where does all the extra current go?
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Posted by anonymous on October 12, 2004, 6:40 am
Please log in for more thread options consider yourself warned. So I have a nice transformer that puts out 25V and 1.5A. I want to step it down to 9V with some voltage regulators. The regulators say they are rated for 1A and 37V max (given proper heat sinking). If the output of the regulator is drawing < 100mA - do I have to worry about the remaining output from the transformer? | ||||||||||||||||||||||
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Posted by colin on October 12, 2004, 6:45 am
Please log in for more thread options > the remaining output from the transformer?
> > > > > No. Colin =^.^= | ||||||||||||||||||||||
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Posted by CFoley1064 on October 12, 2004, 9:07 am
Please log in for more thread options >Subject: where does all the extra current go?
>From: "anonymous" anonymous@catfarm.com >Date: 10/11/2004 10:40 PM Central Daylight Time > >WARNING - dumb question follows - WARNING > >consider yourself warned. > >So I have a nice transformer that puts out 25V and 1.5A. I want to step it >down to 9V with some voltage regulators. The regulators say they are rated >for 1A and 37V max (given proper heat sinking). > >If the output of the regulator is drawing < 100mA - do I have to worry about >the remaining output from the transformer? > Not a dumb question -- just a newbie one. First, I hope your transformer has a center tap on the secondary, because if it doesn't and you're using a 9V linear regulator and are drawing 100mA, you're talking about (37V - 9V) * 0.1A = 2.8 Watts Need a heat sink here. It might be somewhat easier to do it this way if your 25VAC secondary transformer has a center tap (view in fixed font or M$ Notepad): 25.2 VCT Sec ____ | | + o-----. ,------>|--o---o---|7809|--o-----o
)|( | C1|+ |____| +|C2
)|( | --- | --- ) ,---. | --- | --- )|( | | | | | )|( | | | | | o-----' '------>|--o---o-----o-----o-----o
| | -
| === === GND GND created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de Use a 1000uF electrolytic for C1 and a 10uF electrolytic for C2. This will give you about 18VDC peak on C1, which should make your 7809 a lot more comfortable. With total power dissipation less than a watt, you should be able to get away without a heat sink as long as the 7809 is in room temperature ambient. Oh, yes -- transformers are rated for maximum current with resistive load. By definition, a transformer is usually rated so that the output voltage will be within 20% of the rated voltage for the rated current. Usually, if you're loading it lightly, the voltage will be higher than rated, and if you're loading it toward its limit, the voltage will be lower. The transformer is an on-demand machine -- it will produce the voltage as long as the current requirement is within limits., There isn't any extra to take care of. Good luck Chris | ||||||||||||||||||||||
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Posted by dB on October 12, 2004, 9:16 am
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>
> So I have a nice transformer that puts out 25V and 1.5A. > ......... > If the output of the regulator is drawing < 100mA - do I have to worry about > the remaining output from the transformer? No. Your transformer can deliver up to 1.5A but will do so only if its load demands it. Similarly, your domestic a.c. supply can deliver up to "x" kilowatts but will only do so if you switch on enough appliances etc to draw that much power. Or your domestic water supply, which can deliver "x" gallons per minute but will only do so if you open the tap/s sufficiently. So, your "remaining current" doesn't go anywhere - it isn't flowing. If you are drawing 100mA from your 1A transformer it means that the transformer still has a further 900mA capacity which you aren't using | ||||||||||||||||||||||
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Posted by tempus fugit on October 13, 2004, 2:10 am
Please log in for more thread options No. Current is drawn, not pushed. It's kind of like having a well full of
water. You lower a bucket down there and draw what you need. The extra water in the well is not going to force itself into the bucket until it bursts. Think of it as having 1.5A available if you need it. > WARNING - dumb question follows - WARNING
about
> > consider yourself warned. > > So I have a nice transformer that puts out 25V and 1.5A. I want to step it > down to 9V with some voltage regulators. The regulators say they are rated > for 1A and 37V max (given proper heat sinking). > > If the output of the regulator is drawing < 100mA - do I have to worry > the remaining output from the transformer?
> > > > > | ||||||||||||||||||||||
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> consider yourself warned.
>
> So I have a nice transformer that puts out 25V and 1.5A. I want to step it
> down to 9V with some voltage regulators. The regulators say they are rated
> for 1A and 37V max (given proper heat sinking).
>
> If the output of the regulator is drawing < 100mA - do I have to worry