A Bridge Replacement A Good Idea?

bridge

No. With a 1 amp line fuse, the 1N4007 diodes the author recommends against, would do fine. I wouldn't go over a 2 amp fuse with your rectifier. Are you heat sinking it?

Those stud diodes were the only 3A or higher that Mouser's database kept throwing out, and believed me I looked for alternatives. One of the nasties of online shopping, I suppose.

Ron

bridge

Since there is a 10 ohm resistor in series, in addition to the fuse, the surge won't be that big. I think the 30 amp half cycle surge rating of the diodes will handle the surge batter than the 1 amp fuse will.

you ? The

A bridge or doubler built with 1 amp diodes will provide almost 2 amps of average line current (each diode seeing 2 amps half the time), and that will blow the 1 amp fuse if you get that far. I wouldn't expect to get this full rated current out of the 1 amp devices, but the 1 amp fuse will prevent it, anyway and so will the 5 watt 10 ohm resistor (40 watts dissipation at 2 amperes RMS which is well below the almost

2 amps average the diodes could take).

It is *seriously* your turn. :-)

bridge

34A. The 32

How long will that current last if this thing is plugged in at the peak voltage. It is charging 11 uF. Hint: It is no where a half cycle.

The rest of the circuit is separated by a "typical" R3 value of 100 ohms.

using that

No willy nilly, here. I took the whole circuit into account.

you ? The

that application

kind of thing I

You need more practice at analysis, I think.

think you could

saw several

Good for you. By the way, do you remember the coil current on the diodes that failed?

for nothing !

But that "absolute max current" includes the additional spec that it is allowed to take place over 8.3 ms. That isn't going to come close to happening with this circuit. By the way, a cold 1 amp slow blow fuse typically has almost 1 ohm of resistance. More as it gets hot. That additional ohm lowers the peak current to 340/11=30.9 amps.

(snip)

hobbyist situations it's

I appreciate that you are a conservative guy. But I think you should back your conservatism up with actual reasons, not just hunches, when you are giving advice.

What do you think of the peak 10,000 or so watts dumped into that 5 watt resistor during the same start up current pulse we have been discussing? Do you think that might exceed its absolute maximum rating?

I think that part is a lot closer to destruction that 1 amp diodes are, but obviously survives, or the author would have had to pick something else. I suspect the author blew up some 1 amp diodes in early experiments with bigger front end capacitors and learned more from the lesson than there was in it.

The peak heating (not the peak current, but I*T, which is roughly what heats diodes) is about the same, because set up for 120 volts, it charges 22 uF on the first half cycle, instead of 11 uF.

think you could

saw several

coils though. Not

I want to see you blow up a 1N4148 with 42 mA, interleaved by 12 volts reverse, of any duration or number of cycles you are willing to wait around for. It ain't going to happen before we get struck by lightning or swallowed up by an earthquake. I have seen literally thousands in such service with no unexpected failure rate.

I designed quite a few circuits that pulse such diodes with hundreds of mA for 10s of microsecond durations and adequately low duty cycles, also with good reliability.

I might be wrong, but that is what I understand to be the case. If you witnessed a failure under those conditions and didn't investigate enough to claim your Nobel Prize, you missed a great opportunity.

If this failure took place on a breadboard with test clips being moved around and parts reused till the leads broke off, that is another matter, entirely. I've blown up lots of stuff, that way.

motto

There are certainly fuses designed to do exactly that. They are called rectifier fuses or semiconductor protection fuses.

For instance:

I'm not saying this one is intended to protect these diodes, but that it will do it anyway for any normal operation of this circuit. After

1 or 2 other component failures, all bets are off as to what's left after the fuse blows.

for nothing !

;-) It is very unusual. By the way, 34 amperes will charge 11 uF to

340 volts in 125 us.

(snip)

often - but with a boring

not worth it. I've been

it was the client who

As long as you understand exactly how close to that edge you are, and are not just playing a hunch, I agree.

while back I had a small

metal film R as combined R

sub-contractor couldn't find

'blowing', so it's back to a

Prove it. ;-)

though.

I was talking about that first half cycle, not long term. I think we both agree that the circuit will work fine, long term with 1 amp diodes. Only the inrush has been under "serious" discussion.

The sad thing is that I think this sort of discussion is a fine way to spend between 1 and 3 A.M. on a Sunday morning, when the missus is out of town. But it is beginning to cut into tomorrows (well, later today's) motorcycle riding.

think you could

I saw several

coils though. Not

been a similar 24V relay

then the heating effect.

might explain something.

Every few minutes, round the clock. One application was a thermocouple data logger that cycled hundreds of relays to select thermocouples to be connected to the amplifier. No diode failures.

similar swap drom 4148s

1N4148 diodes are rated for 200 mA, average rectified current at 25C: That implies peaks higher than that. 42 mA peak (relay coil discharge currents are roughly triangles that ramp to zero in some milliseconds) isn't going to hurt them with thermal cycles at any repetition rate that a relay will be operated at.

Part substituted - no

Go for that prize, man!

part if

Evidently you don't deal with equipment that has big power semiconductors in it, worth hundreds of dollars, each. Something like:

In that kind of equipment, the fuses are expected to save the semiconductors, and do so, repeatedly, when things malfunction.

If an SCR in a big drive misfires and a fuse doesn't blow and protect the semiconductors, it isn't long before the equipment manufacturer is on the line discussing why the design is so poor, and who is going to pay for the outage. And the person who selected that equipment is explaining why the factory is sitting idle, so long while all those big devices are being unbolted and replaced and re torqued, along with their fuses.

