are dim bulb current limiters dangerous?

[georage]

since getting into the hobby of amp fixing and building ive been advised countless times that "you need a current limiter". i built one and its a great piece of kit. i selected a 300 watt bulb and it does a fine job of detecting and limiting overcurrents.
The thing is, no one ever talks about the danger of using a current limiting device like a dim-bulb. it limits the current and prevents a short circuit, but in doing so it defeats the circuit breaker's ability to clear a fault.
It could be that a human body has too much resistance to trip a 15amp mains breaker anyway and that a current limiter can make a shock safer, but ive never tested that... personally. i would just be interested in hearing any opinions on the matter of dim-bulb safety and use.

[The Dude]

I'm not sure I get the concern. The purpose of the LBL is to limit current in cases of extreme current draw. It's job is to prevent further damage to the gear you're working on and yes prevent constant fuse blowing or breaker tripping. So yes, "...... it defeats the circuit breaker's ability to clear a fault", but that's part of the LBL's job. As far as shock risk, you should be careful not to stick your fingers in dangerous spots whether you use a limiter or not. Remember this isn't end user stuff we're doing. It's part of the troubleshooting process and for techs only.

[Chuck H]

The bulb current limiter is a detection device for troubleshooting usually used by tech's. The Dude outlined it well above. No mention of personal safety has ever been attached to the light bulb current limiter circuit that I know of. Any assumption about that could only be an assumption. And anyone making that assumption has no business inside a tube amp. A diy novice following instruction is fine, But anyone assuming knowledge of limited current equaling personal safety is in over their head. This has never been implied at all.

JM2C

[glebert]

The LBL also only limits current from the AC mains. It does nothing to impede electrical flow from the big capacitors. Not a safety aid, unless it keeps the amp from catching fire.

[Mick Bailey]

300W is to large for a current limiter on music electronics - 100W is plenty, or 60W for smaller amps. It's a troubleshooting device in order to allow enough current to flow to operate the equipment just enough to take measurements at idle or under low-power operating conditions.

The output from a bulb limiter is just as dangerous as the unlimited mains as far as human contact is concerned. When working on defective equipment you have to assume and consider that there are fault conditions that can render a chassis or any other part 'live'. Also to consider, in the case of SMPS, part of the main PCB is always at mains potential and not isolated. A bulb limiter will not protect you from shock in any way, nor will it limit current to anywhere near the degree that would be safe for humans. The bulb's resistance is just a few ohms when hot, and drops when cold (the positive temperature coefficient is what makes it so useful). A 15A breaker won't trip anyhow with human contact. As you observe - the human body has too much resistance to draw enough current. That's why leakage trips which operate at a few mA (ours is 20 mA) are also included in mains supplies. My bench supply also has a dedicated earth leakage trip as a precaution.

The intention of a bulb limiter is to prevent the equipment from repeatedly blowing its own fuses. A fault that would trip a 15A breaker would be quite serious and be on the inlet side of the equipment's mains fuse, indicating perhaps a dead short. This type of fault must be approached with extreme caution and I wouldn't consider using a bulb limiter to troubleshoot this type of situation. The cause should be fairly straightforward to establish with simple observation or measurement with a DMM on unpowered equipment, and the equipment shouldn't be powered up until it's fixed.

[nosaj]

I never use more than a 40watt bulb and the reason being. When I do wiring change or new caps I put it on the dim bulb limiter power on get bright light then dimms down It's good then I switch over to regular power . That's all i ever use it for a safety device for my idiot errors.
nosaj

[Chuck H]

I never use more than a 40watt bulb and the reason being. When I do wiring change or new caps I put it on the dim bulb limiter power on get bright light then dimms down It's good then I switch over to regular power . That's all i ever use it for a safety device for my idiot errors.
nosaj

Once, in a remote location from my bench and under odd circumstances, I used a hastily assembled LBCL to slow charge an amp with old filters that had been sitting for two decades. It actually worked, sort of. The amp we fired up sans this treatment blew electrolyte all over it's innards almost instantly where the slow charged amp was actually playable. But this is the only case I've had where I had to use this circuit creatively. Otherwise it's a diagnostic tool.

[nosaj]

I was going to mention reforming caps, but decided not to. not sure why though.
nosaj

[Chuck H]

I was going to mention reforming caps, but decided not to. not sure why though.
nosaj

To be clear, it did NOT work to re form the caps into a useful condition. But these were twenty five year old gray IC caps!!! Surely crystalized into dust. The slow charged amp (six hours on the limiter) was playable, but not good. There were still obvious filter cap problems. The only up side is that the amp stayed intact. I replaced all the filters in that amp two days later.

[nosaj]

Kinda of like inflating a dry rotted tire, go slow and it pump up won't last but ...
nosaj

[georage]

if someone did want to turn their current limiter into a fuse, it would be simple enough to screw a house fuse into the bulb holder if they use the medium base. my limiter also has a big paddle switch on it for emergency shut downs and im considering building a gfci device into it one day so that if someone did recieve a shock it would click off. even a gfci wouldnt protect against a shock from the secondary of the transformer or the capacitors.

[The Dude]

FWIW: Just posting this so that people know there are options. I don't use a LBL. I use a current limiting variac. If you do repairs for a living, it's a good investment.

[nosaj]

FWIW: Just posting this so that people know there are options. I don't use a LBL. I use a current limiting variac. If you do repairs for a living, it's a good investment.

Could you provide a link?
I was under the impression you vary the voltage based on what the ammeter is showing.
Where as some of my dc power supplies have a current limiting feature where I can set the limit of current.
thanks,
nosaj

[The Dude]

OK, but I'll preface this by saying most of my test gear is Sencore brand. That is because Sencore is right here in my city and it's easy to get service and parts. IMO, they do make great test gear, but I want to be sure that I'm not perceived as a salesman and there are plenty of other great brands/products out there.

That said, I have a PR570.

Also, in answer to your question, you can monitor current while you bring up the voltage. Because of this feature, the current limit rarely trips because you usually kill power before you reach that point. Once you see that a repair piece is drawing lots of current as you bring up the voltage, you know to stop bringing the voltage up.

[Mick Bailey]

I use a sometimes use a LBL, or a regular Variac with current monitoring. Both are useful, and both can be equally misleading with some SS amps where the power amp DC supply will stick to one rail when run in this way (thinking Trace and Sessionette among others).