I've read this before, but only saw it now...
I'm building a PSU for my Preamp, dual 5v and 9v
Starting it up I noticed that the -5 regulator reads -9v.
I checked for shorts and something melting/burning, nothing.
Then I put a 10K load on it. ~-7V.
With a 1K load on it it gives -5.03V
All the other regs read perfect with no load.
This regulator is the only one from ST, the other is fairchild and ON semi.
Just thought I'd share.
I'm building a PSU for my Preamp, dual 5v and 9v
Starting it up I noticed that the -5 regulator reads -9v.
I checked for shorts and something melting/burning, nothing.
Then I put a 10K load on it. ~-7V.
With a 1K load on it it gives -5.03V
All the other regs read perfect with no load.
This regulator is the only one from ST, the other is fairchild and ON semi.
Just thought I'd share.
Wynand said:
Checkout the pins of the negative regulator.
If it is the 3 pin type of 7905 alike,
the negative regulator PIN configuration is often DIFFERENT from positive regulator IC.
At least this is the case of the adjustable LM317, positive and LM337, negative regulators.
I think it is same issue, regardless if it is TO92 or TO220 case.
Datasheets:
http://www.fairchildsemi.com/ds/LM/LM7905.pdf
http://www.fairchildsemi.com/ds/LM/LM7805.pdf
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edit:
The correct solution is in post below.
These 3 pin regulators will need normally
a minimum load of like 5 mA.
If your negative part draws too little current
you need to put in a load resistor,
to reach a total load of those ~ 5 mA.
There are some IC regulators, for example bigger TO3,
that even recommends a minimum load of 10 mA.
lineup
The positive chips voltages are ok, the -9v regulator is fine, it's just the -5v regulator that read -9v.
The negative regulators share the input and gnd connections. I've checked for shorts and there's none. With the reisistor on From the output of the -5v reg to GND the reading is fine.
That means the regulator will conduc fine with a load of 5V/1000R=0.005A
This wont be an issue because I'll have a LCD, MCU and two LM1973 volume control Ic's running off the regulator, I'll even get another -5v regulator if it runs too hot. I'll probable change the regulator though for another brand if it bothers me further.
The negative regulators share the input and gnd connections. I've checked for shorts and there's none. With the reisistor on From the output of the -5v reg to GND the reading is fine.
That means the regulator will conduc fine with a load of 5V/1000R=0.005A
This wont be an issue because I'll have a LCD, MCU and two LM1973 volume control Ic's running off the regulator, I'll even get another -5v regulator if it runs too hot. I'll probable change the regulator though for another brand if it bothers me further.
richie00boy said:
richie00boy is right!
These 3 pin regulators will need normally
a minimum load of like 5 mA.
If your negative part draws too little current
you need to put in a load resistor,
to reach a total load of those ~ 5 mA.
There are some IC regulators, for example bigger TO3,
that even recommends a minimum load of 10 mA.
They need it to start working okay.
lineup
three terminal regulator ( 78/79 series )
Hi, Guys
With my experience in 78/79 series regulator, it is better to parallel a .1uf cap. with the input & output filter cap.
If in the cct like split supply say + & - dc out in cascade, a diode like 1n4004 shunted in the output side, thus protects the +ve side voltage flows to the -ve side and vise vera. therefore U should put two diode in both + & - side with reverse polarity direction. that means the the diode is not in the dc shorting direction.
another way is to give a load like 10 ma to both + & - output, It makes the regulator more stable. the best way is to connect 2 leds with 2 resistors to limit the current flow thru the leds. usually led draws about 10 ma current.
hope the above method can give u a Help.
Hi, Guys
With my experience in 78/79 series regulator, it is better to parallel a .1uf cap. with the input & output filter cap.
If in the cct like split supply say + & - dc out in cascade, a diode like 1n4004 shunted in the output side, thus protects the +ve side voltage flows to the -ve side and vise vera. therefore U should put two diode in both + & - side with reverse polarity direction. that means the the diode is not in the dc shorting direction.
another way is to give a load like 10 ma to both + & - output, It makes the regulator more stable. the best way is to connect 2 leds with 2 resistors to limit the current flow thru the leds. usually led draws about 10 ma current.
hope the above method can give u a Help.
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