are a couple of things to consider:

The AC voltage may be 120 volts RMS, but that translates into peaks that are over 160volts ( sqrt (2) x 120 ). Also, the AC voltage varies considerably. So, you'd need a lot more "ballast" resistor to regulate your current. Perhaps R = 60 volts / .025Amp, R=2.4k at a couple of watts.

LED's are diodes, but they're not meant to block 160V. If some diodes leak more current than others, than you'll be faced with LEDs being driven into avalanche. I don't know whether they can take that. The safe thing to do would be to add a couple of 1n4006 rectifiers. One across the diode chain to protect it from reverse leakage currents and another in series with the resistor to act as the rectifier.
 
  • #3
Thanks for the response mike!

First point: I see what you mean about 160V being too much. But I'm questioning your solution (no offense, haha). If I put in a bigger resistor, sure it will handle the peak better. But what about the valley? Won't it draw the current well below the 25mA rating and make the LEDs super dim, if they turn on at all. I wouldn't normally care since the polarity is switching so fast, but my circuit already has them turned on only half the time. Wouldn't putting a larger resistor on result in them only being on... about 1/4 of the time?

Second point: I was under the impression that since I have 42 LEDs in series, no single diode is responsible for stopping all 160V that need to be rectified at peak. Is that a misunderstanding. Furthermore, I don't understand what you mean by put one across the LED chain AND one in series. So I have a chain of leds with a resistor at the end, I put one in series with that to act as a rectifier, the other one goes across to the other chains or what? Please elaborate more on your second point, I seem to misunderstanding a lot here.

Thank you muchly!
 
  • #4
You could rectify the mains with a bridge rectifier and then filter the DC resulting. Bridge rectifiers are very cheap and you can get them rated at 1000 volts and 30 amps for a small cost.

Assuming a load of 1600 ohms, (160 volts and 100 mA) a simulation indicates that you need a capacitor of about 50 uF to get reasonable filtering without having too much current charging the capacitor. This results in about 15 volts of ripple on the 160 volt supply, so that would probably not be visible.

The extra brightness this should give the LEDs would make up for the extra complexity of the circuit.

You would need 4 resistors each in series with 42 LEDs. These resistors should be about 1.8 K and about 5 watts rating, to be safe.

When designing LED strings, it is a good rule to allow about 25% of the supply voltage across the series resistor. Otherwise, the LED current is too dependent on the exact value of the supply voltage.

However, using high voltages always carries some risk. The 160 volts DC is quite capable of giving a bad shock, so proper construction practice to keep the high voltage covered securely when power is applied is essential.
I remind you of this because other people view this and an extra warning never goes to waste.
All wiring, including the connections to the LEDs, needs to be enclosed in a rugged plastic or metal box. Metal boxes need to be grounded.
 
  • #5
@vk6kro
So, you're proposing that I hook up my AC power supply to a bridge rectifier as show on wikipedia's article http://en.wikipedia.org/wiki/Diode_bridge" [Broken]. Then I connect a 50 uF smoothing capacitor across my two inputs. That will result in a relatively smooth 160V DC input you claim? Can you show me how to do the capacitor calculations?

As for the resistors, I put a 1.8k in each chain of 42 LEDs. Unfortunately, I've noticed that resistors @5W are pretty expensive. So correct me if I'm wrong but if I put 2x900 ohm resistors, that will do the same job of limiting current, but allow me lower power ratings on the resistors yeah? I like your point about the 25% rule, I noticed that high dependence in my calculations earlier, but sort of just accepted it as a fact of life haha.

Lastly, what do you mean by bad shock? I plan to use a standard plug from the wall which has plastic sheathing and solder the leads onto a PCB. Then do the rectifier on the PCB too. The LEDs I plained to just wire the leads to each other and daisy chain them. I will use heatshrink to cover almost all of the open wire so it shouldn't be a problem, but I am an amateur. So I''m worried my heatshrink will not be 100% safe. Is a bad shock like it will hurt a lot, or like it will send me to the hospital? If it's hurt a lot, I think I'll take the risk with my decent heatshrink option. If it's hospital, I might need to look into another option.

