Re: why is my full wave bridge overheating?


Back on earth, I found a cement block power resistor in my box, a 10W
2.7 ohm. I put it in series between the bridge's positive DC output
and the regulator. Surprisingly the cement resistor does not even get
warm (but it obviously is not blown because the circuit powers up). I
think I may have this wrong as I have seen a load resistor on some
schematics that connects to ground. Should it go one end to the bridge
and one end to ground or in series as I have it now?

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It's where it should be, but it's not dropping enough voltage
because it's too small.
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The transformer is actually a 26v AC 300ma output and it feels OK to
the touch with no appreciable increase in temperature.

My 7805 is in the TO-220 package and the heatsink is one of those small
clip-on types. I cannot seem to locate a picture of this type.
Something similar would be this:

http://www.ladyada.net/make/x0xb0x/fab/images/to220-heatsink_t.jpg

I do have some white thermal contact paste on it.

There is a reason I'm using a 24v AC transformer. Mainly it's because
I don't want to run two power supplies and the item ultimately being
powered via relay requires 24v AC. I have to be cautious as to how
much I reveal of the entire circuit because -sooner or later it seems
like someone always winds up saying "just go out and buy one off the
shelf, Pokey, you idiot" and I'd like to make this work.

It would be nice if I had a way to measure the current being consumed
but my cheap DMM does not have this function.

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Yes, it does. Measure the resistance of the 2.7 ohm resistor, then
measure the voltage dropped by the resistor with the circuit
operating and the relay closed, then divide that by the resistance
you measured, and that'll be the current through the regulator,
including its operating current.
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I found these specs from
the relay's data ***:

Minimum Contact Load: 10mA @ 5VDC.
Initial Contact Resistance: 100 milliohms max. @ 100mA, 6VDC.

Nominal Coil Power: Contact rating 3 = 200mW.
Contact rating 10 = 450mW.

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I don't understand what the coil power has to do with the contact
ratings. Do you have a URL for the data *** or can you scan it
and post it?
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Throw in a friendly 2n2222, a midrange pic processor, and a max233 and
you pretty much have it. I will try to work on a complete schematic
with the crappy but free software that I have.

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So, you're supplying the PIC, the MAX233, and the relay coil with 5V
and you're using the transistor as a low side switch to turn on the
relay?
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Speaking of DMM, why do I read 10V AC at the 5V DC regulator output? I
thought the bridge and filter cap knocked off any AC.

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It's probably oscillating. You need to connect the smoothing cap
and the cap on the output as close to the 7805 package as you can
and make all your wiring as short as you can. In particular, make
sure that all your ground wiring is _solid_.
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The term "red hot" is misleading as the part does not actually glow. I
will now reference it as "bitch hot" which should clear things up.

Also, this isn't "let's go spend $50 at digikey" to fix the problem. In
my dreams, I order my $3500 Scopemeter from digikey and become a cadet.
This is more like "let's see if Pokey can dig enough parts out of his
junk box to make this relatively simple thing work reasonably".

BTW, the new reading after doing all of above is 170F at the heatsink
tab so there has been an improvement, but still seems high. This is
measured with a non contact thermal thermometer so accuracy is what it
is.

I think I may have a 12V or 13V zener diode around. What about
replacing two of the 1N4004s used in the bridge with two zeners? I
think this would then reduce the voltage to the regulator.

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Don't do it.


--
John Fields
Professional Circuit Designer
.

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