Protect Yourself Too...
Even more useful is to connect a lamp socket in series with the input side of the variac, and a decent power capable isolation transformer on the output of the variac.
The lamp socket can be bypassed by a switch, and various sized lamps fitted for protecting differing power loads - in practice, a 60W lamp is useful for protecting most loads.
I have used exactly this setup for decades and it has proven its worth countless times when running up newly built or rebuilt amplifiers.
Dave.
Even more useful is to connect a lamp socket in series with the input side of the variac, and a decent power capable isolation transformer on the output of the variac.
The lamp socket can be bypassed by a switch, and various sized lamps fitted for protecting differing power loads - in practice, a 60W lamp is useful for protecting most loads.
I have used exactly this setup for decades and it has proven its worth countless times when running up newly built or rebuilt amplifiers.
Dave.
Hi Dave
So the lamp is acting as a fuse? but can be bypassed? I have a fuse on the input side of the variac. I'm not sure how this is better😕
Can you upload a photo?
Cheers
So the lamp is acting as a fuse? but can be bypassed? I have a fuse on the input side of the variac. I'm not sure how this is better😕
Can you upload a photo?
Cheers
The lamp filament is a PTC.
If it carries a significant current it heats up and absorbs most of the mains voltage, leaving only a small AC voltage across your Variac or across your mains transformer.
This massively reduces the destructive capability of your PSU to damage the mis-wired equipment downstream. It even protects the mains fuse from blowing !
If it carries a significant current it heats up and absorbs most of the mains voltage, leaving only a small AC voltage across your Variac or across your mains transformer.
This massively reduces the destructive capability of your PSU to damage the mis-wired equipment downstream. It even protects the mains fuse from blowing !
read Decibel Dungeon.
He shows a bulb tester with all the bells and whistles, i.e. two extra switches.
My bulb tester has no switches. Both styles do exactly the same job. Insert a PTC into the mains feed of a suspect piece of equipment.
He shows a bulb tester with all the bells and whistles, i.e. two extra switches.
My bulb tester has no switches. Both styles do exactly the same job. Insert a PTC into the mains feed of a suspect piece of equipment.
Cheap And Effective..
The point of the globe is that it acts as a PTC current limiter.
Most amplifiers (except class A) amplifiers will cause the globe to barely glow at idle if operating conditions are correct.
However if an amplifier is faulty it will usually attempt to draw max current, and in this case most of the dissipation will be in the globe and not in the output stages, thus saving output transistors from (expensive and needless) destruction.
When it is established that an amplifier is working correctly at no signal and low level signal, the lamp can then be bypassed by a switch and then run up to full power.
Keeping various rated lamps in series with the mains supply is also useful for exploring how amplifiers behave with varying AC supply voltage versus output power.
When first time running up of very high power amplifiers this protection is mandatory to avoid expensive destruction of multiple output transistors.
If an amplifier is faulty this current/power limiting is also useful for running the amplifier stage long term whilst diagnosing faults.
Understand and try this method and you won't go back.
Dave.
The point of the globe is that it acts as a PTC current limiter.
Most amplifiers (except class A) amplifiers will cause the globe to barely glow at idle if operating conditions are correct.
However if an amplifier is faulty it will usually attempt to draw max current, and in this case most of the dissipation will be in the globe and not in the output stages, thus saving output transistors from (expensive and needless) destruction.
When it is established that an amplifier is working correctly at no signal and low level signal, the lamp can then be bypassed by a switch and then run up to full power.
Keeping various rated lamps in series with the mains supply is also useful for exploring how amplifiers behave with varying AC supply voltage versus output power.
When first time running up of very high power amplifiers this protection is mandatory to avoid expensive destruction of multiple output transistors.
If an amplifier is faulty this current/power limiting is also useful for running the amplifier stage long term whilst diagnosing faults.
Understand and try this method and you won't go back.
Dave.
Guys, thank you all for the advice.
After all your posts I decided that I should sort this out sooner rather than later. I'm keeping an eye out for a 100va (would 50va 230v output be ok for most applications?) transformer on local auction site. I have added a lamp socket in series with the variac feed and also a bypass switch. Thanks again Andrew I did have a read on the Decibel Dungeon site and see why you are so keen on this.
I tested the power supply tonight on a project that has been on the bench for a while. It is DAC from http://members.chello.nl/~m.heijligers/DAChtml/dactop.htm
I was fairly sure there was an issue with it and so tested it on the new supply, no blown fuse just a glowing 40w lamp! I see the benefit now, as I will be able to break the job down into parts and test them individually until I find the error.
So thanks again all
After all your posts I decided that I should sort this out sooner rather than later. I'm keeping an eye out for a 100va (would 50va 230v output be ok for most applications?) transformer on local auction site. I have added a lamp socket in series with the variac feed and also a bypass switch. Thanks again Andrew I did have a read on the Decibel Dungeon site and see why you are so keen on this.
