# Capacitor too big?

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#### Ed.

##### New Member
Hi, I found another old 240V transformer in my shed and would like to make a 36-39V DC power supply, the transformer outputs 39V AC unloaded, when I connected a 1000V 50A full wave rectifier bridge to the DC output it drops a bit to about 37V DC. However when I connected a 63V, 6800 uF capacitor to the bridges' DC output the DC voltage jumped up to 55.4V DC unloaded.

So my questions are:
How does the capacitance value figure into it and do I match the capacitor V to the voltage I want or is there something simple that I need to do.

Thanks.

#### audioguru

##### Well-Known Member
39VAC has a peak voltage of 55.2V and a bridge rectifier will drop it to about 54V which the filter capacitor charges to with no load. The capacitance value affects how much ripple the output has with a load since the rectifiers charge the capacitor at the peaks and the load discharges the capacitor between the peaks. Usually the voltage rating of the capacitor is higher than the output voltage to allow for occasional overvoltage and reliability.

#### ronsimpson

##### Well-Known Member
Welcome,

Here is an example; 12.6VAC transformer. A meter on the transformer will read 12V in AC mode. Just like it should.
In this case, there is no capacitor, and the voltage will read about 12V-(2X0.7V). Remember the diodes causes a small loss. So a DC meter should read the average voltage of about 11.4V.

Same picture but I added a capacitor. Because of the diodes and capacitor, you get the peak voltage stored on the cap.
So the voltage will be, in this case about 18V. (peak is 1.414 more than average)

The capacitor is charged to the very top of the sign wave and then holds at nearly that voltage during the valleys.

#### Ed.

##### New Member
Thanks for the info guys, being a complete newb when it comes to electronics, I gather then that getting it down to 36-39V DC will not be possible with this transformer then, I am trying to charge 3 x 12V 18aH batteries in series which will have a heavy load on them, and hoping to top them up so that they last a bit longer. I thought that this would be a simple way of doing it. Any simple way around this issue? Thanks.

#### JimB

##### Super Moderator
If you are charging lead acid batteries, you do not need the capacitor.

JimB

#### MikeMl

##### Well-Known Member
If you are charging lead acid batteries, you do not need the capacitor...
However, can whatever load is connected to the batteries tolerate the ripple from the transformer/rectifier?

#### Ed.

##### New Member
JimB, That I don't know, it is a Induction heater module, it draws a lot of current and the higher the voltage the more it draws, up to 50A at 40V. I was thinking that the batteries would smooth out the ripples to a certain extent, but wanted the capacitor just in case it they didn't.

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#### spec

##### Well-Known Member
Hi Ed,

Here are some points for you to consider:

(1) How much current will your transformer supply (the 50A rating of the bridge rectifier is only the maximum average current that the bridge rectifier will will conduct without damage)?

(2) How much current are you intending to charge the batteries with?

(3) If you charge/discharge batteries in series you will need a charge balancing circuit.

(4) Assuming that the induction heater takes 50A, that means that at 18Ah capacity the batteries would only have a theoretical duration of 18Ah/50A = 0.36 hours but, depending on the batteries again, you may only get a third of that duration, ie 0.12 hours, or 7.2 minutes duration. Is that roughly your experience.

(5) Depending on your batteries, draining 50A from an 18 Ah would be stressing the batteries.

(6) If the batteries are automobile types, as opposed to deep discharge types, they will not last long.

But on a more optimistic note, charging three lead acid batteries in series would be quite feasible with a transformer with a secondary voltage that you state.

spec

#### Ed.

##### New Member
Hi "Spec",

2) I am not sure what the amp output of the transformer is, it is 16cm W x 13cm D x 14cm H, and weighs 9.7Kg.

3) I just want to "top" up the batteries during the discharge only.

4) 10 min is more than enough time for the heater to do it's job so with the transformer top up it should be enough time not to discharge too deeply, I hope.

5) There are three batteries in series so total of 18aH capacity @ 36V

6) The batteries are UPS AGM type, once the heater has done it's job then I can individually charge the batteries to full with my 4 stage battery charger or just leave the transformer attached. But I am not sure what the extra voltage will do to the 3 batteries, although I measured the output with no load but I just don't know what the voltage will drop down to once connected to the batteries and or the heater.

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#### spec

##### Well-Known Member
Hi Ed,

Thanks for your concise, helpful, and prompt reply to my many questions- that makes life much easier and more interesting for us responding to your request.

2) I am not sure what the amp output of the transformer is, it is 16cm W x 13cm D x 14cm H, and weighs 9.7Kg.
That is a serious transformer and I would guess that it would be capable of delivering a 10A charge into the three batteries. For the time being assume that figure is correct.

3) I just want to "top" up the batteries during the discharge only.
Excellent- that simplifies matters no end.

4) 10 min is more than enough time for the heater to do it's job so with the transformer top up it should be enough time not to discharge too deeply.
Good news! As long as you do not leave lead acid batteries in a discharged state they are unlikely to deteriorate, so the best approach is to re charge as soon as possible.

