Hello all,

I'm playing around with Multisim and diodes (particularly constructing a bridge rectifier), although what I'm not quite understanding is the voltage drop across a resistor (amongst other things). So, if somebody wouldn't mind helping me with the following questions.

Please see the attached image for the circuit and the multimeter outputs.

1) I can't see anywhere in multisim on how to change the properties of a virtual diode, is there something I am missing here?

2) The voltage drop across the resistor is 4.091V, is this a little excessive?

3) Regarding the transformer (and from comments in a previous question) it seems that the transformer can handle voltages above its specified input voltage, but there must be some very maximum voltage which if exceeded, nasty things happen to the transformer, yet I can't find a datasheet that indicates this?


Miscellaneous bridge rectifier questions:

I've looked at the Farnell site, (very sorry to keep mentioning it but from what I gather it seems one of the largest component sites in the UK) and their bridge rectifiers (http://uk.farnell.com/bridge-rectifiers) have quite a number of parameters and have a couple of questions on these:

3) Are these descriptions correct:

Max Voltage Vf

The maximum forward voltage drop.

Av Current If

The average current that can be passed through the forward bias diode.

Voltage Vrrm

The maximum voltage the diode can stand in reverse bias before is breaks down.

4) It seems the vast majority of DC circuits are 24V or under, so I'd expect projects would use the appropriate transformer to move the voltage down to a suitable level, yet all of the bridge rectifiers on the Farnell website have a Vrrm value starting at 40V, most popular ones are 100V and 600V. Why is this as the transformer is expected to output a reasonable voltage (for DC projects 24V or under)?

Thank you very much!
Gump.

 

The reverse voltages are high because silicon diodes in general have a decently high reverse break down voltage.
If you look at Schottky diodes the most common ones are the kind of low voltages you're talking about.

There are many DC circuits with higher voltages than 24V. In an audio amplifier the power output you can have is directly related to the supply voltage. +/-35V and higher is very common. To use tubes you need a few hundred voltages.

 

Note that you have an unfiltered DC output so the measurement across R1 is from about 1.4 volts thais needed by diode drops up to peak.

If you put it on an oscilloscope you would see something like this picture of a comparsion of the AC input and the rectifier output.

4 Volts seems high but averaging for RMS is related to power and higher voltage is more potential power so you get closer to the sine peak than you might think.

 

I haven't actually seen any discrete diodes with Vrrm less than about 50V (excluding Schottky diodes), so that may explain why you find it difficult to get Vrrm less than 50V.

Vrrm is the maximum repetitive reverse voltage. You can exceed this slightly if you are not pulsing it a lot (only occasional overloads); but overall this description is correct.

Vf is usually given at 1 amp. It gives you one point on the IV graph, which can be used to plot the other points to work out Vf at say 5 amps or 500mA.

 

Hi Ghar,

The reverse voltages are high because silicon diodes in general have a decently high reverse break down voltage. If you look at Schottky diodes the most common ones are the kind of low voltages you're talking about.

Ah okay, so it just so happens that the silicon diodes are just very good at their job. Why would somebody use Schottky diodes in place of silicon ones?

Hi Potato Pudding,

Note that you have an unfiltered DC output so the measurement across R1 is from about 1.4 volts thais needed by diode drops up to peak.

Got to say using Multisim really makes things easier to grasp having a few meters in a circuit.

What exactly do you mean "unfiltered DC output"?

I've rigged up an oscillator into the circuit and am getting the same results as you with it and can see that the peak to peak difference in voltage between the AC side (ignoring the negative) of the rectifier and the varying DC side of the rectifier is 1.4V. What I cannot see is why when I had two multimeters was I getting 2V (6V-4V) drop over the bridge rectifier, is this because the multimeters use the RMS value?


Tom66 hello,

Vf is usually given at 1 amp. It gives you one point on the IV graph, which can be used to plot the other points to work out Vf at say 5 amps or 500mA.

Ah okay, so that's what was meant when in the previous question that the diode is not a constant 0.7V output. So if I am pulling very little current through the rectifier the forward voltage drop will be very minimal?


What I'm still not quite understanding is how can Multisim simulate a model of the diode without giving any parameters that can be edited?

Also, does anybody know anything about question 3 in the original post?


Thanks very much,
Gump.

