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Old 14th Jun 2020, 11:33 am   #1
Jimbo
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Default Tingling T-03

I purchased a homebrew variable voltage power supply at a Radio Rally for the princely sum of a pound and had in mind to just re-use the aluminium case as the front panel was removeable and there were not too many holes elsewhere! Opening it up, however, revealed a rather nice 17 volt AC, 170VA toroidal transformer, and the T03 version of an LM338 5 Amp regulator on a decent heat sink. This seemed too good to waste so I decided to change my plans and tidy it up a bit to make a general usage bench PSU.

The original builder had used a model circuit straight out of the Texas Instruments data sheet for the LM338 so after a few safety checks I turned it on and found the voltage adjustment zone was set between about 9.5 and 12 volts.

The off-load voltage across the smoothing capacitor was 23.5 volts dropping to 21.6 volts with a 12 volt 21 watt incandescent car bulb as the load. I was slightly surprised by this as 21 watts is but a small fraction of the alleged 170VA capacity of the transformer.

After ten minutes with this load the regulator and heat sink were warm to the touch but I also got a definite ‘tingle’ from the case of the LM338 and my voltmeter showed about 4 volts AC between the LM338 case and chassis [mains earth]. Neither the positive or the negative side of the DC part of the circuit is grounded.

The bridge rectifier is a heavy duty metal bodied device bolted to [but isolated from], the chassis with large quarter-inch tags for spade connectors. Is it possible that there is a partial breakdown of this component that is giving some leakage of AC through to rest of the circuit and ultimately to the case of the regulator?

I will probably replace the rectifier and smoothing capacitor as part of the re-furbishment anyway, but I have not encountered the ‘tingle’ phenomenon before and would be grateful for any thoughts or guidance.


Jim
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Old 14th Jun 2020, 11:43 am   #2
GrimJosef
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Default Re: Tingling T-03

If you're getting a tingle then I'd be suspicious of the 4 VAC number. That's not a large enough voltage to give you a tingle you can feel (except on your tongue, which you should never put near anything electrical). Are you absolutely certain that the chassis is securely grounded ? It really needs to be.

Cheers,

GJ
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Old 14th Jun 2020, 11:48 am   #3
Cobaltblue
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Default Re: Tingling T-03

The volts drop across the capacitor will not be so much to do with the transformer but the average voltage of across the capacitor which on load will have increased ripple.

If you want an idea of the transformers regulation you need to be looking at the change in the AC output of the transformer off load and on load.

Cheers

Mike T
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Old 14th Jun 2020, 12:05 pm   #4
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Default Re: Tingling T-03

With the whole secondary side of the PSU floating and a large toroid with appreciable primary-secondary capacitance, I suspect that the secondary circuit is being floated to a few tens of volts of high-impedance AC, hence the tingle, and that the impedance is sufficiently high to not harm you and to drop to 4VAC when the meter is applied. Quite often in a situation like this where the output side needs to be uncommitted to earth, you still find a parallel RC combination from some point in the circuit to chassis to prevent things like this and also untoward static accumulation.
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Old 14th Jun 2020, 1:14 pm   #5
ChristianFletcher
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Default Re: Tingling T-03

My quick answer is ground the case and also check you plug socket for an earth fault etc. I typically ground one side of the secondary in my power supplies unless you have a particular reason to have it floating.
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Old 14th Jun 2020, 1:35 pm   #6
Chrispy57
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Default Re: Tingling T-03

Hi Jimbo,
it would be interesting to check the voltage to chassis from each side of the DC output, then, as suggested above, ensure that the chassis grounding is perfect - clean metal to metal contact at that point, and less than 0.1 Ohms back to the mains plug earth pin (also cleaned up if necessary).

If there is no good reason for having a floating output it makes sense to ground the negative side of the output (as shown in the data sheet).

Good luck
Chris
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Old 14th Jun 2020, 2:46 pm   #7
Jimbo
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Default Re: Tingling T-03

Thank you all for your prompt and helpful responses.

How fortunate we are to have a forum with so much accumulated knowledge and experience, and better still, users who give freely of their time to share that knowledge with the 'rest of us'!

I will do a back-to-basics check on all the grounding connections, [including the fact that my workbench is remote from a ring main connection so I am doing everything at the end of 15 metres of extension cable.....you never know...!], and I will report back on my findings.

Thank you again

Jim
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Old 16th Jun 2020, 5:28 pm   #8
Jimbo
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Default Re: Tingling T-03

An update on progress, and a lesson in reading data sheets!

I don't possess a precision meter for reading very low resistances, but the plug-top to chassis resistance of the earth lead on my PSU hovered between zero and 1 ohm which is the smallest readable increment on my meter, and since both the live and the neutral conductors gave similar readings, I did not feel there were any gross faults present. [My 15m extension lead -salvaged from a Qualcast Electric Mower nearly 50 years ago- registered about 3 ohms on each conductor which bore reasonable comparison with other leads of similar length. The conductors were red, black and green so a P.A.T Inspector would probably have apoplexy, but the old cable is of the type that has three lengths of string twisted in with the wires which helps to avoid kinking so I have always kept it!]

Grim Josef was right to question the validity of the 4v AC that I had observed in my first post. I noticed that when I first touched the metal case of the T-03 regulator that my digital volt meter momentarily flickered with a much higher value before settling down to the 4v that I had noted. Similarly, I followed Chrispy57’s suggestion and measured from chassis to each side of the DC output and found exactly the same phenomena.

My cheap digital volt meter only has 200 and 600 AC volts ranges so I was beginning to understand that the AC voltage that was present [enough to give me a tingle] was of high impedance, as suggested by Turretslug, and was being damped by the meter. [I know that we reckon DVMs to be of high impedance on DC volts ranges but I don’t know if this the case on AC ranges as well, -particularly with budget type equipment].

My good old Telequipment Oscilloscope, [probably only just a bit younger than my extension lead!] started to give me some sensible answers at last.

The peak to peak AC voltage on the case of the regulator and either of the two DC output terminals was of the order of 55 to 60 volts and was a slightly noisy, but nevertheless distinctive 50 Hz sinewave. This was probably just enough to give the tingle, [although, subjectively, this was less noticeable standing in slippers at my carpeted first floor ‘radio bench’ than it had been at my concrete floored garage work bench wearing safety-trainers]. It also seemed to confirm that the source of the AC voltage was being inductively / capacitively coupled from one part of the circuit to another rather than ‘leaking’ from the bridge rectifier as I had initially surmised.

Closer inspection of the construction showed that the original builder had provided the suggested protection diodes across the input/output and output/adjust terminals of the regulator, but not the input to ground stabilising capacitor. Most of the construction was point-to-point wiring on the terminals of the voltage adjustment potentiometer on the front panel which thus necessitated long wires to the input, adjust and output terminals of the regulator mounted on its heat sink on the rear panel of the case, and all three long wires trailed full length of the case, directly over the top of the transformer…….I think I may have found the source of the problem!

I will still salvage the nice toroidal transformer and the LM338 regulator, but will now intend to rebuild the PSU and reconfigure the component layout to take into account the regulator manufacturer’s guidance on positioning of certain elements to avoid these unwanted secondary and stability effects.

This has been a most interesting learning exercise for me, and I am most grateful to you all for encouraging me to be a bit more analytical in my fault finding approach.
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Old 17th Jun 2020, 1:26 pm   #9
Chrispy57
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Default Re: Tingling T-03

Hi Jimbo,
well done for sticking with it and understanding and overcoming the problem rather than ignoring it. It is no doubt a very satisfying educational experience that will serve you well in the future too.

Cheers
Chris
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