Bridge Rectifiers

[VK2FAK]

Hi all...

Wondering if anyone has had trouble with the 35A Bridge Rectifiers...My old style linear Power Supply died this evening and first look it seems as though the Bridge is at fault...will double check in the morning.....But has anyone had issues with these things....its probably about 20yrs old..
The problem is the 240v fuse keeps blowing...and does not seem to be the regulator section, including Caps..or the Transformer.

John

[VK5TM]

I've had several go in various things from power supplies to pinball games.

Despite the appearance of being quite hefty & strong, they do not like spikes on the incoming line.

Most of mine failed short circuit AC terminal to AC terminal.

[VK7JG]

Hello John.

Yes it is a common fault with older power supplies .
Remove the Wires to the bride and a quick continuity or diode check will soon tell you the problem.
I have found a lot of early supplies used 35A 100 Volt bridges.

Replace it with a 35A 600V one and you should not have any further problems .

Check the transformer first , this can be done once you disconnect the bridge . just apply power and measure the out put of the transformer should be somewhere between 17 and 25 Volts .

Hope this helps.
73
Joe
VK7JG

[VK2FAK]

Hi All..

Thanks for the advice Joe, yeah did the basics last night but did not have a spare to replace it so just left it..

Pulled it out this morning and yes, short from "AC" to "+" ...

And I thought I would compare it to one in a PS waiting repair,,,I had not looked into this one as yet as I did not need it, but was now having my thoughts to the Bridge..
And Interesting enough I pulled it out and it has a short "AC" to "-" go figure...done no more tests on this one so not sure if other damage yet...

Off to Jaycar this morning, but they only have the 400v 35A which should be ok..

John

[VK2FAK]

Hi all.....

I know life could not be that easy, replacing the Rectifier fixed the fuse blowing issue but not quite fixing the PS, it now has the input voltage turning up on the output......lucky I checked before using it..hihi..

Should not be a big problem to repair...and I guess I should have bought some extra parts today....

Will do all that tomorrow....

John

[VK7JG]

Hello John.

Must be a "Cheap" supply with no over voltage protection.
You will most likely find the Pass Transistors have gone S/C (2n3055) are the most common , I am sure that you will get to the bottom of the problem.

I often use my bench supply as the DC input to the regulator circuit when fixing faulty power supplies . My bench supply has variable voltage and current limiting so you don't destroy more things along the way if there happens to be a S/C to ground .

Regards.
Joe
VK7JG

[VK2AAH]

It isn't a fault restricted to cheap supplies... the Imark 5000 supplies are notorious for blowing bridge rectifiers due to spikes. Sometimes protection circuits don't work...

Cheers


Richard
VK2AAH

[VK3ZGC]

The other issue with these is heat!

My 25Amp "VK Powermaster" took out two bridges in the space of 2 years, but only on a hot day under load!

Discovered that the original mounting place on the floor of the aluminium case was inadequate when things got hot.
I ended up mounting the bridge on a separate heat sink and haven't blown one up since.

Gary
VK3ZGC

[VK2OMD]

The other issue with these is heat!

Do not overlook that with a capacitor input filter, the RMS current (ie the heating effect) is many times the DC (average) current... and these things are usually rated for "average current".

The ratio of RMS/DC depends on a bunch of things transformer, capacitors, load current etc.

Some poorly designed supplies use large filter capacitance, that causes shorter pulses of higher current in the diodes and yes, more heat.

Depending on all the associated bits, a 35A bridge may not be big enough for a 20A continuous DC supply and in any event may dissipate a lot more heat than expected.

Above is a derating curve from an old PW40 datasheet.

You can see that at a case temperature of 75° and capacitive load, the 35A bridge might not be good for much more than 18A (60% rating). Interpreting the chart, at the stated 2°/W Rthj-c, the dissipation is (175-75)/2=50W... yes, that is 50W dissipation with 18A DC output using a highly capacitive load. To get rid of 50W from a 75°case in say 40°ambient, you need Rthc-a (75-40)/50=0.7°/W... that is a pretty substantial heatsink!

In my experience, the smaller and larger VK Powermate designs were overrated, they overheat and in the event of failure have poor protection and expensive parts are destroyed. The current protection scheme is naively simple, copied from a datasheet but the 723 is very susceptible to damage from current overloads due to the naive current sense circuit.

Consider a muffin fan to assist cooling. I know that fan noise is a nuisance, but you can thermostatically control it so it only kicks in when really needed. Here is a little customisable AVR based controller: http://www.vk1od.net/equipment/EP613/fanctl.htm .

I echo someone's earlier comment of using a surge supressor. A good sized MOV isn't very expensive and helps to protect the diodes in these supplies. Protecting the diodes helps to protect everything else in them.

Owen

[VK5PJ]

Thanks Owen,
I had often wondered why the power supplies we had used for the beacons in Alice Springs had regular failures and it was always the bridge rectifier. These were 20A rated PSU's by CODAN and TRACKER for use with their 100W HF radios. Now a long running riddle is answered, as the spot where the beacons was did get quite warm.

The other issue with these is heat!

Do not overlook that with a capacitor input filter, the RMS current (ie the heating effect) is many times the DC (average) current... and these things are usually rated for "average current".

The ratio of RMS/DC depends on a bunch of things transformer, capacitors, load current etc.

Some poorly designed supplies use large filter capacitance, that causes shorter pulses of higher current in the diodes and yes, more heat.

Depending on all the associated bits, a 35A bridge may not be big enough for a 20A continuous DC supply and in any event may dissipate a lot more heat than expected.

You can see that at a case temperature of 75° and capacitive load, the 35A bridge might not be good for much more than 18A (50% rating). Interpreting the chart, at the stated 2°/W Rthj-c, the dissipation is (175-75)/2=50W... yes, that is 50W dissipation with 18A DC output using a highly capacitive load. To get rid of 50W from a 75°case in say 40°ambient, you need Rthc-a (75-40)/50=0.7°/W... that is a pretty substantial heatsink!

Owen

Regards,
Peter, vk5pj

[VK2AXL]

I repair swimming pool chlorinators at work, which are really just 7V 20A PSUs, and I go through quite a few 35A bridge rectifiers. Heat is the killer. Use heatsink compound and be wary of bad connections in crimp connectors on the rectifier. On some troublesome chorinators I cut the crimps off and soler the wires directly to the rectifier. Its messy but they seem to last after that.