Thanks a lot for your reply, wish I had found these forums earlier in my design process =)

Edit: Also, can you link me to some parts? I found a 600V, 25A bridge rectifier on mouser for $2, and 910Ohm resistors @3W (assuming I use 2 of those per chain) at mouser for $0.45 each. Those parts sound good?
http://www.mouser.com/ProductDetail/Fairchild-Semiconductor/DFB2560/?qs=sGAEpiMZZMuL5UzuAmrlvNNa//Gjhnx3" [Broken]
http://www.mouser.com/ProductDetail/Panasonic-Electronic-Components/ERG-3SJ911V/?qs=sGAEpiMZZMvhlCB8CTbT5F%252bwr9A4KnIaWy6ZdnRYoso%3d" [Broken]
 
Last edited by a moderator:
  • #6
So, you're proposing that I hook up my AC power supply to a bridge rectifier as show on wikipedia's article here. Then I connect a 50 uF smoothing capacitor across my two inputs. That will result in a relatively smooth 160V DC input you claim? Can you show me how to do the capacitor calculations?

Yes, that is the circuit.
I used a simulator, but there are simple formulae to do this calculation.
This is one of them:
Capacitance in µF = (Load current * 0.0083 * 10^6) / ripple voltage
C= 0.1 amps * 0.0083 * 10 ^6 / 15 volts = 55 µF

The 0.0083 figure is for 120 Hz, full wave rectified. It is just the period of a 120 Hz sinewave. You could rewite the formula like this:
Capacitance= (Load current * 8300) / ripple voltage

I found a 600V, 25A bridge rectifier on mouser for $2, and 910Ohm resistors @3W (assuming I use 2 of those per chain) at mouser for $0.45 each. Those parts sound good?
Yes those parts sound OK. I was thinking of the ceramic 5 watt types which are very reliable although a bit more expensive.

Lastly, what do you mean by bad shock? I plan to use a standard plug from the wall which has plastic sheathing and solder the leads onto a PCB. Then do the rectifier on the PCB too. The LEDs I plained to just wire the leads to each other and daisy chain them. I will use heatshrink to cover almost all of the open wire so it shouldn't be a problem, but I am an amateur. So I''m worried my heatshrink will not be 100% safe. Is a bad shock like it will hurt a lot, or like it will send me to the hospital? If it's hurt a lot, I think I'll take the risk with my decent heatshrink option. If it's hospital, I might need to look into another option.

If I could achieve one thing today, it would be to convince you that ANY risk of electric shock is totally unacceptable.
160 volts DC from a 50 uF capacitor could kill you. If it killed somone else, you could get a lawsuit that would ruin you for years to come.
You might end up in hospital, but electric shocks can stop your heart working and even if it starts again, parts of it could be dead and you could be crippled for life because of this.

So, I would urge you to think about safety. Heatshrink is not good enough for this voltage.
 
  • #7
If I could achieve one thing today, it would be to convince you that ANY risk of electric shock is totally unacceptable.
160 volts DC from a 50 uF capacitor could kill you. If it killed somone else, you could get a lawsuit that would ruin you for years to come.
You might end up in hospital, but electric shocks can stop your heart working and even if it starts again, parts of it could be dead and you could be crippled for life because of this.

So, I would urge you to think about safety. Heatshrink is not good enough for this voltage.


Greeeaaat. So let's talk remedies. The way I see it... here are my options:
  1. Drop the voltage down to a safer level
  2. Better protection solution

I'd prefer the second option but I'm lost as to how I should pursue this. Let me give a little more detail on my setup. So I'm living in a collegetown apartment where things could get messy. We're not throwing huge parties or anything, but I'm sure there will be a drunken idiot near this contraption at least a few times.

As for details on the contraption, it will be a fold up pong table. So its a 2' x 8' piece of wood with folding legs on the bottom. I drilled 3mm holes in it and planned to just wedge the LEDs in so they're flush with the top of the table. Then I would do all the wiring underneath the table and cover all bare wire with heatshrink. The PCB would be taped to the bottom of the table with the plug running to the wall.

Any ideas on better protection or should I start looking into getting a 24V AC/DC converter?
 
