Effects of capacitor on AC input leads to a transformer.

Thread Starter

Ed.

Joined Nov 16, 2016
19
Hi Guys, if possible I would prefer a simple answer as I only have a very limited and basic knowledge of electronics, I recently built a power source to do a fast boost charge on a 150A battery, I made it from a Microwave Oven Transformer which I rewired the secondary and connected a 6800uF capacitor and a 100A full bridge rectifier to the output leads, they produce about 14.3A DC. The transformer input voltage is about 2.75A at 240V AC 50hz with a 5A fuse.

Anyway I put the build up on the web and a guy posted a suggestion to add a 20-30uF capacitor to the AC input leads, something to do with power correction, not that I know what that is, as far as I know from reading, doing that would offset the current with regards to the voltage timing, so what effect or benefit will that do? I do have a few capacitors in that range lying around in my shed so no problem fitting one in, just wondering if I should as the unit seems to be working fine, is there any benefit to doing so?, apart from the transformer getting a bit warm, and so I added a fan to cool it and the bridge rectifiers' heat sink down.

Unfortunately I have no idea of this persons knowledge or background of electronics, and from some of other comments I have read on other posts, some posters have no clue (like myself), so before I do, I thought I would ask for some advice from you guys as most of you are into these sort of things.

Cheers.
 

Dodgydave

Joined Jun 22, 2012
9,200
Do you mean output voltage is 14.3V dc?

So if your transformer is taking 2.75A@ 240Vac, thats 660 W, output voltage is 14.3V so max current is 46amps.
 

Picbuster

Joined Dec 2, 2013
1,008
Hi Guys, if possible I would prefer a simple answer as I only have a very limited and basic knowledge of electronics, I recently built a power source to do a fast boost charge on a 150A battery, I made it from a Microwave Oven Transformer which I rewired the secondary and connected a 6800uF capacitor and a 100A full bridge rectifier to the output leads, they produce about 14.3A DC. The transformer input voltage is about 2.75A at 240V AC 50hz with a 5A fuse.

Anyway I put the build up on the web and a guy posted a suggestion to add a 20-30uF capacitor to the AC input leads, something to do with power correction, not that I know what that is, as far as I know from reading, doing that would offset the current with regards to the voltage timing, so what effect or benefit will that do? I do have a few capacitors in that range lying around in my shed so no problem fitting one in, just wondering if I should as the unit seems to be working fine, is there any benefit to doing so?, apart from the transformer getting a bit warm, and so I added a fan to cool it and the bridge rectifiers' heat sink down.

Unfortunately I have no idea of this persons knowledge or background of electronics, and from some of other comments I have read on other posts, some posters have no clue (like myself), so before I do, I thought I would ask for some advice from you guys as most of you are into these sort of things.

Cheers.
Putting 10uF to 240V 50Hz parallel to the mains will create an impedance of 1 / ( 2 x Pi x freq x 10 x 10^-6)
I am not sure but I think that you have to scrape this cap from your ceiling.
Picbuster.
 

CadenAAA

Joined May 4, 2017
3
As I see you are using the full wave rectifier to reverse the negative side of a AC wave, and then you use the 6800uf capacitor and perhaps a resistor too to get the DC output. Thus, the equivalent circuit for your whole design can be described as an AC source in series with a DC component (R) and an AC component jX. Your power calculation is actually a Complex power S=VI, (the V, I here are vectors)


You actually need a negative Q added to the system, which yields the power to be just the bottom line. upload_2017-5-8_21-13-57.png
This is called power factor correction which can make your source more efficient(more percentage of real power transfered from the source to the load). But this capacitor should be in paralell with the load, not in series.

I dont know if I am right, just a surmise.
 

MrChips

Joined Oct 2, 2009
21,303
This is called Power Factor Correction.
Power Factor is the ratio of True Power over Apparent Power.

When the Reactive Power is zero, Apparent Power will be equal to True Power and the Power Factor will be 1.

Before attempting to bring the power factor to 1, you should ask what is the current power factor in your situation?
What are the benefits of correcting the power factor to 1?
 

Thread Starter

Ed.

Joined Nov 16, 2016
19
Hi Guys, thank you for your efforts in replying , unfortunately I do not understand most of your answers as I do not know much about electronics, I had a look at the wikipedia link that JohnlnTX posted and all I got out of reading that listing is that a capacitor on the AC input leads would make my setup a bit more efficient in regards to reducing the current going in to the transformer but still producing the same output, is that correct?

So please dumb this down to my level with a simple yes or no answer, should I go ahead and put the capacitor on the AC leads or not bother? Thanks.
 

tcmtech

Joined Nov 4, 2013
2,867
Hi Guys, thank you for your efforts in replying , unfortunately I do not understand most of your answers as I do not know much about electronics, I had a look at the wikipedia link that JohnlnTX posted and all I got out of reading that listing is that a capacitor on the AC input leads would make my setup a bit more efficient in regards to reducing the current going in to the transformer but still producing the same output, is that correct?

So please dumb this down to my level with a simple yes or no answer, should I go ahead and put the capacitor on the AC leads or not bother? Thanks.
Yes and no.

Yes it will reduce the line side amperage. But, no. It will not improve system efficiency.
 

Dodgydave

Joined Jun 22, 2012
9,200
The heat will be due to the eddy current saturating the transformer core, putting a 10uF capacitor on the primary side won't be of any use, only to help lower your power factor wasted current from the mains.

