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| Is my 15KV neon light transformer broken ? |
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| BravoV:
Just scored this for free, and to be honest I was suspicious and curious why it's free ? :-// But I couldn't measure it as I scored it while I was on traveling, so I paid just the shipping charge to send it back home. With primary winding is rated for 220VAC at 2.25 Amp, while secondary with middle tap connected to chassis with 15KVAC at 30 mA, photo & details below. Took out my LCR meter (4 W) and started to measure all windings with these results : Question : - Is it normal for the primary winding's DCR at that low ? :-// - Also while still on DCR mode, measured the resistance between primary vs secondary, the LCR meter showed infinite resistance, but sometimes once in a while flipped/flickered to 50 to 60 Mega Ohm reading, shorted inter-windings ? |
| rsjsouza:
I don't have my NST here to test, but 2.5 ohms on a 450VA transformer is not really unusual. The AC impedance would be higher. The resistance between pri-sec is a bit more worrisome, but at those levels the ohmmeter can be very susceptible to external factors. If you have a Variac, all your questions will be answered. :) |
| Fraser:
This thread may be of interest. You can calculate the inductive reactance of the primary. https://www.physicsforums.com/threads/mains-transformer-with-1-4-ohm-primary-winding.924900/ HT transformers can arc across to the primary winding if they are not on separate bobbins and this used to happen in oscilloscope transformers. I would not expect a readable resistance at the voltage used in your meter test though. More likely it is your meter reading an infinite resistance and responding in an unpredictable way due to RFI etc. If in doubt, connect a 240V 25W light bulb in series with the primary winding when first applying power to the transformer. If the secondary RCA’s over to the primary no great harm will occur. It is not uncommon for these Neon drive transformers or output connecting cable insulation to fail, causing the classic flickering Neon sign. Yours may well be OK. Fraser |
| BravoV:
--- Quote from: rsjsouza on March 20, 2019, 01:33:58 pm ---The resistance between pri-sec is a bit more worrisome, but at those levels the ohmmeter can be very susceptible to external factors. --- End quote --- Yeah, cause I was expecting pri-sec should have infinite resistance, but what external factors are you talking about ? As the transformer was not connected to anything except the LCR meter. --- Quote from: rsjsouza on March 20, 2019, 01:33:58 pm ---If you have a Variac, all your questions will be answered. :) --- End quote --- Start at really low voltage at primary using variac, and slowly crank up while measure the secondary ? To be honest, all my meters are maxed out at 1KV, and quite scare as this is lethal voltage, still thinking how to properly isolate the output terminals before I start powering it. |
| BravoV:
--- Quote from: Fraser on March 20, 2019, 01:36:06 pm ---This thread may be of interest. You can calculate the inductive reactance of the primary. https://www.physicsforums.com/threads/mains-transformer-with-1-4-ohm-primary-winding.924900/ --- End quote --- Thanks for the link. So, my primary reactance based on my measurements above ... Reactance : = 2 x pi x freq x inductance = 2 x 3.14159 x 50 Hz x 0.414 Henry = 130.06 \$\Omega\$ So at 220 VAC the primary current will be -> 220V / 130.06 \$\Omega\$ = 1.69A ? Much lower than the rated current (2.25A) as printed at the label isn't it ? :-// --- Quote from: Fraser on March 20, 2019, 01:36:06 pm ---HT transformers can arc across to the primary winding if they are not on separate bobbins and this used to happen in oscilloscope transformers. I would not expect a readable resistance at the voltage used in your meter test though. More likely it is your meter reading an infinite resistance and responding in an unpredictable way due to RFI etc. --- End quote --- Are you saying at both isolated windings behave like sort of antenna, that caused that flickered high resistance reading as I experienced ? --- Quote from: Fraser on March 20, 2019, 01:36:06 pm ---If in doubt, connect a 240V 25W light bulb in series with the primary winding when first applying power to the transformer. If the secondary RCA’s over to the primary no great harm will occur. It is not uncommon for these Neon drive transformers or output connecting cable insulation to fail, causing the classic flickering Neon sign. Yours may well be OK. --- End quote --- Noted, will do, but as previous post, I will find a safe isolation caps 1st at that two HV terminals before powering this thing. Also currently I don't have any HV probe to test it's output, really tempting to build an adhoc Jacob Ladder though, just to test it's output. :palm: |
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