cradens@uceng.UC.EDU (carl radens) (12/30/89)
What is a good technique for running 100V Japanese products (audio) on a 120V US power line ? Apparently, some units have a selection switch in the back, or at the power supply section; but if not, what to do ? This topic was just chewed over recently in this group. What was the scoop ? Carl Radens, Cincinnati cradens@uceng.uc.edu
ee5391aa@hydra.unm.edu (Duke McMullan n5gax) (12/30/89)
In article <3215@uceng.UC.EDU> cradens@uceng.UC.EDU (carl radens) writes: >What is a good technique for running 100V Japanese products (audio) on >a 120V US power line ? Apparently, some units have a selection switch >in the back, or at the power supply section; but if not, what to do ? Let's assume you don't want to dedicate a variable autotransformer (that's what a Variac (TM) really is) forever to this task. OK, go to your junk box, and pull out an old 24 volt transformer. You DO have an old 24V xformer in you junk box, DON'T YOU? Sigh. OK, buy one at Radio Schlock if you must. Just come up with an ap- propriate transformer. Wire as follows: o__________ || | )|| | (|| _________________________________o | )||( | (||) 24v secondary | )||(_____________ | (|| | __)_____)|| | | | || | o--| | | | | | |_)_____________________| | |__________________________________________o Now, here's what's happening: The 110v (or 115v, or 120v, let's not quibble) on the primary generates about 24v at the secondary. This 24v is wired IN SERIES with the 110v line. The 110v is applied to the left-hand pair of terminals. You need to look at the voltage across the right-hand terminals now. A meter isn't necessary; a regular 110v light bulb will do the trick. (Just in case, use a bulb you don't mind losing.) The phase (like polarity, only AC) of the secondary is critical to this working properly. Wire one way, this will ADD the secondary voltage to the line, giving about 150v (VERY bright bulb); the other way, the voltages subtract, giving you a dim bulb at a little under 100v. Wire appropriately; don't exceed the current rating of the secondary (primary current won't be a problem). 1) I hope the picture survives. If your tabs=8 spaces, we're probably OK. 2) This circuit IS NOT an isolation transformer. 3) I trust you aren't offended by the detail and low-tech description. I know nothing of your electronics background, so I assumed VERY little. 4) For the record, when you wire the thing this way, it is an autotransformer. 5) Don't try this trick with a neon-sign transformer. ;^) Here's hoping it helps, d "You should never threaten a man you are not going to fight, and if war is your intention, it is best to get on with it." -- F.J. Lovret Duke McMullan n5gax nss13429r phon505-255-4642 ee5391aa@hydra.unm.edu
kawaguch@girtab.usc.edu (Atsushi Kawaguchi) (12/30/89)
In article <3215@uceng.UC.EDU> cradens@uceng.UC.EDU (carl radens) writes: >What is a good technique for running 100V Japanese products (audio) on >a 120V US power line ? Apparently, some units have a selection switch >in the back, or at the power supply section; but if not, what to do ? Almost all electronic appliances have +-10% tolerance on power line, so anywhere between 90 to 110VAC is guaranteed to work, but I doubt any appliance can be damaged by 'pushing' a bit further to 120v. I've been using Japanese spec stereos, TV, etc. rated at 100v for years, and nothing bad happened. There are transformers sold at some places just to convert US AC line to Japanese AC line, but I don't think it's necessary. U U SSSS CCCC Atsushi Jun Kawaguchi U US C INTERNET: kawaguch@girtab.usc.edu U U SSS C U U SC UUU SSSS CCCC
kehres@lance.tis.llnl.gov (Tim Kehres) (12/31/89)
In article <7200@chaph.usc.edu> kawaguch@girtab.usc.edu (Atsushi Kawaguchi) writes: > Almost all electronic appliances have +-10% tolerance on power line, > so anywhere between 90 to 110VAC is guaranteed to work, but I doubt > any appliance can be damaged by 'pushing' a bit further to 120v. I've > been using Japanese spec stereos, TV, etc. rated at 100v for years, and > nothing bad happened. There are transformers sold at some places just to > convert US AC line to Japanese AC line, but I don't think it's necessary. A colleague of mine picked up some equipment in Tokyo a little over a year ago and did just this and ended up with a piece of equipment that did not work about 6 months later. The description of what had failed sounded like a failed power supply. You need to be careful when doing this. If the equipment is speced for 90-120 VAC, then there should not be any problem. Unfortunately however much of the equipment for the Japanese market is designed for only 100 VAC, 50/60 Hz. For this equipment, unless you don't mind the possibility of damaging the equipment, it is much better to just spend the $15.00 or so for the step down transformer. Tim Kehres
dbell@cup.portal.com (David J Bell) (01/01/90)
Duke McMullan responded with a neat little bootstrap transformer circuit, and
eventually warned us:
. . . Wire appropriately; don't exceed the current
rating of the secondary (primary current won't be a problem).