5) There are three batteries in series so total of 18aH capacity @ 36V
Thanks. The 36V is nominal. The actual voltage will depend on battery charge state, current drain, and temperature

6[1]) The batteries are UPS AGM type,
Thanks- do you have a part number and/or a link to the manufacturers data sheet- not essential

6[2])once the heater has done it's job then I can individually charge the batteries to full with my 4 stage battery charger
Excellent- providing that the transformer charging is sorted, you would probably only need to do a conditioning charge with your pro charger every ten or so induction heating sessions. Of course, the more often the better

6[3]) But I am not sure what the extra voltage will do to the 3 batteries, although I measured the output with no load but I just don't know what the voltage will drop down to once connected to the batteries and or the heater.
Don't worry about that- it can all be fixed.

Just one more question that needs to be asked: what is the maximum input voltage that the induction heater can take?

From what you say, I imagine that we can sort your transformer charging quite simply, possibly by using just the transformer, bridge rectifier, and a power resistor.

spec

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#### Ed.

##### New Member
The batteries are Powersonic brand PS-12180 NB 12V 18aH
The induction heater works from 12-40V, here is a direct cut and paste from a sales page on E-Bay.

Features:

The DC power supply voltage is 12V~40V.
Recommended input voltage range DC 24V~36V.
Maximum input current 50A (short), Maximum power 1000W.
Equipped with a fan.
High quality module board for induction heating machine.

Notice:
When used in heating supply power must be large enough!
24V power above the recommended maximum current of 20A.
12V power supply maximum current must be more than 10A.

Specifications:
Power Supply: DC 12V~40V
Max. Current: 50A
Max. Power: 1000W

The only must have spec is that the amperage must be instantly available from the time the module is switched on otherwise it won't be able to resonate the current through the looped copper coil and build up the large magnetic field, which is why I am using the batteries.
It doesn't take long to start heating metal, I managed to heat up a 19mm steel rod till it started to smoke in about 20 seconds before I got distracted and blew out all the modules' mosfets on the circuit board. I ordered 4 replacements and soldered them in, but one of the new ones was faulty so now have to wait a few more weeks for it's replacement to arrive. Once that is fitted I will find out if there is any more damage to the module. If there is I will have to buy a new module, they are about $52 to$60 dollars delivered. That will teach me for not paying attention!

In the mean time while I am waiting for the part to arrive I would like to sort out the power supply.

#### spec

##### Well-Known Member
The batteries are Powersonic brand PS-12180 NB 12V 18aH
The induction heater works from 12-40V, here is a direct cut and paste from a sales page on E-Bay.

Features:

The DC power supply voltage is 12V~40V.
Recommended input voltage range DC 24V~36V.
Maximum input current 50A (short), Maximum power 1000W.
Equipped with a fan.
High quality module board for induction heating machine.

Notice:
When used in heating supply power must be large enough!
24V power above the recommended maximum current of 20A.
12V power supply maximum current must be more than 10A.

Specifications:
Power Supply: DC 12V~40V
Max. Current: 50A
Max. Power: 1000W

The only must have spec is that the amperage must be instantly available from the time the module is switched on otherwise it won't be able to resonate the current through the looped copper coil and build up the large magnetic field, which is why I am using the batteries.

It doesn't take long to start heating metal, I managed to heat up a 19mm steel rod till it started to smoke in about 20 seconds before I got distracted and blew out all the modules' mosfets on the circuit board. I ordered 4 replacements and soldered them in, but one of the new ones was faulty so now have to wait a few more weeks for it's replacement to arrive.

Once that is fitted I will find out if there is any more damage to the module. If there is I will have to buy a new module, they are about $52 to$60 dollars delivered. That will teach me for not paying attention!

In the mean time while I am waiting for the part to arrive I would like to sort out the power supply.
Hi Ed,

Wow- great information.

Just a word of caution, it is best to get critical components from a main-line supplier. Many 'expensive' components are counterfeit and not up to specification. Some main-line suppliers are, DigiKey, Mouser, Element 14 (Farnell), RS, Arrow. Very often you can fit more rugged MOSFETs as replacements.

I take it that those are Australian dollars.

In the meantime, until you get the induction heater working again, we can work out a charging arrangement.

I will give it some thought.

spec

#### Ed.

##### New Member
Thanks for all the help. I tried to get those mosfets from RS online, unfortunately they only sold them in in packs of 25 and they wanted about $6 Australian each, hence the reason I got them off ebay. When you pay$8 for 4 of them delivered I expect to get a few duds every now and then. As I wanted to be sure that there no other damaged components, I didn't want an expensive outlay for 25 of them not knowing if they may fix the module or not, and 21 of them would just be wasted.

