[tajord]

A little help here, I hooked my kit up, but didn't put power to the board as yet (safety reasons, good thing too), first thing happened is the four amp fuse blew, tried it again.......blew again, now before i go any further i have to tell you that i didn't use the supplied switch cause it was so small that the plastic on the switch started to melt before i could get the wires soldered, so i used another switch, this one was rated @ 250V, 16A, I figured this is what caused my problem, so i got a smaller switch rated @ 125V 6A, didn't have any more 4A fuses so i tried 3A.....blew, 5A.......blew, 10A.....worked, both switches worked with 10A, now move on to the next problem, out of my DIY rewinded microwave tranformer i'm getting 26.8VAC roughly, after being rectified i'm getting 43.3VDC, this does not compute, I can undo a few windings but don't want to till i find the problem or unless it's really called for, has anyone had similar issues or can assist me in resolving this.

[audioandy1762]

Hi

A rectified AC voltage is roughly 1.4 times greater so in your case
26.8 X 1.4= 37.5 volts. If you want a certain voltage you can use this formula to work it backwards, ie if you require 35 volts DC divide 35 by the 1.4 (this is the RMS value) = 25 volts AC


With regard to blowing fuses so long you have checked and double checked your connections you will probably find it is the inrush current to the transformer and nothing to do with the switches you have used.

Hope this helps


Andy

[tajord]

yes i understand the rectifier conversion, which is why i can't figure out why i'm getting that output voltage, and i kinda assumed the transformer was drawing more than 4Amps.

[audioandy1762]

Where are you measuring your voltage? At the bridge rectifier or after your smoothing caps,I have noticed before with power supplies the output voltage can be affected with size of caps, one of the more knowledgeable members might be able to give a more scientific explanation, I am only drawing on my past experience

Andy

[tajord]

I'm measuring after the smoothing caps.

[rally.sport]

you may find this works had the same problem with a 2a fuse on our system
the original fuse blew, i believe it was something to do with my poking around to see what it does!
i replaced it and it keeps blowing, oh what have i done!
well the fuse that came out was a 2at 250vac
the important "t" is timelag it allows a start up over current on switch on
replace it with a T rated fuse never had a problem in 2 years

one other thing i found out is that all these thing work on magic grey smoke and once it puffs up escapes and goes away it will never work again!

[awright]

The rating of your switch can have no influence whatsover on the fuses blowing unless it fails in a condition putting a short across the line, which has obviously not happened. You could have a giant motor starting switch in place of the original switch and it would cause no problems. If the switch was unable to carry the inrush current or break the running current it would fail, but not in a fashion to cause fuses to blow unless a massive meltdown caused a short across the line - extremely unlikely. Failure would normally occur after a long period of deterioration of the contacts over many switching cycles by erosion until the contacts welded closed or couldn't make contact.

Excessive inrush current to your system is probably caused by having a large capacitive load on the rectified output of your transformer. A discharged capacitor is essentially a dead short on the transformer output winding for the first half cycle, with initial charging current limited only by the resistance of the transformer secondary winding and wiring and the voltage drop of the rectifier diodes - all very small. Perhaps your home-wound transformer has lower than normal winding resistance - a good thing for efficiency, but leading to your problem.

Putting in larger and larger fuses until they stop blowing is not a proper solution to a problem of this nature. You end up with a fuse that will not blow with the inrush current, but which provides almost no protection for other more common types of circuit failures. This is akin to the common problem of people putting 30 amp fuses in their 15 amp circuits "...because the fuses kept blowing..." and having their house burn down because they didn't find the real problem which was an overloaded circuit.

Assuming the problem is capacitor charging upon turn-on and not a wiring or fundamental design error, the proper solution is to provide a low value resistor in series with either the primary or the secondary winding of your transformer and provide a time delay relay (or, much less desirably, a manual switch) to short the resistor after the initial charging of the capacitor bank. There are also negative temperature coefficient inrush limiters that you simply wire in series with your transformer primary, but selection requires more sophistication than I can offer and detailed evaluation of the circuit operating conditions.

I can't advise on the appropriate resistor value in the absence of more info on your circuit, but a few ohms at a few watts would probably do the trick. Values are very non-critical. Remember that a discrete resistor having ten times the resistance of your secondary circuit loop will reduce peak inrush current to about 10% of what it was initially. The resistor should be a higher value if used in the primary circuit than if used in the secondary circuit by about the voltage ratio of the transformer. Helps a lot to have a 'scope or a peak holding DVM to monitor what is happening with the peak current as you experiment.

