kolstad@jomby.cs.wisc.edu (Joel Kolstad) (03/23/91)
Hi. I have a project going that uses a good sized toroid transformer in it. Thinking that it'd be a good idea for a little safety, I added a 5A breaker going through the "hot" line on the 120V side. Unfortunately, when you first apply 120V to the primary, the breaker immediately trips (even with no load on the secondary!) since the lack of a magenetic field in the toroid causes the primary windings to look like a short. So... the question is: How do I prevent the breaker from tripping? I'd rather not change the breaker, since it has already been mounted in the front panel, and is a rather strange rectangular shape that I don't think is readily availiable. I know that some people use thermistors in front of (incandesent) lamps to limit inrush current. Is this a possibility? (Yes, I realize the source of the initial "short" is completely different). Please mail responses to kolstad@jomby.cs.wisc.edu. Thanks! ---Joel Kolstad
ahahma@polaris.utu.fi (Arno Hahma) (03/24/91)
In article <1991Mar23.024309.12334@spool.cs.wisc.edu> kolstad@jomby.cs.wisc.edu (Joel Kolstad) writes: >immediately trips (even with no load on the secondary!) since the lack of >a magenetic field in the toroid causes the primary windings to look like >a short. I had a similar problem with a transformer, the 16 A fuse would blow unless there was a protection circuit. I use a relay and a resistor. Connect a 100 ohm resistor in series with the transformer primary. Connect the relay to short the resistor, i.e. to let the current flow directly to the transformer. Then use a simple RC-timer with a transistor (or whatever delay) to connect the relay on, say, half a second after the power has been switched on. The transformer will then have time to "warm up" before the full voltage is connected. >I know that some people use thermistors in front of (incandesent) lamps to >limit inrush current. Is this a possibility? (Yes, I realize the source of Yes it is, if you find suitable thermistors, that can stand the current the transformer takes when you have your load on the secondary. Also, the thermistor should have a small resistance, as it is warm. ______________________________________________________________________________ R' \ /======\ / \ O / ArNO \ // \\ 2 //--N+ \\ // \ \\______// O- / R
brianr@tekig5.PEN.TEK.COM (Brian E Rhodefer) (03/26/91)
In article <1991Mar23.024309.12334@spool.cs.wisc.edu> kolstad@jomby.cs.wisc.edu (Joel Kolstad) writes: >immediately trips (even with no load on the secondary!) since the lack of >a magenetic field in the toroid causes the primary windings to look like >a short. Not immediately. It doesn't look like a short until the core saturates. The only explaination I can think of for a core saturating *only* at startup is that it comes so freaking close to saturation under normal operating conditions that on the first halfcycle, the flux change that usually starts out at -Phi/2 begins at zero instead, and heads for +Phi instead of just +Phi/2. Even this is very unlikely to cause problems, since most 60Hz power transformers use metallic cores that saturate gracefully (as opposed to ferrites, anyway). Other posters have suggested some nonsense about using thermistor inrush limiting. This might work, but it is a far, FAR less desirable solution than having a properly designed transformer that won't saturate on startup in the first place. Another bandaid fix would be to employ electronic switching to ensure that the primary was only switched on at a supply voltage peak voltage (oddly, the least stressful turn-on condition). Brian Rhodefer
pierson@cimnet.enet.dec.com (dave pierson) (03/28/91)
In article <1991Mar23.024309.12334@spool.cs.wisc.edu> kolstad@jomby.cs.wisc.edu (Joel Kolstad) concerning a transformer tripping a breaker: >>immediately trips (even with no load on the secondary!) since the lack of >>a magenetic field in the toroid causes the primary windings to look like >>a short. I think he's right. the usual explantion is that it takes a finite amount of time for the field to build up (penetrate) the core. During this time, the apparent inductance (reactance) setting the surge changes in value from that of the windings alone (essentially air core, very small) to the steady state value. Its not uncommon to see a 10X peak on the first half cycle (though it may take a few tries to capture it.) The "best" fix is a breaker with a different delay curve, though i gather that's not practical, here. thanks dave pierson |the facts, as accurately as i can manage, Digital Equipment Corporation |the opinions, my own. 600 Nickerson Rd Marlboro, Mass 01752 pierson@cimnet.enet.dec.com "He has read everything, and, to his credit, written nothing." A J Raffles
billp@col.hp.com (Bill Pherigo) (03/29/91)
Are you sure that the power surge isn't because you're trying to charge up a large cap on the (rectified) secondary of the transformer. If so, you can use a slow-blow fuse. I tested many of them and found some that can handle 15 times their rated current for 1 Sec. At this current, the 1 inch size fuses often had over 20 volts ACROSS THEMSELVES due to IR drop. The 20mm type fuses were much more "ideal", having only 2.5 volts across themselves. Bill Pherigo <billp@col.hp.com> (719) 590-3420