 

Multisim will be using a generic diode, one which has a 700mV-800mV drop at 1 amp, and an infinite reverse breakdown voltage and infinite current handling capability.

Your discrepancy in voltage drop is most likely due to multimeter error. They are generally not very accurate in the low volts a.c. range.

Unfiltered = lots of ripple. Note the voltage goes to zero. If you powered a device from this, it would switch on and off 100 or 120 times a second. Not good. Putting a ~10µF to ~10000µF, depending on load, will help filter the output.

Q3. The transformer will have a voltage at which its windings will arc. This voltage is usually in the 1,000s of volts. I think one standard suggested 5,000 volts insulation voltage. Generally, you do not need to worry about this. Also, a higher primary voltage will generally induce a higher secondary voltage, inducing higher currents which could cause the transformer to overheat.

 

Tom, that's excellent thanks very much. Sorry about not replying sooner, I hadn't received an e-mail saying I'd got notification.

Also, I tried to PM you although it seems you have it disabled, but I couldn't help but notice in the projects forum you mentioned your GCSE project, yet despite your age you really seem to know this sort of thing inside and out. Mind me asking what resources you used?

Thanks.
Gump.

 

I'm just a student having recently left secondary school and going onto college and hopefully university. I've learned all this from the 'net (from this website as well), from books, from other forum members, and some is based on interpretations (like I say Vf is at 1 amp. I'd always assumed this, but before I wrote it, I checked.) I've also used my own knowledge gained from when I was younger, I liked to take apart things and it really helped me understand and get interested in electronics. I'm also mildly autistic (aspergers.) Don't know if that helps, maybe it does.

I'll enable PMs. EDIT: It's already enabled, maybe new members (50-100 posts) can't send PMs? That setting was enabled on another vBulletin board I used to visit.

 

I've learned all this from the 'net... from books

I don't suppose there was anything specific that really stood out? Did school ever really help, as I can't imagine a GCSE student knowing half of what you know... unless the standards have been raised quite a bit in the few years I've been away.

I'm also mildly autistic

Hehe, same here. I think it does help, I've been told by a few people that because of my mindset I've got an advantage over other people when it comes to software development, but doesn't seem to help much when it comes to electronics, perhaps things'll get quite a bit easier when I reach FPGA stuff.

maybe new members (50-100 posts) can't send PMs?

Quite possible, I seem to be able to have the option of sending PMs to moderators and such, but yeah could well be a way of preventing spam.


Thanks again,
Gump.

 

Before my GCSE course was completed I didn't actually know a lot about components and electronics. I knew that resistors "resisted" current, capacitors stored it, transformers changed AC. I knew transistors switched and amplified signals, but I didn't know how. I didn't have a clue about op-amps, or much about the 555 timer. That was about it. Once I learnt ohms law, capacitors, 555 timers etc. everything fell into place.

Also, Wikipedia has some excellent electronics articles. Rivalling some professionally written books. I highly recommend you refer to Wikipedia for a quick answer. The theory is often included, which is quite complex even for me p) but I guess when I come more onto electronics theory I'll understand it. A knowledge of science is required to understand some of the things on there. Sometimes I will just click a link to a Wikipedia article or Google it, and in an hour's time I will have 20 tabs open in Firefox.

It also helps to have creativity. Any fool can put together components, it takes creativity to make a good circuit or program. Luckily, creativity is not a rare thing in homo sapiens.

Maths is also a requirement. I'm pretty good at maths. Mental maths is one of my weak points, but I can do quick addition and multiplication... generally I can't do long division or proper addition. I got a GCSE A in Statistics, which I'm pretty proud of, because now I understand standard deviation and ranking, among a plethora of other things.

Aspergers also makes you concentrate a lot on a single subject. Maths and electronics are my favourites, though I do like computers and programming. (These are subjects which all involve creativity...)

 

I'm pretty good at maths.

Urgh, one of my most hated subjects (maybe next to something like History)... I somehow got a B at A-Level, don't have a clue how, absolutely hate maths with a passion.

Quite possible, I seem to be able to have the option of sending PMs to moderators and such, but yeah could well be a way of preventing spam.

Seems I've passed the post limit and can now use PMs.

