Re: International standards

Hi,

I'm using an LM358 op-amp to amplify a signal. The signal is AC and has a peak-to-peak value of 5mV. The op-amp is setup to to amplify the signal by

Thanks for any help.

has a

signal by

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the 2K

double, but

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the

Look at the 'open loop frequency response' of the datasheet given here:

You note that the open loop gain at 35kHz is less than 40dB, which means its less than 100. Op-amps are internally compensated to do this, so they don't oscillate. Thats the most gain you can get at a particular frequency with this opamp.

Using a negative feedback scheme like you are using, the open-loop gain should be much higher than the final gain, or there will be distortion. Thus, if the open loop gain is 100, your target gain should be more like 10.

Regards, Bob Monsen

Hi, Mike. An op amp is a gain block whose total open loop gain is dependent on several factors, including the built-in compensation capacitor on the op amp die. Because of that, if you set up a circuit like this with an LM358 (view in fixed font or M$ notepad):

Open Loop Gain Input Signal |\| o---------|+\ Output Signal | >-------o .---|-/ | |/| | === GND

created by Andy´s ASCII-Circuit v1.24.140803 Beta

and you apply a small amplitude signal at 1 MHz, the output will have a gain of about 1. If you apply a small signal at 100 KHz, the gain will be about 10. If you apply a small signal at 10 KHz, the gain will be about 100. This product of gain and bandwidth for an open-loop op amp will be the same for any given frequency, and is called the gain-bandwidth product. For an LM358, the gain-bandwidth product is 1 MHz, which means you can't expect more than a gain of 30 at 35 KHz, no matter what feedback resistors you use. As a practical matter, the theory of negative feedback you're using to set gain requires excess gain (minimum of 20 dB or at least 10 times), so you actually can't set it up for 35 KHz with more than a gain of 3 and expect to get any fidelity in amplification (your distortion will be too great). So, even using both op amps of the LM358, you can't really expect to get more than a gain of 10 at 35 KHz (gain of 3.2 * gain of 3.2). So your voltage gain of 10 means you've actually got an overachiever for an op amp. GBW product is specified min./typ., not max.

If you can find a faster dual op amp (GBW >= 10MHz), you can set it up so each op amp has a gain of 22, which will give you a total gain of 500. At that gain, you'll also be concerned about noise amplification from different sources (such as your resistors and the op amp itself), so choose a low noise op amp and try to stay away from those megohm resistors. Also try to do a good job on power supply bypassing -- with a gain of 500, power supply noise will become a factor. Try something like this:

Good luck Chris

Hello sci.electronics.basics

I hope this post is not off topic. If so my appologies. Also since I dont really know what I am doing I may not express myself that clearly. Please bear with me.

I bought a bunch of PC parts the other day and build a little PC on our guest bed. Now I want to clean it up by putting it in a box or at least mounting it cleanly. I have a hard drive, a tiny little ITX motherboard and a power supply.

I have built PCs before in standard cases, but this time I want to make the housing. My idea is to use some kind of sheet metal (a flat piece of aluminium?) and drill holes in it to mount the parts. Where I need vertical stuff I was thinking that perhaps I could find some little angle brackets and use them to mount strips of metal going vertically. If that works out I might make a persphex cover for the whole thing but that is for part 2 of the project.

After some thought I decided that must be the kind of problem that you electronics project people do all the time.

Could somebody recommend a resource on how to do this? I went to Fry's, Radio Shack and Home Depot (I live in Orange County, CA) but couldnt find the kinds of things I am guessing I need. What is the kind of store that sells things to do this called (a googleable term would be nice).

I dont have a lot of tools or stuff, but am willing to spend some money to be successful in this project. I dont want to go out and buy some premade solution. I want to build it. Also if you want to make some suggestions please make it pretty simple I am starting from scratch here. For example: dont say "cut some holes in the back panel for the ports", say "use an sonic screw driver to cut some holes in the back panel for the ports".

Any advice much appreciated.

Cheers,

David

by

but

wrong?

Come what may, should be getting more than X10 out. Are you sure you haven't got a massive DC offset at the op-amp output?.

opto-isolator

the

Third way. Run the current thru a core of an inductor, which has the winding for an oscillator. The DC varies the coil's inductance which changes the freq of oscillation. Isolation is inherent as in the above.

