I picked up a used Hammond 169J isolation transformer that was wired for 100VAC. I changed it to 120VAC (the taps as follows Green (120VAC) and Blue (0VAC) ) , wired it up to an outlet in a junction box and connected it to my dimbulb tester. I plugged the transformer into my dimbulb tester and it lights up, I am getting 120VAC out of the transformers junction box plugs. Tested the outer junction box and transformer and no voltages. Plugged a table lamp into the transformers junction box outlet and it works fine. Any idea what my issue is if any? Thanks in advance
You will see it light up, the transformer has magnetizing current.
Isolation transformer first, then dim-bulb, then load.
Isolation transformer first, then dim-bulb, then load.
Thanks for the reply
Appreciate the reply, I was trying to test the transformer as I bought used and changed it to 120VAC. I wanted to ensure all was well with it. Thanks for the clarification on its magnetizing current.
Appreciate the reply, I was trying to test the transformer as I bought used and changed it to 120VAC. I wanted to ensure all was well with it. Thanks for the clarification on its magnetizing current.
An input fuse is needed on the transformer for safety, rated somewhat larger than the bulb's requirements.
169J is a 750VA transformer.
A transformer always takes some energy when voltage is applied.
Pencil VERY roughly 10%. That's 75VA, which if resistive would be 75 Watts.
How big is your incandescent lamp?
I sure can believe that the excitation of a 750VA's iron could glow a 40-Watt lamp.
You can still get 150W-200W incandescents in most of 120V land, though now in back/bottom of the lamps aisle. You could try that. Glow should be super dim.
Actually at 750VA I would expect excitation to be like 5% of max, so 37VA or 0.3A@120V. Hmmm, this will still make some glow in a 60W incandescent.
I DO have trouble with you having both glow and 120V out of the winding. A glowing 120V incandescent must be dropping at least 30V or "no glow". So you expect 90V, not 120V. You may be wired wrong.
A transformer always takes some energy when voltage is applied.
Pencil VERY roughly 10%. That's 75VA, which if resistive would be 75 Watts.
How big is your incandescent lamp?
I sure can believe that the excitation of a 750VA's iron could glow a 40-Watt lamp.
You can still get 150W-200W incandescents in most of 120V land, though now in back/bottom of the lamps aisle. You could try that. Glow should be super dim.
Actually at 750VA I would expect excitation to be like 5% of max, so 37VA or 0.3A@120V. Hmmm, this will still make some glow in a 60W incandescent.
I DO have trouble with you having both glow and 120V out of the winding. A glowing 120V incandescent must be dropping at least 30V or "no glow". So you expect 90V, not 120V. You may be wired wrong.
I have several bulbs, I started with a 350Watt bulb and there was no glow but it did heat up a little, warm to the touch. I then tried a 100Watt bulb (not overly bright) and it lit up as well as a 40Watt which lit up what appears to be full on. I will check the taps once again, but am fairly sure that the green and blue wires are for one 120VAC. The colours have faded over the years but I can still distinguish them. The primary side has no ground wire and a live and neutral connected as I originally received it, although I soldered the connections. I have left the transformer on for several hours monitoring it and there has been no heat build up or any voltage changes. Thanks for your reply.
Just to add to my comment, I need to confirm if I measured the voltage without the dimbulb. I will confirm after work and post my findings. I believe I got 120VAC without the dimbulb connected, but will confirm.
I tested my mains to confirm what it provides and I am getting 124VAC. Connecting the isolation transformer directly to mains with the green tap which should provide 120VAC I am getting 132VAC, with the brown tap which should be 115VAC I am getting 126.4VAC, and with the yellow tap which should provide 110VAC I am getting 120.6VAC. This is without any load and the isolation transformer connected directly to mains.
I tried plugging it into my dimbulb tester and with a 100W 120V bulb the voltage dropped to 98VAC, and with a 300W 130V bulb the voltage dropped to 115VAC. This is with the yellow tap providing 120VAC.
Is anyone aware of the reason this could be, and would it be okay to leave it with the yellow tap at about 120VAC?
Thanks for the help.
I tried plugging it into my dimbulb tester and with a 100W 120V bulb the voltage dropped to 98VAC, and with a 300W 130V bulb the voltage dropped to 115VAC. This is with the yellow tap providing 120VAC.
Is anyone aware of the reason this could be, and would it be okay to leave it with the yellow tap at about 120VAC?
Thanks for the help.
Phasors as normal for ac.
For purely reactive and purely resistive combined, its just pythagous.
For purely reactive and purely resistive combined, its just pythagous.
Looks like I am wrong. I asked the Idiot, using approximate values, and got 101V across the transformer.
This does not quite sum-up (19V across lamp violates my 30V assumption) but transformer voltage is indeed high, for these values.
note: the 59-61Hz sweep because this sim doesn't like single-frequency work.
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Open load voltage will be higher than nominal. This transformer is designed to make 120V with a 750VA load. Anything lower and the voltage will increase.
No Worries... Why not connect the 300W lightbulb as a load for the transformer and check the voltage?
Having the dimbulb connected to the transformer provides no change in voltage (with transformer to mains and dimbulb plugged into transformer outlet). Connecting the dimbulb to mains and then plugging in the transformer into the dimbulb, my voltage goes down from 131.8 to 125.8 (currently wired the transformer for 120VAC and reads 131.8 plugged into mains with no load). My real question is at what voltage should the transformer be at with no load for day to day working on stereo receivers? My thought would be 120VAC, but am not sure.
There must be some change with a 300W load though... Transformer to plug, 300W bulb across secondary?
It is like .1 of a volt. Just to confirm the 300Watt bulb is in the dimbulb tester, which is plugged into the transformers secondary. When I get home from work I'll use a lamp fixture with the 300W bulb and let you know the voltage draw.
No insulation transformer is wound with exactly a 1:1 turns ratio. To cope with sag, the secondary usually get's about 5 % more turns. Hence the secondary voltage is higher at idle.
Best regards!
Best regards!