M
Mark Fortune
- Jan 1, 1970
- 0
Hi all,
A couple of months ago I built a circuit that involved connecting a few
LM780x voltage regulators to the same 12v supply. The circuit was very
simple, Gnd pin connected to ground, Input pin connected to +12v and
output was tried both open and with a small load - an LED and resistor.
On thier own, I experienced no problems whatsoever; However, when I
connected two or more LM780xCV regulators in parallel (that is, using
the same + and - rails) I noticed they all got extremely hot (too hot to
touch!). This was the case with whatever combination I used - 2x
LM7805CV 5v regulators, a LM7809cv and LM7812cv, an LM7805CV and
LM7809CV etc etc. This was the case even when there was no load on any
of the regulators.
Originall I just used 2 5v regulators, the idea was to wire two 5v
regulators in parallel so that they could handle a load of up to 2A
instead of just 1A.
I even tried wiring them in series - IE the input of the LM7805CV
connected to the output of an LM7809CV - they both still heated up to
the extreme.
In contrast, when connecting an LM7805CV to the +12v supply, and putting
on fairly large loads only warmed the device - I could still touch the
heatsink and not burn myself.
I'm not entirely sure what the problem is. For reference the power
supply I am using is an old AT computer PSU rated at 300w which i've
modded so that I can tap into various voltage outputs.
One thing I did notice today - I rigged up a LM7805CV for a regulated 5v
output, and hooked up my scope to the output. I noticed a slight voltage
variation of about 200mv on top of the 5v output. On closer inspection
this turned out to be a sine wave of about 15MHz - placing a 10nf
capacitor across the output and gnd seemed to eliminate this, so I was
just seeing a flat 5v.
I'm wondering now, this sine wave I saw... was it something to do with
the regulators internal regulation mechanism? and if this is the case...
could it be why all those months ago my regulators were getting hot
enough to cook my breakfast on? My guess is that the regulators are
competing against each other - with one pulling the supply voltage down
a little... the other compensating by adjusting its output which alters
the supply voltage again, etc etc.. creating some sort of feedback
between the devices?
any ideas? I would love to know what's going on here.
Oh and btw, the output pins of the devices are not connected in any way,
they are driving individual loads or are held open.
Thanks,
Mark
A couple of months ago I built a circuit that involved connecting a few
LM780x voltage regulators to the same 12v supply. The circuit was very
simple, Gnd pin connected to ground, Input pin connected to +12v and
output was tried both open and with a small load - an LED and resistor.
On thier own, I experienced no problems whatsoever; However, when I
connected two or more LM780xCV regulators in parallel (that is, using
the same + and - rails) I noticed they all got extremely hot (too hot to
touch!). This was the case with whatever combination I used - 2x
LM7805CV 5v regulators, a LM7809cv and LM7812cv, an LM7805CV and
LM7809CV etc etc. This was the case even when there was no load on any
of the regulators.
Originall I just used 2 5v regulators, the idea was to wire two 5v
regulators in parallel so that they could handle a load of up to 2A
instead of just 1A.
I even tried wiring them in series - IE the input of the LM7805CV
connected to the output of an LM7809CV - they both still heated up to
the extreme.
In contrast, when connecting an LM7805CV to the +12v supply, and putting
on fairly large loads only warmed the device - I could still touch the
heatsink and not burn myself.
I'm not entirely sure what the problem is. For reference the power
supply I am using is an old AT computer PSU rated at 300w which i've
modded so that I can tap into various voltage outputs.
One thing I did notice today - I rigged up a LM7805CV for a regulated 5v
output, and hooked up my scope to the output. I noticed a slight voltage
variation of about 200mv on top of the 5v output. On closer inspection
this turned out to be a sine wave of about 15MHz - placing a 10nf
capacitor across the output and gnd seemed to eliminate this, so I was
just seeing a flat 5v.
I'm wondering now, this sine wave I saw... was it something to do with
the regulators internal regulation mechanism? and if this is the case...
could it be why all those months ago my regulators were getting hot
enough to cook my breakfast on? My guess is that the regulators are
competing against each other - with one pulling the supply voltage down
a little... the other compensating by adjusting its output which alters
the supply voltage again, etc etc.. creating some sort of feedback
between the devices?
any ideas? I would love to know what's going on here.
Oh and btw, the output pins of the devices are not connected in any way,
they are driving individual loads or are held open.
Thanks,
Mark