Winding transformer

[JohnW2]

I have a few 'large' capacitors, one being 29,000uF, but the voltage is 25V so not quite big enough. ( I have a pair of 25V 13,000uF that could be in series, if they suit) I will need to find the correct size, for me to price it up. I've probably got the rectifiers.

The size of capacitor you need will depend upon how much ripple on the output is acceptable (at maximum current load) - which, in turn, will depend upon what it is feeding.

Kind Regards, John

 

[JohnW2]

I've just bought a second hand 24V 1000W power supply £100 delivered. There may be taxes on top?

Sounds like a sensible move. It's a pity that (I presume) it would not be acceptable to charge less batteries at a time, since you'd then be into the realm of much cheaper transformers/power supplies.

As we've gone this far with the transformer, I'll carry on, with a lesser project. I'll make a variable one instead. If I use what's in my junk boxes! What is the safe voltage for 25V rated capacitors?

Certainly no more than 25V (peak) - I would personally want to see a significant safety margin, so probably would not subject it to more than 20V peak. 25V peak rectified would imply a transformer secondary output voltage of about 17.7V RMS (about 14.1V for 20V peak rectified).

Using capacitors in series to increase working voltage has its problems, since you have to take steps to ensure that voltage is always shared between the capacitors in the desired fashion. The nightmare scenario is a situation in which one fails in a mode that causes excessive voltage on the other one - hence two bangs!

If those large value capacitors you have are old, be very careful ('stand well back') when first applying voltage to them - they might produce a dramatic bang. Probably best to initially gradually increase voltage when you first 'power them up'.

I've got a quite a bit of 22SWG and loads of cotton insulated 21SWG copper wire (shows how long I've been hoarding it) as mentioned before it could be doubled up.

At the voltages we're talking about, insulation is probably not an issue (although note previous comments about possible high voltage 'surges' at switch-on etc.) and the size of the size of the wire obviously depends upon the maximum current. IIRC, a single 21 SWG copper wire has a capacity of about 1.5A.

Kind Regards, John

 

[Jackrae]

22swg = 0.711mm diameter so giving 0.4mm² of area. At the conservative end of 1000A/inch² (1.65A/mm²) that equates to a load capacity of 0.66 Amps

Tables for wire current capacity invariably relate to open ventilated conductors so when tightly wound onto a transformer bobbin you have to make allowances. I'd suggest you limit your load current per strand to 1.5 amps as JW2 indicates. If you are thinking of running two windings in parallel be aware that each winding MUST have the EXACT same number of turns, otherwise you will end up with high internal circulating currents.

edit : damned predictive text changes words - now corrected to what I meant.

 

[ban-all-sheds]

If those large value capacitors you have are old, be very careful ('stand well back') when first applying voltage to them - they might produce a dramatic bang. Probably best to initially gradually increase voltage when you first 'power them up'.

http://www.google.co.uk/search?safe...g+OR+reforming+OR+restoring)+old+electrolytic

 

[ban-all-sheds]

Tables for wire current capacity invariably relate to oven ventilated conductors.

What oven temperature do they assume? Will tables for old Imperial sized cables have it in Gas Mark #?

 

[Jackrae]

Naw, we're still working with "one log or two ?"
Predictive text seemed to prefer 'oven' to 'open' and it obviously wasn't a mistype because V and P are far apart on the keyboard.

 

[ban-all-sheds]

Ah - predictive text, and suggestions from spell checkers. Wouldn't you just love, sometimes, to meet one of the plonkers responsible for some of those round the back of the bike sheds...

I particularly love spell checkers which (and it can only be because of perverse design) omit obvious adjacent-key suggestions.


Mind you, it was you who chose 'oven', or allowed it to go through...

Tools allow us to do things more quickly, or with greater ease, or which we simply could not do without them, but the operator of them retains total responsibility for their correct operation. Some require greater care, more alertness etc to be applied than when not using them - compare a hand-held electric circular saw to a hand saw, for example.

 

[camerart]

Charging multiple cell packs: At the moment I charge the batteries, 3x cell at a time, because of the charger set-up I have, which takes too long and requires the cells to be apart each time. Once they have been made into a pack, I will be able to charge all at once. That's why I'm stepping up the charging set-up.

As I use lots of 12v applications, a variable power supply with 16V max would be a good compromise. I think 5A would also be good, so that sounds to me like 4x strands of cotton covered 21SWG?? Now I need to work out the length of wires, to make up the amount of turns with a bit spare.

I have rectifiers, and a variac for a slow voltage ramp, for the cap. Can I assume that a too big, as in uF will be ok for a variable unit?

Avoid predictive sex! Off out to fly my multi-copter

C.

 

[JohnW2]

Charging multiple cell packs: At the moment I charge the batteries, 3x cell at a time, because of the charger set-up I have, which takes too long and requires the cells to be apart each time. Once they have been made into a pack, I will be able to charge all at once. That's why I'm stepping up the charging set-up.

Fair enough - but there are presumably be 'half-way houses' between 3 cells and 27 cells which would/could have been much cheaper for you.

As I use lots of 12v applications, a variable power supply with 16V max would be a good compromise. I think 5A would also be good, so that sounds to me like 4x strands of cotton covered 21SWG?? Now I need to work out the length of wires, to make up the amount of turns with a bit spare.

As has been said, you need to look into the current-carrying capacity of the wire when it is all wound around a transformer core - the 'ratings' you're most likely to come across will relate to a single strand of cable in free air.

I have rectifiers, and a variac for a slow voltage ramp, for the cap. Can I assume that a too big, as in uF will be ok for a variable unit?

The greater the capacitance of the capacitor, the greater will be the (potentially very high) transient surge of current through the rectifier at switch-on (when a very high capacitance capacitor will look almost like a short circuit for a very short period of time). You would therefore need to ensure that the rectifier you were using was capable of withstanding the (very brief) peak surge current at switch-on which was going to result from the capacitor you were using. You could probably find that maximum surge current from the rectifier's data sheet, but it would probably be much more of a mission to try to estimate what the peak switch-on current through the capacitor would be.

KInd Regards, John

 

[ban-all-sheds]

Avoid predictive sex!

What about proscriptive?

 

[camerart]

Charging multiple cell packs: At the moment I charge the batteries, 3x cell at a time, because of the charger set-up I have, which takes too long and requires the cells to be apart each time. Once they have been made into a pack, I will be able to charge all at once. That's why I'm stepping up the charging set-up.

Fair enough - but there are presumably be 'half-way houses' between 3 cells and 27 cells which would/could have been much cheaper for you.

As I use lots of 12v applications, a variable power supply with 16V max would be a good compromise. I think 5A would also be good, so that sounds to me like 4x strands of cotton covered 21SWG?? Now I need to work out the length of wires, to make up the amount of turns with a bit spare.

As has been said, you need to look into the current-carrying capacity of the wire when it is all wound around a transformer core - the 'ratings' you're most likely to come across will relate to a single strand of cable in free air.

I have rectifiers, and a variac for a slow voltage ramp, for the cap. Can I assume that a too big, as in uF will be ok for a variable unit?

The greater the capacitance of the capacitor, the greater will be the (potentially very high) transient surge of current through the rectifier at switch-on (when a very high capacitance capacitor will look almost like a short circuit for a very short period of time). You would therefore need to ensure that the rectifier you were using was capable of withstanding the (very brief) peak surge current at switch-on which was going to result from the capacitor you were using. You could probably find that maximum surge current from the rectifier's data sheet, but it would probably be much more of a mission to try to estimate what the peak switch-on current through the capacitor would be.

KInd Regards, John

No 27 minimum.

I'll double check the rest, thanks.

C.