Ultimate lamp limiter & Variac setup.

[6SN7WGTB]

How about "NOFGB"?

[stuarth]

For capacitor reforming, you can use a low impedance multimeter on a voltage range as both current limiter and current monitor. For example, an AVO 7 is 2mA fsd (unless you press the divide by 2 button). An AVO 40 is 6mA fsd. Either of those on the 400V range should be a good current limiter for most vintage radio electrolytics.

I have an isolated and variable mains supply with cutout and a pair of moving iron meters (IAVMSWCOAAPOMIM) which is my go to bench mains supply (useful for 110V stuff, live chassis stuff, and turning up the supply slowly on solid state stuff), and I have recently built an adaptor for it which is just a bridge rectifier, cap, bleed resistor and a small voltmeter in a plastic box.

I can use this adaptor for a HT supply for valve projects, valve testing, cap reforming, etc, and I could even power up the HT of a vintage radio without the heaters coming on (a harsh test for some caps further down the supply decoupling, but could happen in real life under some fault conditions, eg an open circuit in the output stage). With this and an old AVO in series, and a high impedance meter to check voltages throughout the circuit, I can reform electrolytics, check “that cap” and others, and safely check for high voltage breakdowns elsewhere in the unit.

I did think of adding the adaptor bits to the (IAVMS etc) box but decided against it because the modifying the box would be difficult, (probably need a new front panel which is reasonably thick steel - it has a 500VA Variac hanging on it), and the only thing it would save is the second voltmeter, and switching the existing meter between an AC supply on a 13A output socket to a DC supply on 4mm terminals would be messy.

I have yet to build a lamp limiter, but it on the list. The circuit from Alistair looks interesting.

Stuart

[Stockden]

How about YALL - Yet Another Lamp Limiter? Those with a computing background will understand the reference!

Hugh

[Lucien Nunes]

I have decided not to build an isolating transformer physically into the CLAVICLE itself. I already have an excellent 1.6kVA blue GRP-cased unit at the bench that the CLAVICLE can plug into. The transformer I was thinking of using is nowhere to be seen and was in any case of a much lower rating. The key to ensuring that the external transfomer is in circuit when needed is something I do already; use a mains lead uniquely colour-coded at both ends that can be identified at a glance as the isolated supply.

I am now considering whether the unit actually needs to be built into an overall box or whether mounting the lampholders, Variac and a control box separately on a board might be simpler. That would solve the puzzle of integrating the Variac which has no provision for panel mounting. I also like a very clear view of the lamps. It should ideally fit in the void at the back of the bench below the lowest shelf, next to the soldering station.

[Herald1360]

I've got one of those CM3000230 isolating transformers too! Mine gives equal prominence to the continuous rating of 1.5kVA as to the 3kVA intermittent rating though so maybe it's a later model?

I used to run my whole shed from it with its own secondary side RCD to avoid (another) serious ear'ole bending from SWMBO back in the day.

[Lucien Nunes]

We should be clear, for the benefit of anyone reading this who is not as familiar with isolating transformer practice, that running multiple pieces of equipment on an electrically separated transformer secondary (i.e. floating, not referenced to earth) is generally a bad idea. It can lead to an increased risk of shock as there is no provision to detect faults before they bite, as there is with normal mains via an RCD.

Referencing the transformer secondary to earth and equipping it with an RCD makes it safe for normal use, but prevents it being used as an electrically separate source for testing live-chassis sets and reducing shock risk during servicing.

I understand what @Herald1360 was getting at in the above post, but so as not to confuse the normal application of an isolating transformer to provide an electrically separated supply for devices under test, we probably shouldn't discuss other special uses in this thread.

In the meantime, various parts have arrived for the CLAVICLE. I will post some pics of trial layouts in the next few days.

[trobbins]

Originally Posted by Lucien Nunes

Referencing the transformer secondary to earth and equipping it with an RCD makes it safe for normal use, but prevents it being used as an electrically separate source for testing live-chassis sets and reducing shock risk during servicing.

That aspect certainly needs noting up front, as some users may infer that a DUT that powers up ok using the CLAVICLE is then ok to connect to a typical earthed mains supply using RCD protection. Post #7 goes to noting subsequent testing that really should also be performed.

[Radio Wrangler]

CLAVICLE is definitely better.

ACME creates mental images of The Road Runner getting frazzled with a finger in a socket.

David

[kellys_eye]

Would it be possible to use a split secondary isolating transformer and earth the centre tap? This way you can still use an RCD on the secondary but your maximum exposure iin the event of a fault would only be half the total mains supply?

[Radio Wrangler]

Any earthing on the secondary of an isolating transformer stops you being able to earth arbitrary places on whatever is being rested, for the purposes of connecting earthed test gear like oscilloscopes. It really needs to be isolated = floating for some tests. For others earthing and an RCD may give some protection, but then so does the RCD on a direct mains feed.

David