What Current / Voltage are you getting on the secondary side when its charging the battery?
 

tcmtech

Joined Nov 4, 2013
2,867
If you have not worked with repurposed MOT's you may find out the overall performance in terms of working wattage they can handle Vs what the oven it came from was rated for is likely way off.

They are designed around an application where they typically only run for a few minutes at a time not continuously plus are also made as cheaply as the manufactures can get away with and still work long enough for a warranty to run out.

As transformers go they are inefficient run hot and have all around poor overloading capacity since by design in their intended applications they are already running near or at their overload limits as is.

Now if you really want a good robust transformer to repurpose find a used HID lighting ballast. They are very similar in design (but way heavier in build for the same rated wattage) to a MOT but designed for continuous duty in harsh hightempurutre environments which for DIY repurposing work they are very hard to kill. Best part is you can usually get one from most scrap yard of electrical repair or contractors for very little.
 

Thread Starter

Ed.

Joined Nov 16, 2016
19
The heat will be due to the eddy current saturating the transformer core, putting a 10uF capacitor on the primary side won't be of any use, only to help lower your power factor wasted current from the mains.

What Current / Voltage are you getting on the secondary side when its charging the battery?
Open circuit voltage on the DC output end is 15.15V.
I had an 18A UPS battery which was @ 12.56V DC
Output from unit dropped to 13.5V and started at 8.5A when connected to the battery
10min later it was 14.25V and 2.8-3.4A going to the battery which is when I disconnected it from the battery.

Previously when I had connected this unit to a 150A deep cycle battery it started at 20A and within 20 minutes it was down to about 6.5A going in and about 14.3V so I left it running for an hour. I do have a multi stage 8A charger, but to do the 150A battery would be an overnight job, so this one gives it a quick boost and then I can connect the multistage charger to it to finish it off which doesn't take too long from then on.
 

Thread Starter

Ed.

Joined Nov 16, 2016
19
Now if you really want a good robust transformer to repurpose find a used HID lighting ballast. They are very similar in design (but way heavier in build for the same rated wattage) to a MOT but designed for continuous duty in harsh high temperature environments which for DIY repurposing work they are very hard to kill. Best part is you can usually get one from most scrap yard of electrical repair or contractors for very little.
Only reason I used it was because it was lying around in the shed. I was originally going to use it to make a small spot welder but in the end I decided to rewire the transformer out of an old 140A arc welder and that one has way more grunt. So rewired the MOT again and used that instead for this power supply.

I actually had one of those HID ballasts lying around for years, had a blown capacitor but after not fixing it in about 5 years and kicking it out of the way in the shed I then sent it off to the recyclers. I still have about 4 more transformers of varying sizes and output capacities in the shed but don't have any ideas on what to do with them so they will sit around till I find a use for them. I will keep an eye out for those HID ballasts if they are that good.
Cheers.
 
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tcmtech

Joined Nov 4, 2013
2,867
I will keep an eye out for those HID ballasts if they are that good.
They are worth keeping around given most decent ones have multi tap primaries, 2 - 6 taps, that are easily disconnected from the over sized secondary (one easily removed connection lead ties them together) which is handy for cheating on winding primary to secondary ratios for odd things or if you just need to make a step up/down auto transformer that can take a fair amount overloading abuse without problems given they are typically insulated with class H or H+ insulation which can take crazy high operating temperatures that most other transformers would start to smoke at. :cool:

That and being so extramly mass produced they are modular section cores like MOT's not interleaved making them easy to disassemble and reassemble without affecting the overall efficiency or performance. ;)
 

Dodgydave

Joined Jun 22, 2012
9,200
Open circuit voltage on the DC output end is 15.15V.
I had an 18A UPS battery which was @ 12.56V DC
Output from unit dropped to 13.5V and started at 8.5A when connected to the battery
10min later it was 14.25V and 2.8-3.4A going to the battery which is when I disconnected it from the battery.

Previously when I had connected this unit to a 150A deep cycle battery it started at 20A and within 20 minutes it was down to about 6.5A going in and about 14.3V so I left it running for an hour. I do have a multi stage 8A charger, but to do the 150A battery would be an overnight job, so this one gives it a quick boost and then I can connect the multistage charger to it to finish it off which doesn't take too long from then on.
Sounds like your transformer is hitting 114W (13.5V@8.5A) at maximum charge at the beginning, could be limited by the wire size of the core, more current output means thicker secondary cable, and that will be limited by the core size, like you said earlier put a cooling fan on.
 

Thread Starter

Ed.

Joined Nov 16, 2016
19
Sounds like your transformer is hitting 114W (13.5V@8.5A) at maximum charge at the beginning, could be limited by the wire size of the core, more current output means thicker secondary cable, and that will be limited by the core size, like you said earlier put a cooling fan on.
It actually hit 20A with the bigger battery when it's voltage was about 12.6V when I did it, so it is at least putting out that much, I rewired the secondary with 8B&S auto wire which is apparently capable of handling 50A but not sure what it's current generating capacity is in the transformer. At this time I don't have anything that will put a heavier load on the transformer to find out it's limit, but at 20A at 13.3V it is at least 266W so still happy with that. I might have to connect a 100w light on the 150A battery and drain it down to 12V and then see what the unit will output when I reconnect it and charge the battery up again. Only way to make more current is for me to find a thicker cable but with reduced insulation thickness as there isn't much room left over going through the core with the insulation thickness of the wire I have used.
 

tcmtech

Joined Nov 4, 2013
2,867
Take the big battery down to around 10 volts. Thats the voltage it will be at when dead and would put the biggest long term load on things.
 
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