Hello All, I have an old Vinyl cutter that can cut rubber using a 24v cont duty cycle solenoid. My question is: Can the solenoid degrade after 10 years and thousands of cycles to not generate the downforce needed to cut the thicker material? I have verified the correct voltage to the solenoid when activated and isolated the solenoid by using a seperate 24V pwr supply. Either way it does not quite have the downforce to get thru the material. So that led me to looking at the linkage from the plunger to the blade holder. I lubricated and have tried many adjustments in the linkage. The solenoid is about $65 when ordered and don't know if buying the replacement will do the trick. I was really wondering if the solenoid can still activate but lose some of the downforce over the years..... Any help would be great... Thanks in advance Greg

If this were a perfect world and the 3 LEDs were exactly identical, then this would work, but LEDs are never exactly alike and one of them would hog the current, leaving it too brite and the others too dim.

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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I have been playing with an old AT computer power supply and found that for the PSU to stay ON, a minimum load of about 1 amp has to be maintained on the 5 Volt supply.

No minimum load is required for the 12 Volt supply or the negative supplies.

Is this typical for this type of supply or do some require a minimum

Thanks.

Rob.

Model Railroad & Misc. Electronics

Allegro Microsystems makes a 100A hall effect sensor ACS752xCA-100. The two heavy leads connect inline with your motor current. You apply 5 volts control supply. The zero-current signal is 2.5 volts. The signal voltage increases for positive current and decreases for negative current. You can send that to an ADC. You can get a sample of the hall effect sensor from Allegro..

TP

"Daniel Rudy"

assuming this is a power supply and not a 24V battery, and assuming it is unregulated: why not just put a standard toroidal pickup on the AC input to the transformer? the resulting output it then rectified, filtered, and calibrated. any quiescent current will be so small in relation to the 100A load as to be insignificant.

of course it might be a 3 phase input which would add to the complexity.

It ought to be embarrassing for a poster to ask a serious question, have it seriously answered, and then reply with an automatic tag file that's sarcastic and demeaning.

But I suppose Mr. Rudy is either too dumb to recognize his automated insult, or too sophomoric to care.

Ed wb6wsn

Well the components are lying there loose now. But now that it works nicely, my plan is to secure them to the flat plate and later build a cover for it. I noticed at Fry's they have metal and plastic project cases. None of them fit my needs. But I'd love to see one with somebody's project already affixed within it.

Sorry, the sonic screwdriver bit is a Dr. Who reference. Please disregard.

Sorry, I didnt mean to infer that I have never had a tool in my hand. And I did try to be as specific about what I planned to do. I just thought there might a standard way to do it that would make it all go a bit smoother.

I want to mount the motherboard to a metal plate using some metal spacers using holes drilled in the plate to secure the spacers. I will do the same with the power supply, but without the spacers. I plan to find a way to put space between the plate and the hard drives too so they have some airflow around them. If I can find a little angle bracket then I can probably cut some strips off the base with a hacksaw and make a couple of little risers that I can screw the hard drive to. Does that sound like a worthwhile direction to try out?

Cheers,

David

I think there are plastic versions on the spacer I was planning to use to mount the motherboard to the metal plate I will use for my base.

I took an old stereo appart today. Thought I might even be able to use that casing. It did have metal boards much like a I am planning on using to mount my motherboard, but they used them mostly to mount the transformer and mechanical parts of the stereo. Also the boards we pressed into funny shapes that pretty much ruled out my using them to mount my computer parts. I noted most of the circuit boards are mounted quite differently to a normal PC motherboard. As you suggest they did not have a lot of metal contact.

Cheers, David

These are available as a commercial product from a Swiss company called LEM. They make a good product line.

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Sounds like you'd find all kinds of stuff for this at your local hardware store. Since you say your Home Depot doesn't have anything appealing, I'm not sure what to suggest. An electrical supply store might have unusual cases and such...

One funky option might be to use plastic zip-ties to hang the motherboard in the case.

If you want a retro look, find an old wooden console record player, strip out the guts and mount your stuff in it. You can put the monitor and keyboard on top, and mount PC speakers where the old speakers were. ;)

My one recommendation is that you mount the motherboard with something non-metallic; if a rough-edged metal mounting bracket rubs the coating off over time, and touches two traces, say goodbye to your mobo.

To use a single resistor with several LEDS, put the LEDs in series. However, unless the LEDs are the same color and from the same batch, the brightness will vary. Also, the supply voltage has to be high enough to actually light the series string.

ISTM that a solenoid (yoke or plunger - I think the plunger would foul thinks up worse) could become magnetized like a relay armature and that might cause it to weaken like a relay (which will fail)