Crowbar circuit?

[Al (astral highway)]

Originally Posted by Diabolical Artificer

Am also a bit puzzled by some of their specs. EG reverse standoff voltage.

Hi Andy, this is just the DC working voltage.

Originally Posted by Diabolical Artificer

Also on one example it says min breakdown voltage 8.33v, max BDV 9.21v..

Ahah, yes, potentially confusing. The maximum breakdown voltage is the highest point in a range for that device where the impedance of the device shifts non-linearly from high, non-conducting, to very low (ie, conduction, dissipating the energy of the transient as heat.) The minimum breakdown voltage is the lowest point for that device where this happens.

It is further confusing that the maximum clamping voltage at the peak clamping current is a lot higher than the maximum breakdown voltage, around 30 or 40 percent higher. So that's how your 12.9V clamping figure arises:

Take your 9.21V, add 35% (say) of 9.21=3.22V,
sum them=9.21+3.22=12.43V, so near enough to illustrate the point.

This maximum clamping voltage is even higher than the stated maximum working voltage. It's transient though. Try to make it stand off-higher voltage for more than a tiny fraction of a blink of an eye, and it's toast.

[Diabolical Artificer]

Ta for the info Al but i discarded the idea of using a TVS. The TL431 also has proved problematic in use, so as I said in my last post I'm using a straight forward zener , comparatively speaking.

Incidently, when buying 1N400X from Farnell and other distributors, it says "not recommended for new designs" - https://www.diodes.com/assets/Datasheets/ds28002.pdf Looks like they're probably being phased out and replaced by a newer diode on the block. Probably not news for forum members with their fingers on the pulse.

Andy.

[Radio Wrangler]

The 1N400x diodes have been on the professional designer's 'Don't touch em with a bargepole' list for decades.

The high voltage members of that family are PIN diodes to get the voltage rating, but their carrier lifetime means they are so slow that they are barely fast enough for 50Hz supplies. They turn off with a nasty snap which creates a burst of RF interference on each cycle.

Amusingly, you can use them as PIN diode RF switches at the output of transmitters, and they work OK as low value, low ratio, variable capacitance diodes.

They're most probably being phased out due to falling turnover and difficulty of getting them from reputable sources. They tend to be a dumping ground for fall-outs of other types.

David

[Restoration73]

Some years ago I modified the crowbar circuit so that it fired a relay with high current
contacts to interrupt the dc output. This circuit is powered via a regulator from the main
smoother cap, or a separate dc supply. It is possible to have a "test" button.

[Radio Wrangler]

My heavy duty 13.8v supply has an on-off switch which does nothing until you press a start button. The button closes two microswitches sequentially. The second-acting one applies mains to the big toroid via a surge limiting resistor (The resistor is bolted to a lump of aluminium with a thermal cutout on it to protect against someone holding the button in when there is a fault) This charges up the main reservoirs while the first-acting microswitch holds the output regulator in shut-off condition. When the reservoir voltage passes 12v, aa relay closes applying full mains to the transformer. When you take your finger off the button the first acting microswitch lets the regulator come on. The beastie is now working. 13.8v with a 40A current limit that will run 30A 'til the cows come home.

If the regulator should fail and the output voltage go high, a monitor will trigger an SCR. The SCR is connected across the reservoirs fed from a low value very high current wirewound resistor. The SCR end of the resistor is also what powers the mains hold-on relay coil, so triggering the SCR not only dumps the reservoir charge in a controlled way, it also shuts off the mains input.

David

[Diabolical Artificer]

as I thought, not news. So if we don't use 1N400X anymore as rectifier's, what are the pro's using instead David?

A simple crowbar is all that's called for here in this application I think and space is tight. However this little circuit has opened up a whole new world of protection circuits for me filled with possibilities. I like these kind of circuits as they are a bit like the Mad Scientists inventions on Vision On, IE a sequential series of event's resulting in a virtual marble popping into a hole.

Andy.

[Radio Wrangler]

Just have a look for anything described as 'Fast' or 'Ultra Fast'/ Fast is relative, but it usually means faster than 1N400x.

