Need help from the electronic gurus

[Ed.]

Hi Guys, need advice with some gear from you electronic gurus as I have very limited experience with electronics.

I have a largish (about 10Kg) 240V AC to 39V AC transformer, the secondary 39V AC (unloaded) output leads consist of a 2x2mm wires each, so I am guessing that it would put out a fair few amps. (it came out of a very large UPS)

Now here is the reason for doing this, I want to power a 2500W max 50A 48V max input Induction heater or a similar one to this.

https://www.ebay.com.au/itm/2500W-12-48V-ZVS-Induction-Heating-Board-Flyback-Tesla-Coil-Module-Driver-Heater/392519509287?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m1438.l2649

I have connected a MAX MO FSB 7510A 75A full bridge rectifier to those leads and get 35.9V DC unloaded and I assume that because of the F/B rectifier I am also guessing it will be at 100hz instead of the straight DC, am I correct so far?

When I connect a capacitor in between the output leads to get rid of the 100hz frequency it then becomes 54V DC. So if I then connect the 54V leads to the heater I think that there may be an issue due to the increase in voltage . The voltage will probably drop down a bit because of the load the heater will draw but it may still be too high for the induction heater and I may blow it up so I am wary of risking it.

My alternative idea:

Is to connect 4 x 18A/hr AGM batteries in series and then connect the 54V DC leads to the batteries (to charge them whilst the heater is running) and then connect the induction heater to 3 of those batteries.

Will the DC 100hz frequency make any difference or damage the batteries. If not, I could then use the 35.9 V DC@100hz straight out of the F/B rectifier (without the capacitor) with 3 x batteries in series, a bit low for 3xAGM batteries but it only needs to top up the batteries whilst the heater is draining them otherwise I will have to go to 4 x batteries. 4 batteries in series would give me more breathing room as the batteries could charge and tapping 3 of them should be about 39V DC which is well within the heaters specs.

Any advice on which way to go and will this work to power the heater or am I barking up the wrong tree? I am guessing that the frequency wont matter and I could just use 3 batteries.

Hope this makes sense!

Thanks.

 

[Seadog]

For £40 or so I'd give the rectified transformer a go, unless you have a bank of batteries available.

Under load the rectified o/p will drop from the measured 54v. You will need quite a large reservoir capacitor to remove the 100Hz ripple which may upset the curcuitry.

Old style battery chargers were nothing more than a transformer and rectifier. You get about 1.4 x measured ac voltage from the transformer, less forward drop across the bridge. Batteries are not worried about the ripple and would remove it.

If you had a largish Variac that would allow you to drop the transformer input voltage.

 

[Agroshield]

Is there any way to drop the secondary voltage by unwinding a few turns?

 

[Seadog]

I suppose that that is a possibility.

 

[Ed.]

Hi Seadog, Thanks for your reply, I do have 4 batteries that I can use which is why I considered that option, so if you are saying that the DC 100hz charging the batteries won't present a problem that may be my safest option as I don't want to risk the heater or batteries, I actually blew up a smaller inductance heater (1000W) up about 2 years ago, connected up the 4 batteries but forgot to tap off from the the 3rd battery to drive it. That let all the magic smoke out and fried all the electronics on it, that one had a max of 40V input so I dont want a repeat performance.

Any suggestions for a capacitor, I have a 340V 40uF and also a 240V 20uF, any of those suitable or what would be better?

I don't have a variac but I do have a reostat ? which was used to control a 3.6KW heating element, if I connected the transformers primary coil to it, would that do the simlar job by reducing the voltage to the transformer and there by reducing the output from it as well?

Cheers

 

[Ed.]

Agrosheild, it wouldn't be feasable to do that for me as I would probably damage the transformer by attempting to reduce the windings.

 

[Seadog]

It sounds like you have plenty of options here. The rheostat is a distinct possibility and the easiest to try. I would try both capacitors in parallel, you may find you need more. I don't know, but would imagine that the current demand will be a constant once the inverter has run up, they'll just be removing the ripple.

The battery scenario is also covered, albeit at a reduced voltage. At the end of the day it depends as to whether or not you need the full rated output from this.

