What does each of the basic crowbar circuit's components do?

Question

I have a general question about the key components of basic crowbar circuit below

I want to use a crowbar circuit for several DC to DC LM78xx linear regulators (various outputs, 7805 7809, etc.) to protect my devices against high voltage. At this point, I am more concerned about the general circuit behavior but I would highly appreciate if you could tell me what I need to pay attention when choosing components for my circuit.

Ok, Forget about components at the left side of the 7805. I know it is for AC to DC.

Please tell me what these components do during normal operation and during overvoltage condition? I want to know what happens during the normal operation while the zener is leaking current.

1. Fuse F1:

I have seen this fuse in various locations in various crowbar protection circuits. Is it important where to put it? I have seen it in this exact position, also after the output capacitor of LM7805 or even before the input capacitor of LM7805. Which place is more optimal?

What type of fuse is the best for these applications? I think Slow Blow fuses (like T1A) are better because I can choose them very close to my actual current consumption and it allows some transient without blowing.

2. Zener Diode D2:

I know how Zener Diodes work. I want to know how I should choose them to be optimal. Do you suggest TVS ? Can Zener in this circuit cause the output voltage of LM78xx raise or drop (I have seen this reported around the web)? Is it possible to use the Zener only configuration + Fuse for the crowbar circuit? Why not?

What are the most important points in selecting this zener diode?

What does the leakage current of the Zener do during the normal operation?

3. Resistors R1 & R2:

What do these resistors do exactly? Why would someone want to put R2 there? I have seen most of the circuits without R2. Is R1 by any chance for protection against gate floatation? If it is why some designs don't have this R2? What are the optimal values of these resistors? Should R1 > R2? or R1/R2 > some value?

4. Capacitor C4:

What does this C4 capacitor do? Should it be polarized? or is it better to be ceramic or film capacitors? How does choosing a high or low value of this C4 can affect the performance? Is it important for it to be low ESR or not?

5. SCR:

I am not familiar with this device. I only know that it conducts when the current gets to it! How should I choose one that is optimal? Can you tell me several models so I can start with?

What are the main benefits of SCR against a thyristor?

6. Limitations, disadvantages, and drawbacks:

What are the limitations, disadvantages, and drawbacks of this circuit?

Relatively unrelated question about C3 > C2?

Is it normal to use C3 > C2? Is there any reason for this? Haven't seen this!!

Thank you very much.

Answer 1

• Fuse F1

The fuse can be in various locations, as long as it is in the path of the current that the crowbar shorts. It could even be in the ground lead, though that is generally not done because it would disconnect the user ground from the source ground.

It could be argued that putting it before the regulator is better, because then it also protects against overheating when the regulator shorts internally.

I would select a fast fuse, but note that it must fuse at or below the guaranteed short-sircuit current that the regulator can supply (for a short time).

• Zener Diode D2:

The purpose of the zener is to conduct (only) when the voltage is above the trigger level. A TVS has a totally different purpose. The crowbar circuit has no effect on the regulators output voltage, that happens wehn you put a zener between the COM terminal of the regulator and the ground. A zender has a somewhat 'soft' characteristic, is not rated for a lot of current and it does not short, hence just a zener is not a good crowbar.

• Resistors R1 & R2:

The purpsoe of R1 is to leak the zener's leakage current to ground without triggering the thyristor. I guess R2 is to limit the SCR's gate current to a safe value.

R1 should be low enough so the zener's leakage at the nowmal voltage does not cause the SCR to trigger.

R2 should be high enough that the (short term) maximum trigger current is not exceeded with the highest expected (over) voltage.

• Capacitor C4:

My guess is that C4 is to 'short' transients that could otherwise trigger the SCR. I would not choose an elcotrolytical (polarized) type, otherwise I think it is not critical.

• SCR

The SCR will not be very critical, if you choose one that can handle the expected voltage and the fuses fusing current it will be OK. Check that the drop voltage at that current must be << 5V, an SCR rated for mains voltage might not be a good choice.

I guess you could use a thyristor, but most thyristors are meant for mains use so they would not be optimal.

• Limitations, disadvantages and draw backs:

This is a rather simple PSU: linear (will keep you warm in the winter). The crowbars trigger voltage is not very well defined (a voltage reference + opamp could do a much better job).

You could also ask why you need the crowbar at all, the only failure mode it seem to protect against is an internal short of the 7805 from input to output. Is that a likely failure mode, worth the extra components (which will have their own small contribution to the failure rate!)?

Answer 2

1. Fuse F1:

Is placed where shown to ensure that the SCR stays on until the fuse is completely blown. Disadvantage of that position is voltage drop across the fuse at high current.

Note that the fuse will only blow if the regulator becomes a dead short from input to output, not just loss of voltage regulation (eg. due to disconnected COM terminal).

2. Zener Diode D2:

Must trigger the SCR when output voltage rises above the maximum permitted. It should sized to drop the required voltage at the current required to trigger the SCR + current through R1.

The SCR probably needs around 0.9V at 2mA to trigger. The BZX83-C6V2 is specified to drop 5.8-6.6V at 5mA, but in this circuit it must pass 0.9V/100Ω = 9mA + SCR trigger current. Therefore the over-voltage trigger point may be 6.6V+0.9V = 7.5V or higher. This may be too high for sensitive ICs that are designed for 5V maximum (many of which break down at 6-7V). It might be safer to use a lower voltage zener, eg. 5.1V or 4.7V.

Zener leakage current is generally small so it shouldn't be a problem unless you use a very high power Zener, and then it won't be passing enough current to obtain its rated voltage anyway. You should select a Zener with I_z specified close to the current it will actually draw.

3. Resistors R1 & R2:

R1 sets Zener current at the SCR trigger voltage point. eg. if 0.9V is required on the Gate then I_zener = 0.9V/100Ω = 9mA (ignoring SCR Gate current).

R2 protects the SCR's Gate from excessive surge currents. It should be large enough to limit Gate current to a safe level, but small enough not to drop too much voltage at the triggering current.

4. Capacitor C4:

Slows down Gate voltage rise to prevent nuisance tripping due to short voltage spikes. If it is a polarized capacitor (eg. electrolytic or Tantalum) then it must be installed with correct polarity to avoid being damaged.

For greatest effectiveness its ESR should be less than the Zener's dynamic resistance. A standard 0.68uF electrolytic may be around 20Ω, which is probably a bit high. A low ESR capacitor will soak up higher voltage surges for longer (but is that what you want?).

5. SCR:

Must be able to take the peak crowbar current without damage, and possibly take 1A+ continuously (if the regulator limits current and prevents the fuse from blowing). However a high current SCR may require too much trigger current. A 3-6A device in To220 package should do the job.

6. Limitations, disadvantages and draw backs:

It adds several components to the power supply that probably aren't necessary, and may themselves cause trouble. The 7805 regulator has internal thermal protection and is unlikely to fail if used properly (I have only seen a few break down to over-voltage out of thousands).

The most likely cause of over-voltage spikes is feedback from the load, which should be dealt to with bypass capacitors, diodes etc. in the load circuit itself.

The crowbar may not operate fast enough to protect sensitive components. Making it more sensitive may lead to nuisance tripping.