Time delay is also non-critical, as the capacitor bank will charge up within a few cycles of the line power upon turn on provided the resistance of your series resistor is not excessive. 0.5 to 5 seconds would be fine.

If you are a good scrounger, you should be able to find a time delay relay or module at low cost in electronic surplus stores or catalogs. You could also build a simple delay circuit controlling a common relay, but that's a whole new project. Common types of time delay relays are thermal relays inside a glass envelope of an octal-base tube, solid-state circuits inside a plastic case with an octal base, or small plastic blocks with terminals sticking out. You can also find time delay relays in Grainger's catalog (www.grainger.com) or the various electronic supply catalogs like DigiKey or Mouser. The contacts only need to be rated for a little more than the normal operating current because the capacitors will have charged up by the time the relay contacts close, shorting out the series resistor.

If the resistor is in the secondary circuit, the TDR contacts must be isolated from the line power, which excludes the less expensive modular time delays designed for delaying motor starting in HVAC systems.

If the series resistor is in the primary circuit, the cheap modules would be useable because you don't require line isolation there. Consider the following:

120 VAC SSAC solid state timer #THC412A. 6 amp max. 3 second on delay relay. 2" x 2". 3/4"-1/4" terminals. Stock #RL2600 $6.95

at www.candhsales.com. Check out the minimum holding current specs with SSAC and measure the minimum input current of your kit before using this type of TDR. They are designed for larger loads and may drop out if the running current is too low. In that case, go with a standard TDR that doel not depend upon load current to maintain contact.

Have fun.

awright

[techman1]

A little help here, I hooked my kit up, but didn't put power to the board as yet (safety reasons, good thing too), first thing happened is the four amp fuse blew, tried it again.......blew again, now before i go any further i have to tell you that i didn't use the supplied switch cause it was so small that the plastic on the switch started to melt before i could get the wires soldered, so i used another switch, this one was rated @ 250V, 16A, I figured this is what caused my problem, so i got a smaller switch rated @ 125V 6A, didn't have any more 4A fuses so i tried 3A.....blew, 5A.......blew, 10A.....worked, both switches worked with 10A, now move on to the next problem, out of my DIY rewinded microwave tranformer i'm getting 26.8VAC roughly, after being rectified i'm getting 43.3VDC, this does not compute, I can undo a few windings but don't want to till i find the problem or unless it's really called for, has anyone had similar issues or can assist me in resolving this.

When you rectify an AC voltage with a full wave rectifier, the DC voltage is 1.7 times the AC voltage. Rectification factor of a sine wave is 1.717.

[tajord]

the rectifier i have is labeled MB356, could this be a full wave?

[Jason Marsha]

Is that the hobbycnc rectfier?
If it is check with Dave.

Jason

[Al_The_Man]

When you rectify an AC voltage with a full wave rectifier, the DC voltage is 1.7 times the AC voltage. Rectification factor of a sine wave is 1.717.

I think you are confusing the inverse which is AC = 0.707 of peak rectified voltage.
The AC peak and therefore max DC max. of full wave rectifier is RMS x 1.414
Al.

[kreutz]

When you rectify an AC voltage with a full wave rectifier, the DC voltage is 1.7 times the AC voltage. Rectification factor of a sine wave is 1.717.

Wrong!, Single phase fullwave rectifier following by a capacitive filter will give a no load DC voltage = 1.41 times the AC voltage (assuming your Diodes are ideal), the 1.41 comes from the rms to peak relationship for a sinuswave.

On a real full wave rectifier circuit with filter you have to substract the Diode's VF voltage in order to find the "no load" output voltage.

[mark c]

I had the same problem with mine. I went to a Slo-Blow fuse and that fixed it.

[bobs bots]

Hi Tajord
I hope you find the response of the forumists helpful, the "awright" persona has contributed a lot of useful information. I can hopefully extend on this a little.

(a) Rewinding a microwave oven transformer (MOT) would not be my first choice, these particular transformers are specifically designed to have a lot of leakage inductance. Apparently the magnetrons need this characteristic (I don't know why, and I have a M.E. and 30 years experience). On the other hand, I guess they are cheap, and it's easy to remove the secondary.