  • #8
Well Id think of better ways than just heatshrink. Possibly bolt up a wooden panel underneath the table to physically block any access to the wiring. Or it might well be easier just to have the converter, you can probably just pick one up for $50 or so.
 
  • #9
25mA x 168 = 4.2 Amps. You could hook them all up in parallel with a 2.8 volt power supply rated at 5 Amps.

168 = 8x3x7=
2x84 = 3x56 = 4x42 =6x28=7x24= 8x21 = 12x14

I would suggest you work in blocks each block of LED's in parallel and the blocks themselves in series.

2 Blocks at 5.6V x 84 LED's at 2.1A ,
3 Blocks at 8.4V x 56 LED's at 1.4A,
4 Blocks at 11.2V x 42 LED's at 1.05A,
6 Blocks at 16.8V x 28 LED's at 0.7A,
etc.

That way, you can use safe voltage and off the shelf power supplies and if one or two LED's fail you won't loose the whole business.
 
  • #10
Thanks jambaugh, that much I could handle on my own =P

Is it reasonable for me to build a converter from 120V AC --> ~9V DC?
 
  • #11
Very low voltages require a lot of extra resistors and more complex wiring.

It is accepted, I think, that voltages below 40 volts are safe.

So, if you had a transformer delivering 28 volts and this was rectified and filtered, you would get a voltage of about 38 volts. (28 times 1.414 minus two diode drops of 1.2 volts)

Take 75% of this to be the voltage across the LEDs so this gives you 10 LEDs per string. So, you could have 17 strings of 10 LEDs

So, you would need 17 * 390 ohm resistors each dissipating 220 mW. Half watt ones would be OK.

The filter capacitor would then have to be about 1000 µF (or more) but it would only need to be rated at 50 volts or so. This is to get 10% ripple.
 
Last edited:
  • #12
@vk6kro
Is there any advantage to using a transformer and leaving it in AC just to rectify as opposed to just using a converter to something like 40V DC?

Also, are the 2 diode drops from the bridge rectifier you want me to put in?
Also also, since I'll have my last string with only 8 LEDs, would the 390 ohm resistor be big enough?

Lastly, what do you guys use to do circuit simulations? Could someone point me to a good beginners tutorial on that?

Thanks so much, this has been a great learning resource already
 
  • #13
I fixed a typo in the previous reply.

Yes, a converter that gave 40 volts out would be OK as long as the output was totally isolated from the primary.

Two diode drops would be 1.2 volts and this is lost in the bridge rectifier because there are two diodes in series with the output at anyone time.

You would have to recalculate the final resistor. I get about 639 ohms. You could put a 560 and an 82 in series, or just try a 680 ohm resistor.

One simulator is the free one from Analog.com. It is called LT Spice 4.
It is very powerful, so it looks scary when you first see it. However the actual controls you use all the time are very simple. It is excellent software so I suggest you get a copy and have a play with it.
 
  • #14
as the output was totally isolated from the primary.

I don't know what that means, could you explain?

Also, could you point me to a good converter? I've never purchased one of those before. Thanks.
 
  • #15
"Fully isolated" means that there is no input wire that goes directly to the output.
Even if this is the neutral wire, this would not be fully isolated.

It is best that you get this near where you live. You should be able to ask if the device has an output that is fully isolated from the mains input.
If they don't know what this means or just say "sure it is", go somewhere else. They should check it for you.
Get an assurance that you can bring it back if you find it isn't fully isolated.
 
  • #16
@vk6kro
Could you explain why not having an input wire connected to the output matters? For example, if I want to take it back for not being fully isolated, how will I know?
Unfortunately, a google search for "fully isolated" did not return any helpful results, so I still don't really know what you're talking about.

Do you think a radio shack would carry them? Maybe microcenter? I'm only just getting into electronics so I don't really know where to buy things besides the online places like mouser, digikey, and jameco.

Thanks!
 
  • #17
jgordo32 said:
@vk6kro
Is there any advantage to using a transformer and leaving it in AC just to rectify as opposed to just using a converter to something like 40V DC?
...

