0
\$\begingroup\$

I'm currently using a circuit breaker for low voltage (6V) but high current (60A) electrochemistry projects. My bridge rectifier is rated at 60A max. I'm aware that circuit breakers work in counter intuitive ways (they don't trip at exactly their rated amperage and instead are the Thermal - Magnetic type) and have read the NEC regarding their use and have the trip curve for my breaker.

In my applications, the amperage never exceeds 40A due to the resistance of the solution. The real problems are with:

The current circuit breaker I've selected is designed to trip immediately at over 50A (Thus saving the bridge in the event of a short of the electrodes), but trips intermittently during use due to its low rating.

I've been playing around with an idea, but I'm not sure if it will fry the bridge or not. In the trip curve it says that the breaker will trip in half a cycle or so above a certain amperage (lets say 200A) due to the magnetic force generated. If I have a circuit breaker rated at 50A and the transformer will put out 300A on a dead short will this protect the bridge if the solution's resistance prohibits such currents from flowing typically, aside for a short of the transformer secondary?

EDIT 0: The transformer secondary is connected to the breaker in series (before) the bridge (for clarity, the bridge is a full bridge rectifier with the typical 2 AC input, DC+ and DC- output model), then the DC current is used for electrochemistry and passed thorough a solution and back into the bridge and into the transformer secondary.

\$\endgroup\$
  • \$\begingroup\$ Please include a schematic of your idea. How is the transformer connected to the breaker? \$\endgroup\$ – The Photon Mar 21 '16 at 0:59
  • \$\begingroup\$ @ThePhoton Edited for breaker location. \$\endgroup\$ – Sarah Szabo Mar 21 '16 at 1:47
1
\$\begingroup\$

As your breakers trip on short duration high currents (e.g. 300A for half a cycle), one possible next step is to look more closely at the rating of your bridge rectifier, to determine if exceeding 60A for a short duration is fatal to it...

In the interest of having more eyes on your problem it'll be worth adding links to datasheets for your bridge and breaker.

However, in their absence ...

The normal use for a bridge rectifier is AC to DC conversion, feeding into a large reservoir capacitor to reduce AC ripple on the DC output. A consequence of this is that, on startup, there is a large surge of current until the capacitor is charged, and that current is limited by the sum of:

  • the capacitor ESR
  • the transformer winding impedance (resistance and leakage inductance)
  • the bridge rectifier diodes' slope impedance
  • the wiring resistance
  • the source impedance of the AC supply.

All of which are quite low. This amounts to a huge short-term stress on the bridge rectifier in the form of a high current pulse ... how does it survive?

Looking at the datasheet for one 60A bridge rectifier we find this specification:

Maximum Non-Repetitive Surge Current (1/2 Cycle, 60 Hz) [A] I TSM 800

So that's how. It can tolerate 800A (more than 10x its continuous rating) for half a cycle. Which is typical for a bridge rectifier (or even a bit on the mean side).


So, find the equivalent specification for your bridge rectifier, compare it to the trip envelope (current * time graphs) for your breaker, and you should have your answer, which is probably...

TL/DR : you're probably OK with just the breaker and bridge with no further complications.

If you want to test it by deliberately shorting electrodes, bear in mind you're playing with a small spot welder there and take appropriate safety precautions.

I can't comment on RDrast's suggestion of a fuse as an alternative to a breaker. Fuses have their own Time/Current envelopes, and a fuse may well be a sensible precaution in addition to the breaker.

\$\endgroup\$
  • \$\begingroup\$ I typically agree with the fuse concept. It's just that I have such a transformer and circuit breaker and was just curious if the bridge would survive in such an event. I think I'll go for the fuse idea, although not having to replace a fuse is a nice addition. I'm very careful to ensure that the electrodes never short anyway, it's only happened once so far (the electrode fell out of its housing), and I've been using the system for a few months. \$\endgroup\$ – Sarah Szabo Mar 21 '16 at 16:53
2
\$\begingroup\$

Circuit breakers are extremely poor choices for "Protecting Devices". They are properly used to protect from overloading wiring, which causes fires.

You would be much better off to use an ultra fast blow fuse, if you really want to protect a solid state device. Of course, then there is the cost of fuse replacement if it clears, but semiconductor rated fuses do work for protecting rectifiers/transistors/SCR's, et al.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.