Didn't know that, interestingI see your drawing but I need to log on. If I don't log on I see only text.
Just how much is too much? Is my reasoning above correct? 300mA fine, 400mA too much for a 3VA 6Vac transformer.* You can use a 6V transformer if your load doesn't draw too much current.
So does it mean it can blow my bridge? How should I correctly choose a bridge then?a. While charging it draws higher current (surge)
b. It can exceed the rating of the bridge used (as is seen in data sheet at the given link)
I thought of two because I can't tell for sure (given the nonpolarized plug) which is the live and which is the neutral wire.Q-3 Fuses are not that critical, why two fuses why not one, what trade off allowed, you can go ahead with what you have.
Mmm, not sure I get this. Won't a large cap limit the ripple at the cost of drawing more current from the transformer than it can safely supply?Q-4 It is matter of permissible ripple, as you are using three terminal regulator this is not major concern.
It is not good practise to parallel transformer windings as if there is any difference in the number of turns there will be an increased primary current perhaps, leading to overheating. Its a linear law 1% error in number of turns = 1% increase in primary current or 1% less in secondary current. .
When the incoming mains is at a peak, it sends current through two diodes - hence twice the 1 V drop.
" C= 300mA / (2 * 50Hz * 4.73) = 0.63mF ", I like this formula but I don't understand it. C(MF) = amps/ 2 * frequency * voltage ripple. Should there not be a PI in there somewhere?where did you get it from ?
Frank
Just how much is too much? Is my reasoning above correct? 300mA fine, 400mA too much for a 3VA 6Vac transformer.
So does it mean it can blow my bridge? How should I correctly choose a bridge then?
The datasheet mentions a surge overload rate of 50A.
I thought of two because I can't tell for sure (given the nonpolarized plug) which is the live and which is the neutral wire.
Mmm, not sure I get this. Won't a large cap limit the ripple at the cost of drawing more current from the transformer than it can safely supply?
After AC is rectified, and you put a capacitor across the power leads, your supply can rise to 1.414 x AC nominal minus 2 diode drops. That's with no load.
Once you add load, the supply voltage drops to a level of equilibrium. The greater the load, the more the supply drops.
With your 3W transformer you can expect to get 6 V @ 500mA. Notice that 5% variation in house voltage will give you 5% variation in your supply V.
The way I get the source spec is that the diodes will withstand a surge of 50A, that is a very short burst of current of up to 50A, if the burst is long they'll fry, and if the burst needs more than 50A, they'll also fry. Isn't it this way?The surge spec is a short burst through a diode. Your diodes won't have 50A.
so would you go with the 9V 5VA or the 6V 3VA transformer in the above example if I need 300mA?But diodes will get a surge because the capacitor draws a lot of current on power-up. How much surge current is hard to predict. It depends on transformer resistance, windings, etc. It's only for a fraction of a second.
A good rule of thumb is to size components for 2x expected use. Therefore a 1A bridge rectifier should be sufficient.
All any of us can say is, if the bridge blows someday, replace it with a higher rating.
I had to do that with my power supply. Transformer is 12V @ 3 A. I powered an automobile headlight for several minutes. Blew the 4A diode bridge. I replaced it with a 6A bridge. Used heat sink grease (as I should have with the 4A bridge). No problems in the years since.
In the old days there were charts showing the parameters for power supply design e.g.
Dayal M. (1964): Power Rectification with Silicon Diodes; Mullard Technical Communication 7,
230-262, and 9, 46-47, 1966.
Unvala B.A. (1968): Power Supply Circuits Using Silicon Rectifiers; Texas Instruments
Application Report D4.
but these may be difficult to find online. However, since circuit simulation is now so accessible, get hold of a free simulator and run transient simulations to give you all the information you need. Try for example the Linear Technology version available online at Linear Technology - Home Page.
Do you mean that if the 7805 is trying to draw 400mA then the voltage will drop even if I put a huge filter cap and the transformer will not overheat?
Anyway, my reasoning is: if it can provide 6V at 500mA, then if I am trying to draw 7V, then the maximum current will be 420mA. if I am trying to draw 8V, so that I get 7 after the diode drop, then max current will be 375mA. Is this correct?
The way I get the source spec is that the diodes will withstand a surge of 50A, that is a very short burst of current of up to 50A, if the burst is long they'll fry, and if the burst needs more than 50A, they'll also fry. Isn't it this way?
so would you go with the 9V 5VA or the 6V 3VA transformer in the above example if I need 300mA?
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