Using AC directly will cause a couple of problems. One is you're only getting half the cycle as light and so effectively you're halving the value of your LED's. What's the price of a brig rec compared to the price of half your LED's? The other is you'll get a 60Hz flicker which can cause eye strain, headaches and in some very rare cases epileptic seizures. http://en.wikipedia.org/wiki/Flicker_(screen)" [Broken]
http://books.google.com/books?id=_5mdSJfur8cC&pg=PA290&lpg=PA290&dq=60hz+flicker+induced+seizures&source=bl&ots=gY3askFcns&sig=8yIG1vnhHprpU2dO44M56LgQYms&hl=en&ei=ZFJlTKqMAcWclgf_z-3VDg&sa=X&oi=book_result&ct=result&resnum=5&ved=0CCkQ6AEwBA#v=onepage&q&f=false" [Broken]
 
Last edited by a moderator:
  • #18
Could you explain why not having an input wire connected to the output matters? For example, if I want to take it back for not being fully isolated, how will I know?
Unfortunately, a google search for "fully isolated" did not return any helpful results, so I still don't really know what you're talking about.


You would measure this with a multimeter on "ohms". If you don't already have one, they are available cheaply at almost any electronics store.
http://www.harborfreight.com/7-function-digital-multimeter-92020.html
Just measure for any connection between the output wires and the input plug.
Often the packaging for these power supplies would give information about whether the secondary is isolated from the primary, or not. This may be in the form of a warning not to touch the output wires.

It matters because if you are not very careful, you can end up with the active side of your mains supply connecting to your LEDs. Radio Shack sell an adapter for running grounded 3 pin plug devices on a 2 pin outlet.

Radio Shack and other stores have their own websites and you can easily check these for availability or prices. I couldn't see anything suitable from Radio Shack, but you might find something.
Frys have a 24 volt 40 watt transformer for $11 which would just need a bridge rectifier and a capacitor. FRYS.com #: 4411656 This plugs into the mains so you don't need to do any mains wiring.
It doesn't say if the secondary is isolated from the primary. You could ask that.
 
  • #19
If you get a DC adapter/power supply that has UL markings along with other safety agencies, it is a pretty safe bet that the output is isolated. This might be one of your safest bets - especially considering this is going on a table (liquid spills, etc.).

If you wanted to cut down on your overall power consumption, you could use a processor or other circuit to activate alternating strings very rapidly - this would make only half active at the same time (depending on how you do it). You had mentioned you planned on running the LEDs at 25mA which is their rated current - is this their maximum current rating? If so, you might want to consider around 10mA or so...
 
  • #20
@mdjensen22
No 25mA is not their maximum. Here's their http://www.hebeiltd.com.cn/led.datasheet/330LB7C.pdf" [Broken]

I'm thinking about doing microcontroller animations later in my project. Although not with the intention of saving power =P However, I just want to get the thing safe and working before I start messing with that.

@vk6kro and others
Ok, I'm convinced... I'm going to buy a DC converter. I think I'll do 28 chains of 6 LEDs. I'll need a 24V DC output. I'll be using 16.8W (.025A X 24V * 28chains = 16.8W).

So am I correct in thinking that all my circuit needs now is the DC adapter (can anybody help me find a good one online so I know what to look for?), my LEDs, and 28 x ~288 ohm 1/2W resistors?

Oh also, do the DC adapters have builtin surge protectors? How good are the surge protectors in the power strips from like target that cost $15? Should I buy a fuse or something to protect against surges?

Thank you!
 
Last edited by a moderator:
  • #21
jgordo32 said:
@mdjensen22
No 25mA is not their maximum. Here's their http://www.hebeiltd.com.cn/led.datasheet/330LB7C.pdf" [Broken]

I'm thinking about doing microcontroller animations later in my project. Although not with the intention of saving power =P However, I just want to get the thing safe and working before I start messing with that.

@vk6kro and others
Ok, I'm convinced... I'm going to buy a DC converter. I think I'll do 28 chains of 6 LEDs. I'll need a 24V DC output. I'll be using 16.8W (.025A X 24V * 28chains = 16.8W).

So am I correct in thinking that all my circuit needs now is the DC adapter (can anybody help me find a good one online so I know what to look for?), my LEDs, and 28 x ~288 ohm 1/2W resistors?

