Bridge Rectifier SMOKING HOT - BR310

[MarshallPlexi]

Guys I have a circuit that is working fine but the Full Wave Bridge is getting SMOKING hot. I'm using this circuit to power a cooling fan, voltage regulator and the amps Relay and LED.

The fan is getting: 13.5VDC
The preamp tubes are getting 6.5VDC
The LED and Relay are getting 5VDC

The total draw from the regulator is 1.62A well below the BR310's max rating of 3A. Any ideas why this bridge is unbelievably hot? I measured it at roughly 86 Degrees C.

Here is the circuit:



Thanks for any council. I've used this circuit before and I don't recall it ever getting this hot before. I had to put a heatsink on the bridge and it's still way too hot. Any ideas?

Thanks again!

[Enzo]

You are saying that 1.6A flows through the fuse, as measured there?

Does the bridge get hot with the fuse removed? if so, it is likley bad.

12VAC rectified and filtered ought to make about 17VDC, not 13. probably not enopugh to kill a 16gv cap, but still beyond its rating. Disconnect the cap and sub in another.

Disconnect the input lead to the regulator. Still get hot?

Pull the tubes, still get hot?

Are those measured voltages, or are they calculated?

All in all it seems the bridge is getting hot because something is loading it down.

[MarshallPlexi]

Those voltages are with the amp working properly and measured under actual conditions. I'll pull the fuse and see if she still gets hot. If that doesn't work I'll pull the fan and see if she gets hot; then the regulator.

[Wilder Amplification]

12VAC rectified and filtered ought to make about 17VDC, not 13.

Actually, about 15.5 - 16 volts. You forgot to take into account the 1.2V drop across two of the diodes in a BR circuit.

probably not enopugh to kill a 16gv cap, but still beyond its rating. Disconnect the cap and sub in another.

Agreed...this supply puts that cap right at the limit. I'd change it out to a 25V or so.

Also, how many preamp heaters are you trying to run off of that thing?

[Enzo]

You're right, I didn;t subtract for that. I thought about it, but decided noy to cloud the issue. I also did not know if that was a measured 12VAC or a 12v rated transformer, which could have been higher. But even so, it appears loaded.

[Wilder Amplification]

You're right, I didn;t subtract for that. I thought about it, but decided noy to cloud the issue. I also did not know if that was a measured 12VAC or a 12v rated transformer, which could have been higher. But even so, it appears loaded.

That it does. I'm thinkin' the BR is shorted, although I could be wrong. Also, that fuse needs to be rated no higher than 2 amps since that is a 2 amp regulator.

[nickb]

Seems to me thats about what I would expect.

In operation the drop across each diode is about 1V @ 1.5A (from data sheet).
Thus the power dissipated in the bridge is 2 x 1 x 1.62 = 3.24 Watts.
The thermal restance from ambient to case is 19 DegC/W (again from data sheet) making

case temperature = ambient temp (say 25C) + 3.24 x 19 = 86C.

I'm making some simplifications but It'l be close enough.

Going further, the junction temperature will be 11 x 33.24 + 86 = 122C since the junction/case thermal resistance is 11 DegC/W. The max junction temp allowed is 150C.

So all is OK. Personnaly I would fit a heatsink just in case ambient gets any higher - and it will.

[jrfrond]

If all checks out, then just get yourself an MP3510 bridge, which is in a metal case with a bolt hole, and bolt it outboard to the chassis for heat sinking.

However, I do agree that something seems amiss, and you should not need a tank of a bridge rectifier to handle it.

The cap really should be a 25V rating. 3300 or 4700 @25V will be easy to find.

Do you have a DVM with an ammeter function? If so, pull the fuse and read the current draw in series with the load. If you don't have an ammeter, then clip a 1 ohm/5W resistor across the fuse terminals to read voltage drop across it, and then do your Ohms Law to figure out your current draw. At this point, you need some numbers.

[MarshallPlexi]

Do you have a DVM with an ammeter function? If so, pull the fuse and read the current draw in series with the load. If you don't have an ammeter, then clip a 1 ohm/5W resistor across the fuse terminals to read voltage drop across it, and then do your Ohms Law to figure out your current draw. At this point, you need some numbers.

This is exactly what I did. I measured 1.62A constant across the fuse holder.

[mooreamps]

Guys I have a circuit that is working fine but the Full Wave Bridge is getting SMOKING hot. I'm using this circuit to power a cooling fan, voltage regulator and the amps Relay and LED.

The fan is getting: 13.5VDC
The preamp tubes are getting 6.5VDC
The LED and Relay are getting 5VDC

The total draw from the regulator is 1.62A well below the BR310's max rating of 3A. Any ideas why this bridge is unbelievably hot? I measured it at roughly 86 Degrees C.

Here is the circuit:



Thanks for any council. I've used this circuit before and I don't recall it ever getting this hot before. I had to put a heatsink on the bridge and it's still way too hot. Any ideas?

Thanks again!

Ya, beginners mistake. Use a 5 or a 10 amp bridge.

-g

[MarshallPlexi]

Okay..... so why do I need a 5 or a 10A bridge if the constant load is 1.62A?

[Albert Kreuzer]

You don't.
But the max ratings are usually specified with an appropriate heatsink.
Your 3A bridge will be fine if you mount it to a heatsink with some thermal compound.

[mooreamps]

Okay..... so why do I need a 5 or a 10A bridge if the constant load is 1.62A?

be...cause they don't run as hot.

-g

[nickb]

They will only run less hot if the the thermal resistance is less and or the power dissipated is less. You have no control over the current so you must lower the thermal resistance or lower the forward voltage drop.

Now, a given higher current bridge might have a lower or higher thermal resistance. It might also have a higher or lower forward voltage drop. You won't know without reading the data sheet.

At the end of the day you will still need a heatsink for conservative operation. Just bolt it to the chassis - it's free! Also, since it's still running within spec - just- you won't even need and thermal grease!

[mooreamps]

At the end of the day you will still need a heatsink for conservative operation. Just bolt it to the chassis - it's free! Also, since it's still running within spec - just- you won't even need and thermal grease!

No you won't...

-g