#### spec

##### Well-Known Member
Hi again Ed,
Thanks for all the help. I tried to get those mosfets from RS online, unfortunately they only sold them in in packs of 25 and they wanted about $6 Australian each, hence the reason I got them off ebay. When you pay$8 for 4 of them delivered I expect to get a few duds every now and then. As I wanted to be sure that there no other damaged components, I didn't want an expensive outlay for 25 of them not knowing if they may fix the module or not, and 21 of them would just be wasted.

Yes, I know what you mean about RS- they are either out of stock or want you to buy hundreds.

What a shame you didn't post on ETO because we could have sorted you out some excellent MOSFETs, at a reasonable price. What are the MOSFET types?

As the absolute maximum input to the induction heater is 40V there is a slight problem charging three 12v batteries in series, because the total charging voltage would be 42.30V.

Reading between the lines of the specification for the induction heater I would think that a maximum reliable operating voltage would be around the 30V mark.

With that in mind, what do you think about operating the induction heater from just two batteries in series rather than three batteries?

spec

#### Ed.

##### New Member
The mosfets were IRFP260N, The reason I was hoping to use the 3 batteries was that it was close to the output of the transformer, provided I don't use that capacitor. Seeing as the batteries can be charged at 14.5V, I was hoping that measured voltage might drop a bit once a load was put on the output voltage, so after the load is connected it might end up the right voltage, well that was my theory at least! If I use just the 2 batteries I won't be able to use the transformer to top the batteries up as I use the heater.

I suppose I could just connect it up briefly to the 3 batteries and see what the DC voltage ends up. Is a different sized lower voltage capacitor an option?

#### tcmtech

##### Banned
I suppose I could just connect it up briefly to the 3 batteries and see what the DC voltage ends up. Is a different sized lower voltage capacitor an option?
Just hook it up for a bit and see where the actual battery voltage and input current level off at with no load on the system before worrying about things you don't actually know are real and true or not.

As for the capacitor, 6800 uf isn't much regarding the overall power levels you are working with. It's pretty tiny actually and changing its voltage rating is irrelevant to how it works in the system. it could be a 10,000 volt rated device and it would still work the same as it does now.

#### spec

##### Well-Known Member
Hi Ed,

The MOSFETS were IRFP260N
Hmm, quite tasty NMOSFETs. There are better though.
http://www.mouser.com/ds/2/196/irfp260npbf-937709.pdf
http://au.rs-online.com/web/p/mosfet-transistors/5429771/
http://www.mouser.com/ds/2/196/irfp260npbf-937709.pdf
The reason I was hoping to use the 3 batteries was that it was close to the output of the transformer..
That is good. May I suggest then that you forget about the transformer for the time being and go for a two battery configuration which will give you much more reliability and, hopefully, no more blown MOSFETs.

If I use just the 2 batteries I won't be able to use the transformer to top the batteries up as I use the heater.
That is not necessarily true.

Just to avoid wasting time, please let me know which way you want to go with the batteries.

Thanks

spec

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#### spec

##### Well-Known Member
Hi again Ed,

Just another question, do you actually need the induction heater to be battery powered, or would it be acceptable for the induction heater to be powered from the mains without the use of batteries?

spec

#### Ed.

##### New Member
Hi Spec, I would prefer to use the three batteries as that would give me the instant amps the the heater module requires, as for being mains powered, I would imagine that the 240V transformer to supply a constant 50A at 36V would be quite large, and I do not have one that size lying around. To buy a transformer that size would also be too expensive. Hence I was trying to use what I have. This heater will probably only be used about once a month so don't want to outlay too much for the use that I will give it. What did you have in mind?

The only other "transformer" that I have that is capable of that amperage is my large inverter MIG welder which I can select from 10V to 39V DC in 0.1 of a volt, however there is a problem in that is has a voltage spike which is about 75V for a very short time just before dropping down to the required set voltage. The main problem is using it is, that if something goes wrong and I short something the MIG will vapourise what ever it is powering as it will supply about 24Kw instantly at that voltage. Ideally a variable voltage power supply unit would be ideal as I could also use it to power other little projects I had in mind. and I had thought about using the MIG to do this, but had the spike issue to deal with and the worry of a short circuit.

The blown Mosfet were the result of me not paying attention and occidentally contacting the bottom of the circuit board to my steel table which shorted the lot, with lots of big sparks and and a big bang. It was working well up to that point.

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#### spec

##### Well-Known Member
Hi Ed

The blown MOSFETS were the result of me not paying attention and Accidentally contacting the bottom of the circuit board to my steel table which shorted the lot, with lots of big sparks and and a big bang. It was working well up to that point.
Ah, that indicates that the induction heater is reliable working from about 36V, which is what I imagine the voltage the three batteries would be producing, with a current load of 30A (the current that the induction heater would be taking with a 36v input according to the data your post #11)

Can I request that you just answer my questions without worrying about solutions for the moment. Apologies for to being pedantic, but that is how you arrive at a good design.

Just to reiterate:
(1) I take it that you would be quite happy if the induction heater were mains powered

(2) What is the minimum power you require from the induction heater: would 24V at 20A be acceptable or do you need the present 36v at 30A?

spec

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