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Elektor Wireless RS232 link and current consumption

M

Mike

Jan 1, 1970
0
I've built the Wireless RS232 link as described in Elektor Electronics
Magazine (December 2003) - and I've even done it on their own
(extortionately expensive) board.

When powered up, the 7805 1A REG gets VERY hot - enough to remove the
skin on my finger, and I've traced the high current consumption to the
MAX232 chip. I don't believe this can be right.

However on previous things I've built where I've used a MAX232, I've
also noticed that the voltage regs run a lot hotter than I expect,
though not enough to burn my fingers.

At the moment, the Elektor PCB just has the PSU bits, and the MAX232
chip + caps, and the voltage reg starts getting hot (not just warming
up) after 30 secs or so.

Has anyone out there built one of these things ? So I expect this ? I
don't want to risk the £20 ER400TRS if it's going to blow up !
 
P

petrus bitbyter

Jan 1, 1970
0
Mike said:
I've built the Wireless RS232 link as described in Elektor Electronics
Magazine (December 2003) - and I've even done it on their own
(extortionately expensive) board.

When powered up, the 7805 1A REG gets VERY hot - enough to remove the
skin on my finger, and I've traced the high current consumption to the
MAX232 chip. I don't believe this can be right.

However on previous things I've built where I've used a MAX232, I've
also noticed that the voltage regs run a lot hotter than I expect,
though not enough to burn my fingers.

At the moment, the Elektor PCB just has the PSU bits, and the MAX232
chip + caps, and the voltage reg starts getting hot (not just warming
up) after 30 secs or so.

Has anyone out there built one of these things ? So I expect this ? I
don't want to risk the £20 ER400TRS if it's going to blow up !

The power requirements of the MAX232 can be read from the datasheet. When it
uses (much) more, you know there's something wrong. So if nothing but your
7805 regulator becomes that hot, it may be defective itself.

BTW what is the voltage of the regulators input? It is the power (I*U*t)
that's warming up the regulator. You may need a heatsink.

petrus bitbyter
 
R

Ryan Wheeler

Jan 1, 1970
0
petrus said:
The power requirements of the MAX232 can be read from the datasheet.
When it uses (much) more, you know there's something wrong. So if
nothing but your 7805 regulator becomes that hot, it may be defective
itself.

BTW what is the voltage of the regulators input? It is the power
(I*U*t) that's warming up the regulator. You may need a heatsink.

petrus bitbyter

if you only have the max232 connected to the +5V DC output
of the regulator and the regulator get hot you got something wrong.
I dont get the Elektor mag so dont know how that is wired.
1. check the reg input range, I am used to +12VDC to +18Vdc range.
2. most common mistake are capacitor connections on the MAX232 chip.
3.
 
J

John Jardine

Jan 1, 1970
0
Mike said:
When powered up, the 7805 1A REG gets VERY hot - enough to remove the
skin on my finger, and I've traced the high current consumption to the
MAX232 chip. I don't believe this can be right.

Make sure those 'lytic caps are polarised correctly. I recently spotted some
wrong on an expensive commercial design that was just ready to go into
production.
 
R

Robert C Monsen

Jan 1, 1970
0
Mike said:
I've built the Wireless RS232 link as described in Elektor Electronics
Magazine (December 2003) - and I've even done it on their own
(extortionately expensive) board.

When powered up, the 7805 1A REG gets VERY hot - enough to remove the
skin on my finger, and I've traced the high current consumption to the
MAX232 chip. I don't believe this can be right.

However on previous things I've built where I've used a MAX232, I've
also noticed that the voltage regs run a lot hotter than I expect,
though not enough to burn my fingers.

At the moment, the Elektor PCB just has the PSU bits, and the MAX232
chip + caps, and the voltage reg starts getting hot (not just warming
up) after 30 secs or so.

Has anyone out there built one of these things ? So I expect this ? I
don't want to risk the £20 ER400TRS if it's going to blow up !

The maximum power dissipation of the max232 is (depending on package)
between 450mW and 880mW. That means something like 170mA max current,
assuming the higher number and a 16 pin DIP case.

However, the 7805 is a linear regulator, meaning it'll dissipate more
power with input voltage. If you are powering the circuit with a 12V
supply, for example, and drawing 880mW with the MAX232, then the 7805
will want to dissipate 7/5 of that, or 1.25W, which will generate
about 80C rise above ambient. Thats toasty, but not too hot.

Regards,
Bob Monsen
 
M

Mike

Jan 1, 1970
0
Robert said:
The maximum power dissipation of the max232 is (depending on package)
between 450mW and 880mW. That means something like 170mA max current,
assuming the higher number and a 16 pin DIP case.

However, the 7805 is a linear regulator, meaning it'll dissipate more
power with input voltage. If you are powering the circuit with a 12V
supply, for example, and drawing 880mW with the MAX232, then the 7805
will want to dissipate 7/5 of that, or 1.25W, which will generate
about 80C rise above ambient. Thats toasty, but not too hot.

Regards,
Bob Monsen

Input voltage is 7.5V. It then goes through a diode for input voltage
reversal protection, so there's only 6.9V hitting the voltage reg.

I've checked the elctrolytics. They agree with the printed legend, and
that agrees with the circuit diagram. I've checked the pinout on the MAX232.

There's nothing else on the board to check !

My only other thought is that this 7805 is over 25 years old. I bought
a lot and I'm just reaching the end. It's actually marked up
UA7805/UC7803. It does regulate to 5V, I'm just wondering, if somehow
the spec for these things has changed over the years. But it's pretty
much a standard building block isn't it, 7805, can't have changed ?

To summarise then, with the voltage reg, it's associated capacitors,
and a small LED/resitor to indicate power, it all runs perfectly.

Add MAX232 powered from the regulator, the 4 voltage doubler
electrolytics, and the voltage reg starts cooking.
 
S

Sir Charles W. Shults III

Jan 1, 1970
0
Not certain how your circuit is working, but I know that if at any time,
the output voltage and input voltage show a reversal, a simple 7805 will
start doing odd things because it is trying to protect itself from
destruction.
It can go into thermal shutdown, or in some cases can be tricked into
going into reverse conduction. I have a metal cased one that literally blew
a pin right through the top of the TO-3 case when this happened. Like a
welding arc- a perfect little round hole, pin vanished right out of the
glass opening from the underside and left a perforation in the case so you
could look right through it.
Have a look at some of the protection schemes to be employed with 7805
style regulators so that this will not occur. They are present in the data
sheet or if you have the old style linear data book, it should also be
there.

Cheers!

Sir Charles W. Shults III, K. B. B.
Xenotech Research
321-206-1840
 
R

Ryan Wheeler

Jan 1, 1970
0
Mike said:
Input voltage is 7.5V. It then goes through a diode for input
voltage reversal protection, so there's only 6.9V hitting the voltage
reg.
and this is probaly your trouble. A 7805 needs at least 8VDC, better
9Vdc on the input to give you a reliable 5VDC output.
 
P

petrus bitbyter

Jan 1, 1970
0
Mike said:
Input voltage is 7.5V. It then goes through a diode for input voltage
reversal protection, so there's only 6.9V hitting the voltage reg.



There you go. I bet your 7805 is oscillating like an alarm clock. 6.9V is to
low for an ordinary 7805 and the diode makes things worse. I advise to
remove the diode and rise either the input voltage to at least 8V or to
replace the 7805 by a low drop regulator.

petrus bitbyter
 

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