I have a Preamp based on the Doug Self Elektor "2012 preamp". I've built it according to his articles and it is functioning properly for a couple years with an occasional problem. I've had one of the diodes fail. On a separate occasion it blew the fuse but I detected no internal cause. It uses 4 MBR1045 diodes to form the full wave rectifier. AC is provided by a
Farnell (Multicomp) toroid transformer with dual 18 v secondaries, rated at 50VA. I have just noticed that the MBR1045 diodes are operating at about 80 C and wonder if that's too hot. The PSU uses LM317/LM337 to produce the +/-17V and a separate LM7815 for 15v to the relays. Heatsink on the ICs stay below 50 C. The MBR1045 diodes do not have a heatsink but I can probably find a way to add one if it's advisable.
I'm attaching a picture of the schematic and the board as built.
Your thoughts appreciated. Thanks.
Farnell (Multicomp) toroid transformer with dual 18 v secondaries, rated at 50VA. I have just noticed that the MBR1045 diodes are operating at about 80 C and wonder if that's too hot. The PSU uses LM317/LM337 to produce the +/-17V and a separate LM7815 for 15v to the relays. Heatsink on the ICs stay below 50 C. The MBR1045 diodes do not have a heatsink but I can probably find a way to add one if it's advisable.
I'm attaching a picture of the schematic and the board as built.
I don't believe they blew due to heat in normal use. For half the time they have 0.7V across them with a few amps, a couple of Watts at the most. 80C is nothing, surely the junction temp is below 100C. 10A continuous capacity? And Tja is only 2C/W.
If it is not a rare bad diode, I would bet a short-term accident causing a short, or a short term exceeding Vmax with a high mains voltage.
You can add a heatsink but it won't prevent a similar "accident".
Jan
If it is not a rare bad diode, I would bet a short-term accident causing a short, or a short term exceeding Vmax with a high mains voltage.
You can add a heatsink but it won't prevent a similar "accident".
Jan
Thanks for the information gents. Much appreciated. I'll probably add heatsink, recognizing that it's overkill - but this is DIY...
thanks for the reminder that the tabs need to be isolated since they are connected to the cathode.
thanks for the reminder that the tabs need to be isolated since they are connected to the cathode.
Thermal resistance junction to ambient is 60C/W, not 2C/W. That's with no heatsink. Thermal resistance junction to case is 2C/W. The diodes are heating up more than I would expect for just powering a preamp. What is the load current on the DC outputs?
Good point. I'll have to measure the draw and get back to you.
For what it's worth, the fuse on the mains is 600mA.
For what it's worth, the fuse on the mains is 600mA.
Jan, I think you mean theta_JC is small. theta_JA is pretty big because a bare TO220 doesn't have much thermal contact with the ambient air.
Motorola, the originator of the Motorola Barrier Rectifier ("MBR") line of Schottky diodes, says theta_JA is 60 degrees C per watt -- datasheet attached.
Motorola, the originator of the Motorola Barrier Rectifier ("MBR") line of Schottky diodes, says theta_JA is 60 degrees C per watt -- datasheet attached.
Make it easy to take it apart. With those too low diode voltage ratings more will die.
Jan
So, over 50.9V peak, 45V rated diode. That's why they die.
The maximum reverse voltage is twice the peak voltage. Peak of 18V is 25.45V. Twice that is 50.9V. A 50VA transformer has at least 10% sag, or 10% un-sag, 56V no-load (and to a 50VA winding, a preamp is almost no load).
If it is going into breakdown 5% of every cycle, no wonder it runs hot.
Using generic diodes, I like at least twice the computed stress. Here, 112V. I remember when a 400V part cost 5X the price of a 100V part; those days are behind us, the 1,000V 1N4007 is same price as the 100V part in DIY quantity.
VS-MBR10T100 may be the 100V part in the MBR1045 family but seems to be out of stock everywhere.
LTSpice actually simulates the situation fairly well. Diode breakdown together with forward power dissipation adds up to almost 1Watt. With a 25C room that would yield 85C junction. The preamp in this case is unusually complicated with many op amps and relays to power. The load is higher than one might think at first. Still, most of the power dissipation in the diode will vanish if the breakdown is stopped.
Attachments
This makes makes sense to me. Looks like the simple solution is a different diode. Anything magic about the MBR series or can I substitute a MUR810. It's a very straight-forward PSU - LM317, LM337 and LM7815. MUR810 has same pinout and is 8A rated instead of 10A but reverse voltage is 100V. Am I missing anything?
I suppose that I could also change to a 15V transformer. That would give me peak reverse voltage of 42.43V. This would be just under rating on the MBR1045. that doesn't allow much margin and doesn't quite generate the 5V above target output voltage (17V). So, unless I'm missing something, the MUR810 would be a better choice.
If it was my preamp, I'd change to MBR1060 and call it a day. Reducing the transformer voltage runs the risk of causing the LM317 and LM337 to drop out due to insufficient input voltage. It may be marginal as it is with the 18V transformer. To check it, using a scope measure the bottom of the input ripple voltage to the LM3x7 devices. If it isn't at least 2.5 volts above the output voltage there is risk of drop-out.
I made the same mistake before in a tube amp, kept popping the rectifier diodes and i couldnt figure out why. Finally realised i had close to 800v reverse voltage on a 600v rated part. I can also say from experience that power transformers make some angry noises when the diodes fail 😆
So you guys are saying that the original article written by Douglas Self, which calls for a 2x18VAC transformer and MBR1045 rectifiers (image in post #1 of this thread), contains a glaring error?
BTW the MBR10100 (10A, 100V, Schottky) is in stock at lots of distributors today, according to (octopart.com)
BTW the MBR10100 (10A, 100V, Schottky) is in stock at lots of distributors today, according to (octopart.com)