|
General Vintage Technology Discussions For general discussions about vintage radio and other vintage electronics etc. |
|
Thread Tools |
9th Apr 2019, 11:15 am | #41 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
|
Re: Maths help....Again...exp?
This is the simulation from PSU Designer. It is not created by a formula, it calculates the outcome in time steps so you can see the initial surge during the first cycle and the settling down towards a steady state. Each component in the circuit obeys simple rules but, as the circuit contains a rectifier and is non-linear, the overall circuit behaviour cannot be accurately calculated.
The formula you are using is assuming that the rectifier is instantaneously charging the reservoir capacitor up to the input voltage on each cycle. It makes no allowance for the transformer regulation but this would be the first place to look when designing a PSU. Calculations on a linear LRC network can be done 100% accurately but anything with a non-linear component will require approximations. Last edited by PJL; 9th Apr 2019 at 11:23 am. |
9th Apr 2019, 11:27 am | #42 | |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,876
|
Re: Maths help....Again...exp?
Quote:
I was talking in terms of simple three-terminal linear regulators, or of series-pass valve regs. When switchers enter the scene, it gets a lot nastier. Their efficiency means they tend to take constant power, so to whatever is feeding them the don't look like a resistive load, they don't look like a constant-current load. they look like a negative resistance load1 These things can make systems go unstable. In an aeroplane more and more of the electrical load is going into 'smart' stuff with switch-mode internal regulators. At some point the generator controller isn't going to cope with the less stable loads. Quite worrying. Real resistive loads have nice stabilising effects on power systems, but they're fast going away. David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
|
9th Apr 2019, 11:32 am | #43 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
Re: Maths help....Again...exp?
I know what exponential decay is in terms of CR and time, we did graphs etc of that at City & Guilds tech.....What I can't do yet is work out the answer using the formula that's under discussion, my own calculator has no e, exp or upward arrow function and if I times something by trying to put a minus in front of it the display just goes AWOL... so I searched the web for an online exp calculator, I couldn't make sense of them either except for one, the one in the link I posted earlier.
Next phase was to try a different approach using the mentioned exp calculator, I did the calc for T/CR on the calculator I have at home, that was no problem for me, I entered the result into that exp calculator and it spewed out a result. Then with my home calculator I divided Vp-0.7 with that result, the result of that was then subtracted from Vp-0.7 and the result of that agreed with the result shown on the website in the link I posted earlier, the result was Vr=0.47volts. I then used the same method for the other examples shown on that website and the results I obtained were in agreement. But that hasn't solved the problem of me not solving the original equation, to that end GeoffK pointed me to a scientific calculator on Windows 10 which I now have..... …..so now it would be nice if someone could talk me through the procedure to obtain the correct answer using the formula under discussion and the Windows 10 scientific calculator that I now have, the inputs are Vp-0.7=14.3 volts....T=0.01667 secs....R=5,000 ohms....C=100uF (I know how to do T/CR but that could be included again in the talk through procedure) Anyone? It would also be good if someone can explain to me the relationship between the method that I used and the mathematical goings on in the formula incase I've already missed it. Formula to crack on the calculator is shown here again for convenience: Vr=(Vp-0.7)(1-exp(-T/CR)) P.S. I haven't replied to all the posts but I appreciate everyone's input so far. Lawrence. |
9th Apr 2019, 12:09 pm | #44 |
Nonode
Join Date: Oct 2014
Location: West Midlands, UK.
Posts: 2,181
|
Re: Maths help....Again...exp?
Looking at the "exp" item, it's most likely another name for log, but as most logs work in base 10 , it's most likely that, but to add to the confusion, some older computers ( heading back to the old speccie days ) used the Naperian base of 2 and a bit.
Easiest way to find if calculator used base 10 is to enter 2 and get the log of that. If the answer is .3010 then it's base 10 . |
9th Apr 2019, 12:15 pm | #45 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,311
|
Re: Maths help....Again...exp?
What you calculated on your home calculator, which gave the correct answer, was to work out
(Vp-0.7) - (Vp-0.7)/exp(T/CR) Do you agree ? We can rewrite this expression by taking (Vp-0.7) out, since it's a factor of both the terms in the expression. That converts your home calculation to (Vp-0.7)(1-1/exp(T/CR)) Does that make sense ? Then all we have to do is to recognise that 1/exp(T/CR) is equal to exp(-T/CR). That was my point about exp of minus something being the reciprocal of exp of plus the same something, quite a few posts ago. Making that change gives us (Vp-0.7)(1-exp(-T/CR)) Hey presto ! In just two steps we've shown that what you calculated on your home calculator turns out to be equal to the original formula, so it's no surprise that your home calculation gave the right answer . Cheers, GJ
__________________
http://www.ampregen.com |
9th Apr 2019, 1:26 pm | #46 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
Re: Maths help....Again...exp?
