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Diode does not limitting current???

 
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ericmar



Joined: 14 Apr 2004
Posts: 279
Location: Singapore
Post04 Oct 2004 10:59   Diode does not limitting current???
hi all,

I was doing something today in my lab and used a 1N4004 diode which i supposed to limit the current source to 1A but it doesnt work! I got my power source from a DC adapter which is about 16V @ 2.2A and the output current from the diode is approximately 2A! Shocked

I hv no idea y this occured. Could anyone explain to me?

Thanks for ur help!

Regards,
Eric
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M!k



Joined: 22 Apr 2002
Posts: 814
Helped: 69
Post04 Oct 2004 11:36   Re: Diode does not limitting current???
That diode doesn't limit the current to 1A. That current is only the maximum allowed for this diode. Reason: there's a voltage drop of about 0.8V. At 1A this means the power loss of the diode is 0.8V * 1A = 0.8W. Now if the current is too high the diode will heat to much, the temperature of the junction increases to >>125°C and the diode will be damaged soon.
You'll need a current limiting circuit for that purpose; evtl an electric bulb will work too.


Mik
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Borber



Joined: 01 Jan 1970
Posts: 1492
Helped: 112
Post04 Oct 2004 12:34   Re: Diode does not limitting current???
Diode itself is not a current limitter. Only appropriate circuits or other elements can limit the current. I think above answer has hit the nail on the head.
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mopengfei



Joined: 10 Aug 2004
Posts: 41
Location: hangzhou of china
Post04 Oct 2004 12:53   Diode does not limitting current???
can u explain "power source from a DC adapter which is about 16V @ 2.2A"
got my power source from a DC adapter which is about 16V @ 2.2A and the output current from the diode is approximately 2A-----i think the currient is lost spending in resistance
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Borber



Joined: 01 Jan 1970
Posts: 1492
Helped: 112
Post04 Oct 2004 22:51   Re: Diode does not limitting current???
Your data 16V(at)2.2A means that output voltage from PS is 16V at 2.2A load current. The output voltage depends on load current if it is not stabilized. Less load current will give you slightly higher output voltage.
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ericmar



Joined: 14 Apr 2004
Posts: 279
Location: Singapore
Post05 Oct 2004 3:59   Re: Diode does not limitting current???
M!k wrote:
That diode doesn't limit the current to 1A. That current is only the maximum allowed for this diode. Reason: there's a voltage drop of about 0.8V. At 1A this means the power loss of the diode is 0.8V * 1A = 0.8W. Now if the current is too high the diode will heat to much, the temperature of the junction increases to >>125°C and the diode will be damaged soon.
You'll need a current limiting circuit for that purpose; evtl an electric bulb will work too.


Mik

Can u show me an example of a current limiting circuit?

Thanks!
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platonas



Joined: 14 Aug 2004
Posts: 165
Helped: 5
Location: Universal Citizen
Post05 Oct 2004 6:45   Re: Diode does not limitting current???
Typical current limiting circuit.
Calculate value of R2 according to your needs and also choose transistor according to power/current requirements.
[/img]

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cent24.3



Joined: 14 Jul 2004
Posts: 12
Helped: 1
Post05 Oct 2004 21:07   Re: Diode does not limitting current???
Hello,

Just to say it : If you need a current regulation you can try diodes like 1N5297 but it's only for low current (1mA for the 5297).

I think the schematic of "platonas" is more what you are looking for but it has got some problems because I can't see it !

But I think he was speeking of this one

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Kevin Weddle



Joined: 10 Jul 2004
Posts: 101
Location: San Antonio, TX
Post06 Oct 2004 22:10   Re: Diode does not limitting current???
Use a thermal fuse. I am sure they make them although I have never used one. You can use a transistor arrangement but it will affect the regulation if you so desire to regulate the voltage of the supply. Let's say you have 16v that is regulated. The regulator will maintain the 16v but a transistor configuration on the output will not regulate effectively. Your power supply should have a regulated current limiting feature.
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cent24.3



Joined: 14 Jul 2004
Posts: 12
Helped: 1
Post07 Oct 2004 6:40   Re: Diode does not limitting current???
Kevin Weddle wrote:
You can use a transistor arrangement but it will affect the regulation if you so desire to regulate the voltage of the supply.


Easy... you just put the current limitor before the voltage regulator not after !
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Kevin Weddle



Joined: 10 Jul 2004
Posts: 101
Location: San Antonio, TX
Post07 Oct 2004 17:24   Re: Diode does not limitting current???
He is using a DC adaptor so the current limiting function will be directly on the AC side. To current limit the AC side wouldn't you need to use a triac. That would require a pulse to turn it off. That might work. The trigger arrangent migh be little tricky.
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