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| Testing back to back power transformers for use as an isolation transformer |
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| malagas_on_fire:
Thats a neat redundant psu soldar. I have two sets of back to back transformers but with 6VA and 12VA only, in back to back since they are reduction transformers 230 to 6V and the other are 230 to 12V . For testing triacs with Dimmable bulbs, capacitive dropper PSU's with small loads they quite do the job. |
| nctnico:
--- Quote from: soldar on May 10, 2019, 02:14:21 pm --- --- Quote from: David Hess on May 09, 2019, 04:34:08 pm ---... they support 120V/240V with split windings, the output side actually provides two 120 volt AC isolated outputs or isolated 240 volts AC if needed from a 120 volt AC line. --- End quote --- I have a couple of similar transformers with two separate primary windings which con be configured in series for 230 V or in parallel for 115 V. Let us call the windings of one transformer A and A' and those of the other transformer B and B'. I connect A and B in series to the 230 V mains and connect A' and B' in series and get 230 V isolated from the mains. It is more efficient than connecting the transformers back to back. --- End quote --- This doesn't sound safe to me because the primary windings won't have a safety isolation barrier at all. This is also true for any regular transformer when considering the primary / seconday windings unless it is rated as being double isolated. Just don't mess with isolation transformers. Differential probes are the way to go because they much safer compared to floating the DUT and they aren't that expensive any more. A good isolation transformer probably costs more. |
| soldar:
--- Quote from: nctnico on May 11, 2019, 01:06:07 pm --- This doesn't sound safe to me because the primary windings won't have a safety isolation barrier at all. --- End quote --- True but in this case I was not really looking for strong isolation as both sides were at mains voltage. I just needed some very basic isolation to prevent GPCI faults. |
| bob91343:
I can see two reasons to isolate. One is the obvious safety concern over fault currents. The other is common sense regarding interconnecting test equipment. The former is important in permanent installations but not so much for test setups. I only use an isolation transformer when running tests on ac-dc radios and similar gear. For all other purposes, it isn't important to my operation. The only exception to this was when I was repairing one of my Boonton Q meters. There is a constant voltage power transformer in it that failed, and I decided I didn't need regulation and so I bypassed it. Mistake. I sacrificed the isolation it provided. I found out inadvertently when feeling a tingle while making some connections. So I bit the bullet and repaired the power supply properly and no more trouble. As with nearly everything in life, common sense is important. Don't fix something that isn't broken, and don't reengineer something unless you really know what you are doing. |
| duak:
--- Quote from: soldar on May 11, 2019, 06:26:01 am --- --- Quote from: duak on May 10, 2019, 07:13:47 pm ---Series/parallel windings are wound bifilar, ie. two wires wound at the same time, to ensure they have the same number of turns and identical coupling inductance. --- End quote --- Definitely not the ones I have. I guess mine were wound by someone who could count :) --- Quote from: duak on May 10, 2019, 07:13:47 pm --- As an aside, I encountered a toroidal power transformer where the primary winding count differed by one turn. It worked fine when the windings were in series, but not in parallel because there was a circulating current flowing that would trip the line breaker. Going back to the build sheet, I found that the designer did not specify bifilar winding. --- End quote --- I doubt that. One turn is not going to blow anything. If the transformer tripped the breaker when the secondaries were connected in parallel it was because they were connected incorrectly, not because there was a one turn difference. --- End quote --- Soldar, the particular transformer I referred to was rated for about 2 KVA. This was 30 years ago so I don't have the build sheet but going by memory, there were something like 80 turns for each 120 V primary winding so each each turn contributed about 1.5 V. The breaker was a 2 pole rated for probably 8 amps and also served as the power switch. The same breaker was used for both 120 and 240 V and was connected in series or parallel along with the transformer when the line voltage strapping was changed. During final test of the overall unit, it would work fine when strapped for 240 V, but would trip after a short time (a minute?) when strapped for 120 V. All voltages measured OK and the line current wasn't significantly higher than spec. The resistance of each winding was a fraction of an ohm so the difference in the number of turns caused a circulating current in the primary windings when they were connected in parallel. I think I used a clamp on ammeter on one of the primary windings and found an excessive current and further tests showed the primary windings had a different number of turns. I would have thought that the manufacturer would have known how to count accurately too, but I guess by specifying the exact number makes them actually wind them and then test afterwards. If memory serves, this transformer was from a batch of 10 or 20 and was the only one that had this problem so I chalked it up to a spec, fab and QC issue. |
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