My bold:
Quote:
Update, I calculated T/RC where T=.01667 secs...R=5,000 ohms....C=100uF....the answer came to 0.03334, I then did the following with the scientific calculator, I entered 1 then - then pressed the exp button then - then entered 0.03334 then hit the equals button, this is what I got... ...0.9666....multiplying 14.3 (Vp-0.7) by that number doesn't give Vr, the ripple voltage, where did I go wrong? Lawrence. |
|
9th Apr 2019, 1:29 pm | #47 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
|
Re: Maths help....Again...exp?
The microsoft scientific calculator is rubbish and won't allow you to do this. Alternatives are:
* Microsoft Excel * Microsoft Mathematics - download here https://www.microsoft.com/en-gb/down....aspx?id=15702 Image of calculation on Microsoft Mathematics below and you enter it like a formula using 'e^' for 'exp'. The '^' symbol does not display correctly in the app. Last edited by PJL; 9th Apr 2019 at 1:44 pm. |
9th Apr 2019, 1:40 pm | #48 | |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
|
Re: Maths help....Again...exp?
Quote:
|
|
9th Apr 2019, 1:44 pm | #49 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
Re: Maths help....Again...exp?
Quote:
Is there a suitable live online calculator I can use instead of a download? I've got enough download clutter on my PC as it is. Lawrence. |
|
9th Apr 2019, 1:52 pm | #50 | ||
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,311
|
Re: Maths help....Again...exp?
Quote:
Cheers, GJ
__________________
http://www.ampregen.com |
||
9th Apr 2019, 1:55 pm | #51 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
|
Re: Maths help....Again...exp?
Microsoft Mathematics looks like a useful free app from Microsoft, far better than the standard Microsoft Calculator that does not even support a sign or a decimal point on exponents on my PC.
|
9th Apr 2019, 3:05 pm | #52 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
Re: Maths help....Again...exp?
Could someone confirm the answer to exp0.03334 on their scientific calculator, the online exp calculator I'm using says 1.0339 the Windows 10 calculator just returns 0.e+3334 which doesn't compute with me.
Lawrence. |
9th Apr 2019, 3:19 pm | #53 |
Heptode
Join Date: Mar 2016
Location: Essex, UK.
Posts: 602
|
Re: Maths help....Again...exp?
The Microsoft calculator has In and pressing Inv at the top row gives e^x
Exp is the exponent i.e. 1^Exp2=100 It is a perfectly good inbuilt calculator, not rubbish at all.
__________________
Geoff |
9th Apr 2019, 3:24 pm | #54 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,311
|
Re: Maths help....Again...exp?
When x is much less then 1, exp(x) is approximately equal to 1+x.
So exp(0.03334) is approximately 1.03334. The online exp calculator's answer is 1.0339 and that's correct (and very close to the approximate answer). Cheers, GJ
__________________
http://www.ampregen.com |
9th Apr 2019, 3:38 pm | #55 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
Re: Maths help....Again...exp?
Quote:
Lawrence. |
|
9th Apr 2019, 3:48 pm | #56 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
Re: Maths help....Again...exp?
Quote:
https://www.medcalc.org/manual/exp_function.php It's the only one I could find that I could make sense of. Lawrence. |
|
9th Apr 2019, 3:49 pm | #57 | |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
|
Re: Maths help....Again...exp?
Quote:
So instructions for Lawrence are: 1. Press [C] 2. Enter 0.03334 3. Press [Inv] 4. Press [ex] 5. Result = 1.033902... Microsoft Mathematics accepts complex expressions and is far more intuitive. |
|
9th Apr 2019, 3:56 pm | #58 | ||
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
Re: Maths help....Again...exp?
Quote:
I don't want to download a separate Microsoft app because when ever I download apps the pop up box rate, the spam junk rate and the general PC slowness and annoyance rate increases, that's just the way it is. Lawrence. |
||
9th Apr 2019, 4:14 pm | #59 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,288
|
Re: Maths help....Again...exp?
I checked this on my ancient CASIO COLLEGE fx-80 Scientific Calculator and got the answer 1.0338606 which of course rounds to 1.0339.
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
9th Apr 2019, 4:32 pm | #60 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
Re: Maths help....Again...exp?
Quote:
Lawrence. |
|