I haven't been in that hot seat, but I have watched others squirm and sweat as they threw the power back on to see if the new set also instantly blew up because they were under the gun and didn't have time to trouble shoot why the first set failed before trying another set.

I'll look into that.

Here is the data sheet for the MDL 0.25" x 1.25" slow blow, 250 volt fuses (Something that might be appropriate in the absence of a specified alternative).

If I read that correctly, the cold resistance MDL-1 is 0.525 ohms. But the typical voltage drop (operating at 1 ampere and 25C ambient) is .995 volts. That is close enough to almost 1 ohm for me.

Please point me to a similarly rated fuse's data sheet that shows less than 0.1 ohm, cold or warm.

Pretty impressive. Very close to less than 100 milliohms. And an impressive I^2*T, also. 26 A^2s versus 11.5 for the MDL.

I wonder how its thermal fatigue resistance compares to the well matured MDL line.

Very nice, but I don't see how they get away with calling this a slow blow fuse. It has a measly 3.3 A^2s melting time.

I hadn't appreciated that there was so big a range. I got caught with a voltage drop problem a while back in a 5 volt circuit that had pulsing current passing through the fuse. I need to go back and see what alternatives are available, besides the few choices I checked and thought must be typical.

This thread has been very information filled so far.

Are we in trouble?

bridge

Where did you get the idea about stud diodes ? I see no mention of them.

Graham

bridge

Ahem ! Are 1N4007s going to stand up to the 32A peak charging current ?

A 4A bridge is very sensible.

You don't *seriously* think a normal 1A mains fuse will 'protect' 1A semis do you ? The semis have to be rated according to the application.

Graham

bridge

On 240V mains Vpk = 340V.

If the caps are charging from zero, only the 10R is in the way. Ipk = 340/10 =

34A. The 32 figure was my mistake.

That's the *non-repetitive* single cycle surge value ( 8.3ms ). You can't go using that figure willy nilly.

do you ? The

Not it *absolutely* won't !

Have you ever seen the I-t fusing curves for fuses ?

I've plenty of practice designing real world products and using a 1N4007 in that application is just far too close to the edge for my liking reliability wise. It's the kind of thing I could see 'working for a bit' before failing.

It's also very poor practice for a pro to give such bad advice to a novice.

Never underestimated the damage you can do to diodes with pulses. I used to think you could use a 1N914/4148 safely as a catch diode on typical small pcb relays until I saw several failures. I fit 1N400Xs there now.

Graham

than

1000V bridge

= 34A. The 32

I was just looking at that. I had assumed a larger value for the storage caps.

I'm still not keen on exceeding Ifsm though. They don't call it absolute max for nothing !

using that

do you ? The

that application

kind of thing I

I think it's poor advice since I reckon it's marginal - as I said. With hobbyist situations it's far better to add some margin.

On 120V supplies with the same 10R it would likely be fine with 1N4007s though.

think you could

saw several

Not offhand and it's a long time back. They would likely have been 12V or 24V coils though. Not the 'sensitive type'.

This kind of thing actually....

12V 285R - 42mA

Graham

btw - fuses are there to stop fires !

Try typing in 2 A, 1000V diode at Mouser and see what ya get...

Ron

That's entirely true. Sod's Law however dictates that some poor bugger will be especially talented at regularly hitting the switch at exactly that point in time ! ;-)

I wouldn't rely on that statistical probability to avoid failures. The old motto 'if it can happen - it will happen' is wise advise here.

In this case with 11uF of reservoir cap, this is indeed rather less stressful than say a typical small SMPS for sure.

No fuse will save the rectifier though. A fuse is there to stop fires.

Graham

caps.

for nothing !

It's true I hadn't initially considered the very low value of reservoir cap.

hobbyist situations it's

I'll back it up with experience. Been bitten too often ( actually not that often

- but with a boring predictability ) when designing close to the edge. False economy. It's simply not worth it. I've been pressured into designing in parts I wasn't entirely happy with too. Thankfully it was the client who insisted ! They can pay for the repairs.

Funnily enough ( I assume you mean the 10R ) that just crossed my mind. A while back I had a small SMPS design that used a 3W 4R7 inrush limiter R. I toyed with using a power metal film R as combined R and fuse ( since it fails flame free ). Worked fine here but the Chinese sub-contractor couldn't find a part with equivalent durability. They had to fit wire-wound to avoid it 'blowing', so it's back to a fuse plus R ( I'm actually using a 'Surge Gard' this time ).

It's all about thermal mass basically. A wirewound will survive the shock.

though.

I don't see the long term heating issue being a problem actually.

think you could

I saw several

24V coils though. Not

Seen it several times with only a couple of units in service. It *may* have been a similar 24V relay but with obviously lower operating current.

Consider the waveform shape and time and consider the diode's intrinsic R and then the heating effect. It *has* to be transient thermal failure following a fatigue period.

How regularly ( time interval ) were your relays being switched btw ? That might explain something.

I've seen similar problems in another configuration too. Just last week did a similar swap drom 4148s to 4004s.

It's *all* about high peak current.

No. This was a fully functioning unit in service. It happened more than once. Part substituted - no more failures.

Graham

I should have said no 'normal' fuse like I did in an earlier post ! Mea culpa.

FF fuses are horribly expensive. Easier and likely cheaper to put in a beefier part if available.

In the end though - fuses are simply there to stop the equipment cable burning.

Graham