Oh also, do the DC adapters have builtin surge protectors? How good are the surge protectors in the power strips from like target that cost $15? Should I buy a fuse or something to protect against surges?

Thank you!

Without pulsing, 25mA is indeed the absolute maximum for these LEDs. They are only rated for 100mA with a pulse of Duty=0.1mS,1kHz. Which means, if you're going to hold them steady on, 25mA will not fair well long term. You should set the current lower via the resistors.

Also, the Vf for these LEDs is 3.2V at 20mA, not 2.8V.
 
Last edited by a moderator:
  • #22
@mdjensen22
Oh true, That's weird, the website where I bought them listed different specs from the datasheet. I think I'll trust datasheet.

Can anyone link me to a 24V (chains of 6) or 12V (chains of 3) AC to DC converter I could use?

Thanks!
 
  • #23
Here's a 24V 2.2A power supply with a universal input and has UL & CE safety markings for $27. It should be pretty easy to mount as well.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=102-1937-ND
 
  • #24
@mdjensen22
Oh nice, thanks. Is it just me or does that say it weighs 14kg? Are they all going to be that big and heavy?
I was thinking I could get one of those wall converters that you use for like a cell phone or something. Something like http://www.microcenter.com/single_product_results.phtml?product_id=0310998". That would work for 12V I think, but I think 24V would be easier to wire so I'm looking for one of those.

Thank you!

P.S. Thats all I need right? Converter, LEDs, and resistors?
 
Last edited by a moderator:
  • #25
For example, would http://products.cui.com/adtemplate_child.asp?brand=v-infinity&p=85690&c=768138&catky=764537&subcatky1=356029&subcatky2=" [Broken] work? It only has 1A output, think that would be good enough? I theoretically only need .7A
 
Last edited by a moderator:
  • #26
I notice in the LED data sheet, that the voltage at a current of 20 mA is 3.2 volts. (See the 1st graph on P3)

So, if you had 24 volts DC out, and allowed 25% for the resistor, the LEDs could have 18 volts.

Dividing this by 3.2 volts gives 5.625 leds. Hmm.
Allow 19.2 volts for the LEDs. You could have 6 of them per string. You want 168 of them so that is 28 strings of 6 LEDs.

The resistor would have to be (24 volts - 19.2 volts) divided by 0.02 amps or 240 ohms. The nearest readily available resistor would be 220 ohms. You would need 28 resistors, all 220 ohms and each in series with 6 LEDs.

The current would be 28 times 0.02 amps, or 0.56 amps.

Also, you need to get a multimeter if you haven't already got one.
 
  • #27
I did this sort of circuit many years ago (1980s) as I designed LED panels having anywhere from 20 to 2500 LEDs.

It seems that it isn't common knowledge these days that a blinking LED is preferred to a steady lit LED. It not only consumes half or less power (lower heat and resistor concerns), but it actually appears brighter to the eye. So the idea of trying to smooth the 150-160v peak from a bridge rectification is not so valuable. Light flicker at 120Hz (full wave rectification) is undetectable to the mind, but causes more effect on the optic cells of the eye. About 100Hz is ideal.

Another consideration, as you decide how many LEDs to parallel rather than series, is the maintenance and cross weakening effects. A single LED becoming weak can disrupt the entire matrix. I recommend, and always used, lower voltages either by saturated transformers or switching SCRs/Triacs, to power an array of shorter series. If you try to string all of them in series, or as many as possible, you start having dimming concerns that force you to one-by-one isolate which LED is causing the variations (the Christmas tree lights effect).

Also, by using a saturation transformer, you further restrain the power source variations that can easily cause substantial dimming as the power company switches loads and circuits. In most cases, a small voltage regulator is advised, perhaps an array of zeners, so as to give room for source variations without affecting performance.

25ma sounds like a lot to me. I never had a case needing any more than 13ma, but maybe you are using some newer high power devices. You do NOT want to run them at their max rating. An LED will last forever if you ensure that it never exceeds its max. If you allow for only a second for it to get to max current, it will slightly melt and become comparatively dim if not burn out entirely.