 

[Ed.]

More power to the heater would just result in a bigger heating effect and whilst that might be nice it isn't that critical, so even if I get 2000W capacity out of it it running at about 39V that will be more that enough for what I want it for, which is primarily to heat up metal for quenching and hardening up. My Oxy-Acet bottles are pretty much empty and can't be refilled due to their age, so I would have to spend $600 to get a bottle of each and that is more than it is worth for me. I only have one of the large caps (silver can next to switch in the top right corner of the pic) but I do have more of the smaller ones.

 

[Seadog]

You battery terminal voltage will rise to about 14V on charge, so that would give you a comfortable 42v available.

 

[voipio]

Some figures to add to the fag packet. Twin 2mm diameter copper secondary should be good for around 20A, discontinuous, assuming output under load to be around 35-37V, plus a further loss of 3V in the rectifier, max continuous output power capable of being delivered to a resistive load, without overheating, will be around 650W. Peak voltage, at 100Hz, would be around 48V.

 

[Ed.]

Is it possible to measure how much this transformer can out put by putting on a load and then using an AC clamp meter to measure the output amperage and keeping track of the transformers temp? I have a clamp meter that can measure up to 200A AC. If so, I am trying to think what I could use that would max it at about 35-40V, anybody got any suggestions?
Judging by the size of this tranny I would have thought that it would put out way more than 20A. It is 3 times the size of a 600W tranny I pulled out of a microwave oven.

 

[Ed.]

Actually thinking about loads, could I use some stove heating elements that I have lying around although they are designed for 240V, would that matter?

 

[Seadog]

A resistive load designed for high wattage would be fine. However, they're designed for 240v so will present a lower load at 40 odd volts. But it's a starting point. You can, of course, increase the loading by adding more elements in parallel.

 

[Ed.]

As it is night now I will have to go to the shed tomorrow morning and see if I can find the stove elements, I know where the 3600W water heating element is but that probably is way too big. If I can find them but I may have thrown them out when I did a clean out a while ago, if not, I will start with the smallest and work my way up, hopefully it may give an indication of the trannies output capacity.

 

[voipio]

A 3600W 240V heating element would be 16 Ohms, giving a current draw of ~2.5A so just a bit less than 100W from 39V.

The 20A current capacity of the transformer output is based on transformer wire ratings data tables. 2mm dia corresponds to 12AWG or 14SWG, which are rated at 9.33A single winding. Doubling up will increase the rating by less than twofold, so 20A was being generous, hence my mention of intermittent use.

 

[Hopefuldave]

I've always used a bunch of headlight bulbs for low-voltage loads - put as many in series as takes it to your trans voltage, so 4x for a 48v supply and e.g. a 5 amp 240w load. Put more strings in parallel to get 5 amp "increments" (or strings of brakelight bulbs for 2 amps, etc.), a decent transformer will have about a 5% regulation factor, meaning the voltage will drop by 5% of the no-load voltage at full rated current - so you can measure the AC voltage it produces for different loads to get a pretty sensible max current rating (when loaded voltage has dropped by 5% Go NO Further)

Dave H. (the other one)

 

[the snooper]

Hi Guys, need advice with some gear from you electronic gurus as I have very limited experience with electronics.

I have a largish (about 10Kg) 240V AC to 39V AC transformer, the secondary 39V AC (unloaded) output leads consist of a 2x2mm wires each, so I am guessing that it would put out a fair few amps. (it came out of a very large UPS)

View attachment 222571


Now here is the reason for doing this, I want to power a 2500W max 50A 48V max input Induction heater or a similar one to this.

https://www.ebay.com.au/itm/2500W-12-48V-ZVS-Induction-Heating-Board-Flyback-Tesla-Coil-Module-Driver-Heater/392519509287?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m1438.l2649

I have connected a MAX MO FSB 7510A 75A full bridge rectifier to those leads and get 35.9V DC unloaded and I assume that because of the F/B rectifier I am also guessing it will be at 100hz instead of the straight DC, am I correct so far?

