I worry a bit about the forward current on the Zener. I guess that with the values put in, the current will be small enough.
Also if you don't mind a little leakage:
Modified version:
It relies on the device capacitances to hold the NPN on through the zero crossing.
MooseFET a écrit :
Under normal working conditions (no tripping/limiting) the supply has no current to provide (just biasing the gate), so the average (or even peak) current can be made 1mA without any pb.
Ok, but it doesn't trip, so you'll need some very serious heat sinking under fault conditions.
-- Thanks, Fred.
I'll put the goggles on.. :) D from BC
That's fast... Fast enough to be a fuse protector.. :)
D from BC
Ooops..I didn't know this was an auto reset breaker.. I was looking for negative feedback..I thought this was a limiter(current regulator).. I didn't recognize the SCRish transistor arrangement..with it's own positive feedback loop.
Biasing the NPN base??
That's from when I mistakened this as a linear current limiting circuit..
This circuit is probably going to keep me busy for hours.. It's all in the bridge :)
What's the history on this circuit? Textbook? Well known? Own invention? Magazine? Old? New?
D from BC
D from BC a écrit :
Uh, do we really need to copy such circuits from somewhere? Just sketched it in 2 minutes.
-- Thanks, Fred.
2 minutes!!
My imagination failed me :( It's a mental block or something..Or I have too much DC in the brain...:) It's the lead.... :P
It's like shorting a bridge out...that's wack!! :) Obviously...I have only used bridges in basic applications..
D from BC
Bussmann TPS fuses clear in less than 5ms. TPS series are 170VDC rated fuses. In our test setup, we shorted out a 2.5F cap bank charged up to
100VDC. Made a tiny nick in the copper shorting block. Peak current was around 2k Amps (don't quite remember the details). Fuse worked well!-- Mark
It should be noted that microsecond turnon is usually a problem for SCRs; either a photoSCR or an integrated SCR/Zener combination (SIDActor type) will perform best with rapid turnon requirement.
Either a fuse, OR a positive-tempco polyfuse should work well; if you use the polyfuse, a few seconds of power-off will reset the protection.
[=2E...]
I've added some "*"s
No, it does trip. Note the resistor marked with the "*"s. When there is a drop on the MOSFET, it biases the NPN on thereby providing the positive feedback needed for the latching action.
MooseFET a écrit :
Ah, yes missed that. Then it's perfect.
-- Thanks, Fred.
You might also need to decouple the supply well in case glitches set of the triacs and blow the fuses.
Of course. I'm getting better at the noise reduction layer in electronics design..
What might be a kickass combo would be the bridge based breaker circuit(as posted in this thread) followed by a triac shunt. But..probably overboard...
I'm currently playing with mutations of the bridge breaker circuit.. Such as: RL current sensing Turn slew control Low current consumption control electronics Single IC solutions D from BC
Have you tried telephoning or emailing any fuse manufacturers? Every time i have asked one they were very helpful.
-- JosephKK Gegen dummheit kampfen die Gotter Selbst, vergebens. --Schiller
Doh!
Hopefully, I won't need any fuses if I cook up an AC limiter/breaker based on posts in this thread.
I've cooked up a 2 op amp latchup circuit for inside the bridge. It has massive positive feedback for latch up. This allows a cool 0.1ohm current sense resistor. Thermal stability and precision current trip point are also benefits. Faster speed can be accomplished with high speed comparators. I've come up with a single IC circuit with no discrete small signal transistors.
D from BC
Don't knock the discrete transistor approach. To turn the Mosfet off fast you need to pull the gate charge out, which takes relatively high peak currents. The discretes give you this capability.
Slightly off topic: when I were but a slip 'o a lad, I worked for a summer in a test lab. One fun bit was helping a Real Engineer test a circuit breaker - a 1A model was the target. First we checked that it tripped properly at the nomial current, then came the fun bit.
A circuit breaker has to break the circuit, especially under fault conditions. That means you have to check that it does so under not just reasonable conditions, but also unreasonable ones.
The Real Engineer's test area was surrounded by a metal cage, and in back a door led into a vault which had double walls, and was filled with a couple of tons of heavy duty batteries, switchgear and massive bussbars through the walls into the test area.
We spent a morning making sure the batteries were all fully charged, and connecting up the poor little unsuspecting Consumer Unit
1A circuit breaker in a safety chamber. We then retired to a "shack" within the test area, and threw the switch. ISTR the current wasa "pass" as it broke the circuit ;-)
Regards Ian
I suggest you use the comparitor. Dollar for dollar, they are usually much faster than op-amps. IIRC, you can get a comparitor and reference in one package from Linear.
IIRC: The LTC1440
It seems to me that if you find the right comparitor, one comparitor will do. You only need to find some way to do the positive feedback. Many comparitors have inverted and non-inverted outputs.
If you use the trick of using the drain voltage of the MOSFET as the place to take the positive feedback from, you may be able to make the circuit reset on each zero crossing.
BTW: I think you also need a stout MOV across the bridge. On an inductive load, the turn off could be a bit exciting.
True...That MOSFET needs to be pulled down hard for fast switch off.. But...I've come up with another solution for that bridge based AC limiter/breaker... Using a Mosfet driver chip! Some of the mosfet driver chips have all the features I need to make an AC breaker/limiter such as:
hard switch off cool resistor current sensing overcurrent latched shutdown.. internal reference error output Power rail ripple rejection Maybe glitch rejection Maybe low idle current??
This reason why I gravitate toward IC solutions is:
About that high peak amp breaker story.. Why did it the breaker circuit blow up but saved the unit? No fuse pop? Slow breaker? Shunt type? Goofy design? D from BC
Breaking an inductive load = zap! So.yeah...MOV for sure..
I might drop the comparator idea and use a mosfet driver IC with current sensing.
But let's say I keep going with comparators... I believe you're right that only one comparator with a positive feedback arrangement can turn off the mosfet. I might use a comparator with differential outputs.
The 2 op amp latching circuit I mentioned is a little goofy.. Op amp 1 works in linear mode and limits the MOSFET current.. There is a lot of negative feedback.. Op amp 2 senses the Mosfet gate in linear mode and with positive feedback (gain^2) shuts down op amp 1 to shut off the mosfet. Manual reset required.. (But that's ok)
The general idea is that a temporary linear mode may clip the current spikes but may not be enough to trip the latch for breaking. It's a way of getting overcurrent protection with fewer interruptions. Note: This is employed in some Mosfet drive IC's.
D from BC
Mosfet driver chip sounds good.
On the circuit breaker test, the breaker has an electrical path, a current sense for that path (usually magnetic), and a switch.
The issue is whether the switch opens at high overcurrent, or if the contacts get welded shut. You have to test a number of breakers at steadily increasing currents to make sure there is no intermediate fault current that produces the welding action. The final test blew the breaker to pieces, but the circuit was broken so the result was a "pass".
Regards Ian
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