I found it quite a surprise when I came across people designing in 'Ultra Fast' rectifier diodes in mains power supplies to reduce RFI. Obviously, you'd expect a faster switching edge to create harmonics to higher frequencies? But the 'slow' diodes hold charge and release it in a fast snap of a pulse in a way reminiscent of step-recovery diodes. This problem bites you whether you're building a switcher where the mais is rectified directly, or if you're building a traditional supply with a nice mains transformer. You'll sometimes see disc ceramic capacitors plastered all over the secondaries of transformers. Most people guess it's to stop muck getting in, but it's to stop noise from the rectifiers getting out and up the mains cable.

I've seen cases where designers have used filtered mains connectors, capacitors all over the place and still not passed emissions tests. Swapping the diodes made more difference than some of the filtering.

David

[McMurdo]

just an aside, TVS diodes are often used commercially to provide overvoltage protection on expensive microprocessor boards. The TVS will either run indefinitely at its clamping voltage or else fail s/c to give you your crowbar effect. Either way the load is protected.

TVS diodes in car ECUs are one reason it's not recommended to disconnect the battery when the engine's running. (actually witnessed someone do that last week...BTW).

[Oldcodger]

Current through valve heaters - tens of milliamps ? So fuse - 100mA. Space tight, I'd just stick a 5w 15v Zener across supply. Voltage exceeds 15v, Zener conducts and fuse blows.
5w /15v zener will handle 330mA, far more than a 100mA fuse.
Crude, but effective.

[Al (astral highway)]

Originally Posted by Oldcodger

Space tight, I'd just stick a 5w 15v Zener across supply. Voltage exceeds 15v, Zener conducts and fuse blows.
5w /15v zener will handle 330mA, far more than a 100mA fuse.
Crude, but effective.

I like it. Least number of components, 100% reliable!

[Station X]

Originally Posted by Oldcodger

Current through valve heaters - tens of milliamps? So fuse - 100mA.

What sort of valves are we talking about and how many?

[mole42uk]

Originally Posted by astral highway

I like it. Least number of components, 100% reliable!

A company I worked for used to make a fairly high-power electronic device for connection with crocodile clips to a 12V lead-acid battery.

My solution to the inevitable "connected it the wrong way around" was the old relay driven through a diode trick. Correct polarity, relay closes and supplies power to the electronics. Wrong polarity, relay doesn't close. A tell-tale LED which only lit if the polarity was wrong was deleted by the cost-controllers.

[Diabolical Artificer]

Were only talking two valves - ECC83 and an ECC82, each wired for series 12.6v then wired in parallel. So 12.6v at 300mA. As said earlier in thread they run off a DC PSU. Protection criteria for 20% over voltage = 13.9v .

Protection needed to be sensitive, don't really want 15v or more on NOS valves. The problem with a very near rated fuse is as you know valves draw more current cold so a 300mA fuse blows, as does a 360mA so have used a 400mA. Also a fuse rating is for current as drawn by circuit in question, but will blow at a higher fault current in X/time. I did the rough figures and felt there was too much uncertainty. It's further complicated by the fuses I have are rated for 250v at 50hz not 12.6v. Not sure how much this effects the sums, the rough ones I did were very complex for a generic 300-400mA "f" fuse.

As I was trying to protect against overvoltage I knocked up a crowbar circuit. It fits on a 1 1/4" x 1" circuit board, I could have made it even smaller using SM components but used through hole as this is what I had to hand. The other positive in favour a crowbar circuit has, that a zener or TVS doesn't, is that it gives the amp user an indication of a fault condition by not working, IE no sound OP. A zener would sit there clamping and the valves would be running at a higher voltage and you wouldn't know a thing about it unless a visual indicator is included. Finally a zener can have a 10% tolerance so for a 15v zener we could end up with 16.5v - 2.6v over our 12.6v 20% limit.

I didn't use a TVS for the same reasons as for a zener, that and I couldn't make head nor tail out the datasheet parameters and if I remember rightly couldn't find a 13v TVS.

Valve datasheets don't give any info about heater tolerances as a rule. I looked at five ECC82/83 datasheets which mentioned nothing. Whilst the two ECC etc valves survived 30 odd volts, the original fault that started this discussion, I felt safer using a crowbar to trigger at 13v.

Thanks for all your suggestions, there's some clever ways to protect circuits in this thread, Richards relay and diode being a good one.

Andy.