I tested the power supply tonight on a project that has been on the bench for a while. It is DAC from http://members.chello.nl/~m.heijligers/DAChtml/dactop.htm
I was fairly sure there was an issue with it and so tested it on the new supply, no blown fuse just a glowing 40w lamp! I see the benefit now, as I will be able to break the job down into parts and test them individually until I find the error.
So thanks again all
Any cheap "working" transformer that isolates your mains supply from your low voltage variable AC output will do. 50VA and 18-0-18Vac is 1.4Aac. That's a lot of current for an auxiliary supply. 150VA 12-0 & 12-0 is 75VA per winding for 6.25Aac.
Thanks again, still working on that one.
I have now striped the dac boards from each other and the transformer and still the lamp is ever so slightly glowing with just the transformer connected
It is brand new I had it wound by Airlink in the UK before I left???
I have checked the 6N1P connected to a old school 6.3v transformer and the lamp glows then dims to nothing, I have check the resistance of each winding (unable to test for shorts as lopt meter is one of those projects on the shelf!). Any reason why this could happen?
I have now striped the dac boards from each other and the transformer and still the lamp is ever so slightly glowing with just the transformer connected
It is brand new I had it wound by Airlink in the UK before I left???
I have checked the 6N1P connected to a old school 6.3v transformer and the lamp glows then dims to nothing, I have check the resistance of each winding (unable to test for shorts as lopt meter is one of those projects on the shelf!). Any reason why this could happen?
What is connected to the transformer? Mains to the Primary and what to the secondary?
What is the power of your test bulb?
I don't think the bulb should be glowing if the secondary is open circuit.
Can you measure the mains voltage dropped across the test bulb and across the transformer primary?
Can you measure the mains voltage dropped across the test bulb and across the transformer primary?
This is the most wisdom , from someone who uses variacs to trouble shoot all forms of gear on a daily basis. The analog ammmeter and re-setable breaker that is. Forget the voltmeter, for most purposes you can almost get away with marking voltages on the dial.
Ok, ammeter is next on the list. I see exactly what Enzo ment now😉
Andrew,
Power supply set on 230Vac with lamp bypassed.
With lamp connected reads 222Vac (so 8Vac drop for the 40 watt lamp)
Connect transformer, primaries read 222Vac on DMM, power supply display reads 216Vac
Connect EZ90 (load) power supply displays 207Vac, transformer primary reads 256Vac on DMM😕
Cheers
Andrew,
Power supply set on 230Vac with lamp bypassed.
With lamp connected reads 222Vac (so 8Vac drop for the 40 watt lamp)
Connect transformer, primaries read 222Vac on DMM, power supply display reads 216Vac
Connect EZ90 (load) power supply displays 207Vac, transformer primary reads 256Vac on DMM😕
Cheers
8Vac across the 40W bulb seems a bit high.
That indicates current passing through the primary and coming out as heat from the primary and heat from the core. There is no heat coming from the secondary/ies, they are open circuit.
Can you replace the bulb with a fixed power resistor (try 10r). Using the Variac to vary primary voltage from Zero to mains voltage +10%. Plot current vs primary voltage.
This is all at mains voltage, find a way to do this safely.
That indicates current passing through the primary and coming out as heat from the primary and heat from the core. There is no heat coming from the secondary/ies, they are open circuit.
Can you replace the bulb with a fixed power resistor (try 10r). Using the Variac to vary primary voltage from Zero to mains voltage +10%. Plot current vs primary voltage.
This is all at mains voltage, find a way to do this safely.
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Ok I will try this tomorrow as it’s Friday night here = beer = not safe to test power supply!
By the way, ended up finishing the Lopt meter this afternoon (stress management!) tried a few different transformers, this one failed so I removed it from the mounting bracket and put it aside. After re-reading a few notes on the dac I decided to test it one more time, this time 8 rings??? Very strange. I tested it a few more times and it passed all of them, could it have been clamped to tightly into the bracket??? So I powered it back up, all seemed ok, lamp was very very dim this time! Ok power it up with a load and back to square one! Can a transformer self heal??? Mad thought.
Ok I will get back to you tomorrow with current verses voltage plot.
Cheers
By the way, ended up finishing the Lopt meter this afternoon (stress management!) tried a few different transformers, this one failed so I removed it from the mounting bracket and put it aside. After re-reading a few notes on the dac I decided to test it one more time, this time 8 rings??? Very strange. I tested it a few more times and it passed all of them, could it have been clamped to tightly into the bracket??? So I powered it back up, all seemed ok, lamp was very very dim this time! Ok power it up with a load and back to square one! Can a transformer self heal??? Mad thought.
Ok I will get back to you tomorrow with current verses voltage plot.
Cheers
Hi, although unlikely, it is possible that two secondary wires at the outside edge bore the brunt of the very tight clamping force and the two enamel surfaces scraped together such that a short between the windings was created. This would act as a shorted turn and increase massively, the primary current. Unclamping would release the load squeezing the winding turns together and now air is acting as the insulator.
Re-clamping just changes the air gap.
If this is an explanation, then sorry I have no way of testing for proof.
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