Right. It'll be the *regulator*, not the bridge that's cooking. Which,
funnily enough is what it says if you 'mouse over' it.
OK, I did not mouse over these things. I was not expecting temperature
readings in the mouseover messages - I see something new so often!
If the regulator is dissipating a fraction of its max power dissipation,
180 F (about 82 C) is actually OK for temperature of the heatsink tab of
the regulator - if it is accurate. I see a lot of thermometers read low
on things like that for any of 3 reasons:
1. The thermometer does not have thermal contact to the part being
measured to overwhelm heatsinking of the temperature-sensing part of the
thermometer by other parts of the thermometer.
2. The thermometer heatsinks the part being measured enough to screw up
the temperature reading.
3. The thermometer is a thermal infrared non contact one that is reading
low from the item of interest being too small (width half an inch plus an
inch per foot of distance from the "muzzle" of the "ray gun" for my
model), or from reading bare metal.
Also the reservoir cap voltage is too low btw !
Good looking - I did not catch that! I only looked at the schematic
for a few seconds... That could make the bridge rectifier hotter, the
transformer overheat, or simply blow! If it fails short, the transformer
could burn up pretty bad!
Come to think of it, 2200 uF is a big filter capacitor for input if the
current is low, especially if a regulator follows. Big filter capacitor
means spikier current waveform through the rectifier and transformer
windings (regardless of whether or not regulation follows the capacitor).
If the current waveform through the transformer windings is very spiky,
then the RMS value of the current can be a few times the average current.
If the DC current draw is 100 mA, then the RMS current flowing through the
transformer secondary could be as much as .4-.5 amp or so, give or take a
fair amount - and a meter that does not brag about being "true RMS" will
not show that.
A 300 mA 24V transformer in this application could overheat if the DC
current draw is 100 mA! (Its winding resistance will probably spread out
the current spikes to get RMS current down to 350 mA I would guess.) A
500 mA one could get close to its limit if the capacitor is on the high
side of its tolerance range or if the load current is more than 100 mA.
And beware that with mild to moderate overload, a transformer may do its
overheating awfully slowly!
I would use a 1000 uF capacitor with a 12V transformer, or a 470 uF one
if I had to use a 24V transformer. Heck, with a 5V regulator and .2 amp
current draw and a 24V transformer, 100 uF is adequate!
And watch for those Radio Shack transformers (I mean ones with wire
leads in and out rather than wallwarts) that get hot when you baby them
and get cooking hot when you are barely within their limits!
Something like a 12 or 9 Volt wall wart would fix it.
- Don Klipstein (
[email protected])