Only a small quibble: the *primary* current is what you have to worry about,
not the secondary: since all the load current flows through both transformer
windings, they get equal current, unlike the usual case where the primary is
low current and the (step-down) secondary is much higher. You'd want to first
find out what the primary current rating *is* before selecting the transformer
to use. To wit:
Power_rating = sec._voltage (24) * sec._current_rating (e.g. 3A) = 72 Watts
Primary_current_rating = Power_rating / 120VAC, or 0.6 A in this example.
If the equipment you want to power doesn't give a current draw, calculate it
from the power spec: Equipment_power / 100 VAC = Equipment_current, which
must be significantly less than the transformer's primary current rating...
Good luck! Dave dbell@cup.portal.comjewett@hpl-opus.HP.COM (Bob Jewett) (01/03/90)
> Only a small quibble: the *primary* current is what you have to worry > about, not the secondary: since all the load current flows through both > transformer windings, ... Nope. The current ratio of secondary to primary is still about 5:1. It is easier to see if the schematic is redrawn: 24V secondary Hot,120VAC 0-------*----------wwwww------------0 Hot,~90V (or 150V!!) | =========== *-------mmmmmmmmmmm-* 120V primary | | | Neurtal,120VAC 0---------------------------*-------0 Neutral The load current flows only in the secondary. If you want to be safe, you also have to worry about fuses and which side of the line is neutral. Bob Jewett
brianr@tekig5.PEN.TEK.COM (Brian Rhodefer) (01/03/90)
In article <25539@cup.portal.com> tekig5!zephyr.ens.tek.com!uunet!portal!cup.portal.com!dbell dbell@cup.portal.com (David J Bell) writes: > >Only a small quibble: the *primary* current is what you have to worry about, >not the secondary: since all the load current flows through both transformer >windings, they get equal current, unlike the usual case where the primary is >low current and the (step-down) secondary is much higher. You'd want to first >find out what the primary current rating *is* before selecting the transformer >to use. To wit: > Dave is greviously mistaken. The original poster was quite correct: the load current determines the transformer's secondary current rating. The same current most emphatically does NOT flow through both the primary and the secondary, as anyone who purports to understand transformers ought to know. Suggesting that the primary and secondary currents of an operating (not saturated, etc) transformer are equal is the same as suggesting that the winding voltages are equal, and I doubt even Dave believes that. (note: excitation currents neglected.) Brian Rhodefer
jgk@osc.COM (Joe Keane) (01/03/90)
In article <25539@cup.portal.com> dbell@cup.portal.com (David J Bell) writes: >Only a small quibble: the *primary* current is what you have to worry about, >not the secondary: since all the load current flows through both transformer >windings, they get equal current, unlike the usual case where the primary is >low current and the (step-down) secondary is much higher. You'd want to first >find out what the primary current rating *is* before selecting the transformer >to use. To wit: > >Power_rating = sec._voltage (24) * sec._current_rating (e.g. 3A) = 72 Watts >Primary_current_rating = Power_rating / 120VAC, or 0.6 A in this example. This is incorrect. The secondary carries the output current, so, assuming we have a 120VAC to 20VAC transformer, the primary must carry only 1/6 of that current. Also note that only 1/5 of the power consumed by the load actually flows through the transformer, from the secondary to the primary. . . .
dbell@cup.portal.com (David J Bell) (01/05/90)
OK OK OK - Stop already!!! :{)
I can only claim it was New Years Eve, and I'd been drinking
my homemade cider for too long when I posted that "correction"...
The original statement about the secondary current being the limitation
was quite correct; somehow, I interpreted it differently. Right now,
it seems obvious!
Thanks (?) to evevrybody who posted or mailed corrections.
Dave dbell@cup.portal.com