When I connect a capacitor in between the output leads to get rid of the 100hz frequency it then becomes 54V DC. So if I then connect the 54V leads to the heater I think that there may be an issue due to the increase in voltage . The voltage will probably drop down a bit because of the load the heater will draw but it may still be too high for the induction heater and I may blow it up so I am wary of risking it.

My alternative idea:

Is to connect 4 x 18A/hr AGM batteries in series and then connect the 54V DC leads to the batteries (to charge them whilst the heater is running) and then connect the induction heater to 3 of those batteries.

Will the DC 100hz frequency make any difference or damage the batteries. If not, I could then use the 35.9 V DC@100hz straight out of the F/B rectifier (without the capacitor) with 3 x batteries in series, a bit low for 3xAGM batteries but it only needs to top up the batteries whilst the heater is draining them otherwise I will have to go to 4 x batteries. 4 batteries in series would give me more breathing room as the batteries could charge and tapping 3 of them should be about 39V DC which is well within the heaters specs.

Any advice on which way to go and will this work to power the heater or am I barking up the wrong tree? I am guessing that the frequency wont matter and I could just use 3 batteries.

Hope this makes sense!

Thanks.

Do you know the model of the ups, if so you may be able to get the specs for the transformer

 

[Ed.]

Voipio, So looks like there is no point me testing it then that way. I was never any good at this sort of stuff so another thing I have learnt now. I thought that the wattage of the element would be constant but apparently not.

The Snooper, That UPS that it originally came from was about 70cm high and weighed about 70Kg, and had a gazzilion 5a/hr batteries in it also had a 15A plug to it. Don't remember much else about it as it was about 20 years ago when I pulled it apart.

I will have to just connect it to the 3-4 batteries in series and see how many amps the tranny can produce. The heater wont be running for that long so it will still be within the draw capability of the 3-4 battery setup. I will order another inline amp meter off E-Bay and when it arrives, will connect the tranny to the batteries. As it is, it will be about 6 weeks at least before the heater arrives from Taiwan so plenty of time to play around, at least it is doable by using 3 batteries without capacitor or 4 batteries with the capacitor and remembering this time to tap off the 3rd battery. Just need to check the voltages and amperage's to make sure everything is within limits.

Out of curiosity, how does the full bridge rectifier that I am using then rate , according to the specs it is rated at 1000V and 75A capacity, so if W=VxI that would mean 75000W, and in my mind there is no way that it can handle that sort of wattage, the terminals would have to have 25mm posts or am I barking up the wrong tree again? or does it mean that that the 1000V limit would get reduced to keep within the 75A limit? Now that I have typed it out it actually makes more sense now and if that is the case, what is the max voltage at 54V or do I need a larger rectifier that is assuming if the tranny can even put out 20A.

 

[Hopefuldave]

The voltage rating is the reverse voltage it can withstand - when it won't be passing any current (or shouldn't!) - the amps rating is the *PEAK* forward current, not continuous, and it'll be dissipating 0.6v (the forward voltage drop) x current in watts - so at 75A it'd need to dissipate 45 watts of heat - BIG heatsink and thermal paste required! If it's passing 20A it'll still need heatsinking for 12 watts to keep it in its temperature range.
Most commercial bridges are rated to 90c, so if the air's at 25c there's a 65c difference, the heatsink will have a degrees per watt figure that lets you work out whether it's big enough - e.g. if it's 10 degrees/watt it'll cool 6.5 watts, if it's 2/w it'll cool 130 watts - so for the 20A you're probably looking at and allowing a max 50c rise you'd need a 2.5 degree/watt heatsink to be on the safe side, fan cooling would help, too.
The diode bridge package has its own temperature limitations too, it'll give those in degrees/watt (buried in the manufacturer's spec sheet) and you need to look at the total for both package and heatsink, e.g. 0.5c/watt for the package, 2.5 for the heatsink is 3c/watt.

Dave H. (the other one)

 

[Ed.]

Hopefuldave, could you dumb it down a bit more for me, so if my tranny could make 15-20A continuous at 50V would this rectifier do the job if I added a large heat sink and fan or do a need a bigger rectifier to start off with as well as a fan and heat sink? Thanks.