 

I also took History. It was a toss up between History and Geography. I just chose History because I like knowledge more than even more science (I was already taking triple science...)

 

Hehehe, I had Geography and triple science - although we called it separate science. Seems as though the options in third year aren't much different from what I had.

 

Huh. Anyway, if you want to ask some more questions you should probably start a new thread, this one is getting off topic.

 

Heh, that's true... I think I'm all questioned out for now. Thanks for your help.

 

Hi Ghar,

Ah okay, so it just so happens that the silicon diodes are just very good at their job. Why would somebody use Schottky diodes in place of silicon ones?

Schottky diodes don't suffer from the charge storage effect which prevents the diode from turning off quickly (a problem in switching circuits) and they can have a much lower voltage drop, around 0.3V or so.

 

Busy topic while I was away.

i- Why would somebody use Schottky diodes in place of silicon ones?

ii-What exactly do you mean "unfiltered DC output"?

iii-I've rigged up an oscillator into the circuit and am getting the same results as you with it and can see that the peak to peak difference in voltage between the AC side (ignoring the negative) of the rectifier and the varying DC side of the rectifier is 1.4V. What I cannot see is why when I had two multimeters was I getting 2V (6V-4V) drop over the bridge rectifier, is this because the multimeters use the RMS value?

iv- Also, does anybody know anything about question 3 in the original post?

(which is)
3) Regarding the transformer (and from comments in a previous question) it seems that the transformer can handle voltages above its specified input voltage, but there must be some very maximum voltage which if exceeded, nasty things happen to the transformer, yet I can't find a datasheet that indicates this?

Thanks very much,
Gump.

i-For your circuit Schottky diodes would be better because of the lower voltage drops which would waste less power and give you a higher output DC level. Schottky are not exactly rare or expensive anymore and with good reason. I would avoid using silicon diodes in new designs for low voltage circuits. In anything handling above 1Khz you also want Schottky for their faster response. Silicon would probably work upto at least 15kHz but you would still have the signal loss across the higher silicon voltage drop which is a circuit problem that makes any signal circuits prefer Schottky. High voltage (over 40 volts) AC Power Rectifiers are the last best place to use silicon.

ii-Parallel your Output Resistor with a Capacitor and it will charge up to near peak voltage (it should have a smoothing coil or a resistor in series to limit the AC-Short surge currents.)

That is called filtered DC because the AC component from the source is reduced. The source is the pulsating DC but a decent filter turns it into a flat DC level or Rail. I hate to use a simile but look at the bridge diodes output like a hose that shoots out blasts of water. The Capacitor is a bucket that you drop that hose into and as the bucket fills you stop seeing the blasts and splashing and end up with a steady level of water that you can siphon out from to get a constant flow of current.

iii- Yes it is the RMS value causing the difference to be a drop of 2 Volts.
RMS is an averaged integral power function of the area under the curve. I hope that telling you that much will be enough techical direction.

Do another circuit but with AC of only 2 volts. After the 2x.7 volt diode drops you would see only a small bit of the peaks. The RMS value with so much of the output at 0 volts with brief pulses is going to be severely reduced. You are already seing some of that space between pulses at 6 AC input volts if you look again at the image I posted.

By the way - I notice nobody caught the mistake I made. The LTSpice SwitcherCad that I used reads AC values as peak values. So I input 5.995V SineAC but I should have used 8.5V to convert the 6V RMS to peak voltage for LTspice. Sorry about that.

iv- Bad idea to discuss overvoltaging transformers because you do not want to do it. Transformers can be badly damaged and dangerous without the damage being readily apparent to a person without expereince. Transformers are overbuilt - I agree they often have tolerances much higher than rated. They should and there is a reason for that. Transformers are notoriously liable to failures. That is why they try to give them extra capacity and protection.

There are are many reasons why transformers can be a problem. It starts with their being the component that sees all of the power needed in the circuit. Power transformers tend to be heavy is another reason that they can be damaged. They can have a tendency to vibrate under load.

Some transformer failures can be nasty. Here is one common example.

A short in the primary (which tends to be very light guage wire that can be easily damaged) can change your turns ratio by jumping across a few hundred turns. It might suddenly go from 600:100 turns to 300:100 turns. Your output would briefly double in voltage and draw too much current which will quickly burn the primary winding open if you are lucky.