Power efficiency should be considered. It is not merely a waste, but an issue of heat production which shortens the life of all components. Arranging the best array matrix reduces the need to use resistive components. Any voltage stepdown will produce heat waste. Switching circuits are easily purchased and are the most power efficient means to reduce voltage. But cheap saturation transformers do the trick although might wear out sooner.

There are many ways to do what you are talking about. I wouldn't worry about trying to produce a steady DC.

Also, realize that typically, LEDs do NOT come matched. Each, although within specs, is still different than the others. Thus don't go assume that every LED will yield the same effects. It won't. If you string 12 together and another 12 together with the exact same voltage applied, you might get substantially different illumination. If you want something that actually appears professional, you need to carefully choose which LEDs you string with which other LEDs. Its a pain, but if you do not current regulate, it is often required.

Current regulation is best handled by purchased IC modules designed, usually with junction diodes, to ensure that you get the proper current regardless of device variations and source variations (within limits of course). LEDs produce illumination proportional to current, not voltage.

In addition, consider what is going to happen when even one LED burns out. Normally an LED will melt into a short circuit before it opens up. If your series string is designed too close to the max rating and the string is short, you will most probably burn and damage the entire series just before it blinks out, having to replace all of the LEDS instead of just the one that first failed. You the "Typical" ratings in your designing, not the max. According to your data sheet, that would be 20ma, not 25ma. But if, for example, you only had 2 LEDs in each string, the second LED that had not failed will suddenly and for a short time, have twice the voltage on it and will melt. Heat is the issue even though you can have as much as 100ma for a very short pulse.

If you design the string using a 20ma expected current and you have 10 LEDs per string, if one shorts out, you will end up with 3.5 volts per LAD in your string. As per your data sheet, that would produce about 50ma through each other LED. That would burn them in pretty short order.

To ensure that you have less than max current even if one shorted out, you need 20 or more in each string. 20 would leave you at 3.38v per LED and about 25ma if one shorted.

You might also want to note that you can buy such strings ready made these days. They make Christmas tree strings with LEDS. You can find them online I'm sure and your cost might be less than trying to build one yourself unless you happen to already have the components.
 
Last edited:
  • #28
Thanks for all of the help guys. I will definitely be returning to these forums many times in the future.

http://www.youtube.com/watch?v=7UkuMnFmx6A" is the result. I'm really pleased with how it turned out, so thanks again everyone!
 
Last edited by a moderator:

1. How do you calculate the total power consumption of this LED array?

To calculate the total power consumption, we can use the formula: Power (W) = Voltage (V) x Current (A). In this case, the voltage is 120V AC and the current is 25mA. Converting milliamps to amps, we get 0.025A. Plugging these values into the formula, we get a total power consumption of 3W.

2. Can this LED array be powered by a battery?

Yes, this LED array can be powered by a battery. However, the battery's voltage and current output must match the specifications of the LED array (120V AC and 25mA). It is important to note that using a battery may not provide the same level of brightness and may drain the battery quickly.

3. Is it safe to touch these LEDs while they are powered?

No, it is not safe to touch these LEDs while they are powered. The voltage and current running through the LEDs can cause electric shock. It is important to follow proper safety precautions when handling powered LEDs.

4. Can the voltage and current of this LED array be adjusted?

The voltage and current of this LED array are fixed at 120V AC and 25mA. However, external components such as resistors can be added to the circuit to adjust the voltage and current to meet the specifications of the LEDs. It is important to use the correct components and consult a professional when making adjustments to the circuit.

5. How long will this LED array last?

The lifespan of an LED array depends on several factors such as the quality of the LEDs and external factors like temperature and humidity. On average, LEDs have a lifespan of 50,000 hours. With proper usage and maintenance, this LED array can last for several years.

Similar threads

Replies
19
Views
2K
  • Electrical Engineering
Replies
10
Views
1K
  • Electrical Engineering
Replies
10
Views
2K
  • Electrical Engineering
Replies
26
Views
3K
  • Electrical Engineering
Replies
3
Views
2K
  • Electrical Engineering
Replies
9
Views
9K
  • Electrical Engineering
Replies
4
Views
2K
Replies
12
Views
1K
  • Electrical Engineering
Replies
13
Views
3K
  • Electrical Engineering
Replies
18
Views
2K
Back
Top