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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Isolating Transformer
I was working on an CRT TV using an isolating transformer. I happened
to measure the voltage between the chassis 'dag' and a nearby metal fixture and was surprised to find 75Vac. Without the transformer the reading was 110Vac. Something is wrong, right? BobH |
#2
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Isolating Transformer
In article , pfm wrote:
I was working on an CRT TV using an isolating transformer. I happened to measure the voltage between the chassis 'dag' and a nearby metal fixture and was surprised to find 75Vac. Without the transformer the reading was 110Vac. Something is wrong, right? BobH You shold be able to trace it out. I'm glad you said isolation transformer rather than isolation transformer. You want ground isolation because the primary is referenced to ground. You can check out some resistances with the AC removed. greg |
#3
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Isolating Transformer
On Dec 18, 3:19*pm, (GregS) wrote:
In article , pfm wrote: I was working on an CRT TV using an isolating transformer. I happened to measure the voltage between the chassis 'dag' and a nearby metal fixture and was surprised to find 75Vac. Without the transformer the reading was 110Vac. Something is wrong, right? BobH You shold be able to trace it out. I'm glad you said isolation transformer rather than isolation transformer. You *want ground isolation because the primary is referenced to ground. You can check out some resistances with the AC removed. greg On Dec 18, 3:19*pm, (GregS) wrote: In article , pfm wrote: I was working on an CRT TV using an isolating transformer. I happened to measure the voltage between the chassis 'dag' and a nearby metal fixture and was surprised to find 75Vac. Without the transformer the reading was 110Vac. Something is wrong, right? BobH You shold be able to trace it out. I'm glad you said isolation transformer rather than isolation transformer. You *want ground isolation because the primary is referenced to ground. You can check out some resistances with the AC removed. greg Thanks Greg but I'm not sure what you are saying. I use the terms 'isolating' and 'isolation' interchangeably. I thought the purpose of an isolating/isolation transformer is to ensure that no potential difference can exist between the device under test and a grounded object such as a concrete floor or a water pipe. The risk of shock is restricted to within the device under test. |
#4
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Isolating Transformer
pfm writes:
On Dec 18, 3:19*pm, (GregS) wrote: In article , pfm wrote: I was working on an CRT TV using an isolating transformer. I happened to measure the voltage between the chassis 'dag' and a nearby metal fixture and was surprised to find 75Vac. Without the transformer the reading was 110Vac. Something is wrong, right? BobH You shold be able to trace it out. I'm glad you said isolation transformer rather than isolation transformer. You *want ground isolation because the primary is referenced to ground. You can check out some resistances with the AC removed. greg On Dec 18, 3:19*pm, (GregS) wrote: In article , pfm wrote: I was working on an CRT TV using an isolating transformer. I happened to measure the voltage between the chassis 'dag' and a nearby metal fixture and was surprised to find 75Vac. Without the transformer the reading was 110Vac. Something is wrong, right? BobH You shold be able to trace it out. I'm glad you said isolation transformer rather than isolation transformer. You *want ground isolation because the primary is referenced to ground. You can check out some resistances with the AC removed. greg Thanks Greg but I'm not sure what you are saying. I use the terms 'isolating' and 'isolation' interchangeably. I thought the purpose of an isolating/isolation transformer is to ensure that no potential difference can exist between the device under test and a grounded object such as a concrete floor or a water pipe. The risk of shock is restricted to within the device under test. There's always capacitive and inudctive coupling. You were probably measuring with a high-Z DMM. Put any sort of load between the meter probes and the voltage will go to 0. If it does not, you do have a serious safety issue. -- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/ Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/ +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs. |
#5
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Isolating Transformer
"pfm" I was working on an CRT TV using an isolating transformer. I happened to measure the voltage between the chassis 'dag' and a nearby metal fixture and was surprised to find 75Vac. Without the transformer the reading was 110Vac. Something is wrong, right? ** Wrong. DMMs typically have 10 Mohms impedance across the leads when in any of their AC volts ranges - so a reading of 75 volts AC indicates there is leakage of about 7.5 uA of AC current. Equates to about 600 pF of capacitance ( primary to secondary) in the iso tranny. Try connecting a 0.1 uF cap from the TVs chassis to ground - that should swamp the effect. ..... Phil |
#6
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Isolating Transformer
On Dec 19, 10:29*pm, "Phil Allison" wrote:
"pfm" I was working on an CRT TV using an isolating transformer. I happened to measure the voltage between the chassis 'dag' and a nearby metal fixture and was surprised to find 75Vac. Without the transformer the reading was 110Vac. Something is wrong, right? ** Wrong. DMMs typically have 10 Mohms impedance across the leads when in any of their AC volts ranges *- *so a reading of 75 volts AC indicates there is leakage of about 7.5 uA of AC current. Equates to about 600 pF of capacitance ( primary to secondary) in the iso tranny. Try connecting a 0.1 uF cap from the TVs chassis to ground *- *that should swamp the effect. .... *Phil Thanks Sam and Phil, you were right, it was just stray voltage. |
#7
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Isolating Transformer
In article , pfm wrote:
On Dec 18, 3:19=A0pm, (GregS) wrote: In article = ..com, pfm wrote: I was working on an CRT TV using an isolating transformer. I happened to measure the voltage between the chassis 'dag' and a nearby metal fixture and was surprised to find 75Vac. Without the transformer the reading was 110Vac. Something is wrong, right? BobH You shold be able to trace it out. I'm glad you said isolation transforme= r rather than isolation transformer. You =A0want ground isolation because t= he primary is referenced to ground. You can check out some resistances with the AC removed. greg On Dec 18, 3:19=A0pm, (GregS) wrote: In article = ..com, pfm wrote: I was working on an CRT TV using an isolating transformer. I happened to measure the voltage between the chassis 'dag' and a nearby metal fixture and was surprised to find 75Vac. Without the transformer the reading was 110Vac. Something is wrong, right? BobH You shold be able to trace it out. I'm glad you said isolation transforme= r rather than isolation transformer. You =A0want ground isolation because t= he primary is referenced to ground. You can check out some resistances with the AC removed. gregI 'isolating' and 'isolation' interchangeably. I thought the purpose of an isolating/isolation transformer is to ensure that no potential difference can exist between the device under test and a grounded object such as a concrete floor or a water pipe. The risk of shock is restricted to within the device under test. I was suggesting that you may be using the wrong kind of transformer. We went all through this recently. Most all isolation transformers sold to the public are NOT isolated. They are for noise reduction. You seem to be correctly talking about a transformer that does not have the secondary tied to ground. It can easily be checked by resistance measurment. I was just trying to troubleshoot your problem, and wanted all the facts. greg |
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