Hey,
(please note, this isn't about high voltage, thats why its not in the high voltage section).
After putting it off for months, I have just wound my first transformer as a practice run for future projects. Some are high voltage, some are low voltage and there are different topologies I plan to use.
This first one however, is a simple flyback transformer (or coupled inductor) wound on a small E-core ala 'flybacks for dummies':
Flyback Converters for Dummies
Since I didn't know the properties of the ferrite, I used an inductance meter I built a while ago to measure the inductance of 20 turns. Then, since I knew I would need an air gap anyway, I slowly filed down one of the centre legs of the E-core's to end up with a very small (maybe 0.1-0.2mm) gap. With 20 turns, surprisingly I got a nice 101.2uH!!
So, since I wanted a 1:12 step up transformer, thats 20 * 12 = 240 turns on the secondary. As I couldn't be bothered to count the actual turns as I did them, I wound one neat layer, counted the turns (roughly 40-50) then added 6 more layers. Now, the reason for the extra layer (50*6= 300, not 240) was because I would rather over-wind and have to 'unwind' it afterwards, than cut the wire, measure, then have to splice more wire and add more.
Now, I have read up on inductance and transformers, but I guess i'm missing something. When I measure the primary now, after I have wound the secondary over it, I get 308uH, instead of 101uH. Also, I measured the secondary inductance as 15.6mH.
Inductance ratio = 15600 / 308 = 50.65. Theoretically, the turns ratio is the square root of this, which is 7.12. I know I wound exactly 20 turns on the primary...so that would mean that, the secondary has around 20 * 7.12 = 142.4. On 7 full layers, that would be 142/7 = 20 turns each layer - I wound waay more than this. (I purposefully added far too many turns).
So, obviously either my inductance meter isn't up to the job (quite possible, although it seems to measure 'known' inductors very accurately). Or, my testing methods are flawed (very probable).
Could someone explain two things:
1. why the primary inductance increased when I added the second winding? Is it simply that the primaries magnetic field is better coupled to the core? since it is now surrounded by a coil.
2. Why are my results for measuring turns ratio (indirectly, by measuring inductance ratio) so off? as my measured results are pretty much half of what was expected. I mean, given my DIY inductance meter, and perhaps my poor experimental practice, I expected some error, but not this much...
I am sticking with 20 turns, but planning to gap the core futher to get 47uH (ish) and perhaps even lower if the saturation of the primary is under 2.5A. This is really 'trial and error', but its better than just going with pure theory
Also, if someone can provide a different method of measuring the ratio, preferably without dumping a 12V square-wave across the primary at a few kHz...just incase the ratio really is 1:14+, I don't trust my winding insulation layer to cope with the massive HV spikes it'll produce.
Thankyou, apologies if this really is basic stuff, I coverd it in university a few years ago, but that was all basic theory.
(please note, this isn't about high voltage, thats why its not in the high voltage section).
After putting it off for months, I have just wound my first transformer as a practice run for future projects. Some are high voltage, some are low voltage and there are different topologies I plan to use.
This first one however, is a simple flyback transformer (or coupled inductor) wound on a small E-core ala 'flybacks for dummies':
Flyback Converters for Dummies
Since I didn't know the properties of the ferrite, I used an inductance meter I built a while ago to measure the inductance of 20 turns. Then, since I knew I would need an air gap anyway, I slowly filed down one of the centre legs of the E-core's to end up with a very small (maybe 0.1-0.2mm) gap. With 20 turns, surprisingly I got a nice 101.2uH!!
So, since I wanted a 1:12 step up transformer, thats 20 * 12 = 240 turns on the secondary. As I couldn't be bothered to count the actual turns as I did them, I wound one neat layer, counted the turns (roughly 40-50) then added 6 more layers. Now, the reason for the extra layer (50*6= 300, not 240) was because I would rather over-wind and have to 'unwind' it afterwards, than cut the wire, measure, then have to splice more wire and add more.
Now, I have read up on inductance and transformers, but I guess i'm missing something. When I measure the primary now, after I have wound the secondary over it, I get 308uH, instead of 101uH. Also, I measured the secondary inductance as 15.6mH.
Inductance ratio = 15600 / 308 = 50.65. Theoretically, the turns ratio is the square root of this, which is 7.12. I know I wound exactly 20 turns on the primary...so that would mean that, the secondary has around 20 * 7.12 = 142.4. On 7 full layers, that would be 142/7 = 20 turns each layer - I wound waay more than this. (I purposefully added far too many turns).
So, obviously either my inductance meter isn't up to the job (quite possible, although it seems to measure 'known' inductors very accurately). Or, my testing methods are flawed (very probable).
Could someone explain two things:
1. why the primary inductance increased when I added the second winding? Is it simply that the primaries magnetic field is better coupled to the core? since it is now surrounded by a coil.
2. Why are my results for measuring turns ratio (indirectly, by measuring inductance ratio) so off? as my measured results are pretty much half of what was expected. I mean, given my DIY inductance meter, and perhaps my poor experimental practice, I expected some error, but not this much...
I am sticking with 20 turns, but planning to gap the core futher to get 47uH (ish) and perhaps even lower if the saturation of the primary is under 2.5A. This is really 'trial and error', but its better than just going with pure theory
Also, if someone can provide a different method of measuring the ratio, preferably without dumping a 12V square-wave across the primary at a few kHz...just incase the ratio really is 1:14+, I don't trust my winding insulation layer to cope with the massive HV spikes it'll produce.
Thankyou, apologies if this really is basic stuff, I coverd it in university a few years ago, but that was all basic theory.