(b) The above characteristic would require a lot of magnetising current (much more than similar normal transformers) , so the best bet is to use the same fuse that came with the microwave oven.

(c) A 600W microwave oven at 110v needs to draw approx 6amps just for the resistive load and probably another 6amps of reactive current. So the total current is around 8.5amps (remember to use vector arithmetic 8.5= sqrt(6^2 + 6^2)). The reactive current (magnetising current) will always be drawn irrespective of what is attached to the secondary.

(d) Fuse facts:
There are 3 commonly used fuse types:

Fast: these have the letter F stamped on them somewhere
Normal: also called time delay, have the letter T stamped somewhere, suitable for most applications with some magnetising current.
Slo-Blo: Have the letter S stamped on them, suitable for high inrush currents and when large capacitors need to charged.

All fuses will carry their rated current continously 24hrs/7days per week
All fuses will tolerate 2x overloads for considerable periods. (so if you are blowing 3amp fuses then the current is obviously in excess of 6amps)

(e) So in your case, and not knowing the exact details, you should probably be using an 8.5A slo-blo fuse (I don't think 8.5A is a prefferred value) or do what most people do (as slo-blo fuses are expensive), just uprate to next higher T rated fuse, e.g. a "10A T" fuse.

(f) Using a soft start circuit (as suggested by other contributors) is also benefical as it reduces the strain on all components.

(g) Because such a high fuse rating is required due to the magnetising current, you will need to ensure that your secondary winding is made of sufficiently thick wire to not act as fuse itself! . i.e. you need to fill up the bobbin with wire. Typically at this power level you will find it easier to put the windings on as 2, 3, or more windings in parallel by winding them simultaneously with multiple strands of wire. (First you need to work out the length of wire, and temporarily wind out multiple lengths onto an empty spool)

(h) The MB356 rectifier is a 35A 600V full bridge rectifier. Unless you have some really unusual wiring you will have a full bridge rectified circuit.
(hh) Note also that the 10A T fuse suggested above will not protect this rectifier from overheating due to excessive current due to a prolonged overload, it will however prevent the transformer bursting into flames when the MB356 eventually shorts out due to a sustained overload. If this bothers you, then perhaps a 30A SloBlo fuse could be inserted upstream of the rectifier.

(i) Assuming a sinusoidal secondary waveform, then the rectified DC voltage across the capacitors will be slightly less than 1.4 x the RMS (root mean square) voltage of the secondary. You lose a bit due to diode voltage drops, the average ripple voltage drop, and the reactance of the transformer. So starting with 27V rms, you would expect 38V DC (from 1.414 x AC), allowing for diode drop 37V, and when you load it up, this will drop to say 36v allowing for ripple, and possible as low as 32V when the effect of transformer reactance is considered.

(j) Your measurement of 43.3 volts DC is certainly anomolous, I can only suggest the following.

* You have a cheap multimeter and it is not reading RMS voltage correctly
* The secondary waveform is not a sinewave due to the high reactance of microwave type transformers (this is the reason for the error above)
* You have a cheap multimeter and had it on AC whilst measuring the DC voltage
* The DC voltage reading is

(k) You may find you need to put some kind of dummy load on your supply to stop the DC (capacitor) voltage floating up when unloaded. The reactance of the MOT will cause more than the usual amount of droop under loading.

I hope this is helpful.
Cheers, BobT

[bobs bots]

oops, cut off a sentence.

* the DC voltage reading is the most accurate reading from a multimeter, (most multimeters have dubious AC measurements).


BobT

[tajord]

hey thanks b bot, you have been quite informative in your explaination, i've been so frustrated i'm telling myself i'm gonna just buy a new transformer to complete my project, but still, this thread looks like it can grow to be really helpfull to anyone who wants to go this route, even if i do buy the xformer, i'd like for this thread to go on as a number of guys out there have rewound these babies, don't know if any actually did it with the hobbycnc kit though, overall i think it can bring solutions to other's potential problems.

thanks again, and to all who gave input.

[geh1]

Some transformers cause a large "inrush current" that will pop fuses and blow circiut breakers (especially with larger toridol transformers). This problem is easily fixed with the addition of an NTC (negetive temp. coefficient) resistor in series in the v source. NTC's are somewhat hard to find so try "google", I believe I got my last set from allied electronics. -G-