Making transformer - results not as expected - need your input

Thread Starter

bushrat

Joined Nov 29, 2014
209
Ok, so I got a hold of some transformer wire (coated), and decided to make my self a HV transformer.

I took a 7" iron bolt, put 2 large washers at the end to keep the coils in place without spilling over, and i used a power drill to turn the bolt for me while I guided the wire neatly. I used few hundred feet of wire, wrapped without any gaps, made some 30-50 layers onto a bolt. One I reached the top, i continued to wrap the wire down in same direction (always left hand rule). The whole process came out rather nicely.
I measured the inductance (rough estimate, not precise), and it shows about 400 mH (rather low), but ok. DC resistance came out to about 38-40 ohms
Let this be my secondary coil.

I wrapped some regular wire on top of the first coil, about 10 turns, and I connected sig generator to it, 1 Vpp. I connected o'scope to the secondary coil (the one with thousands of turns) and I was expecting voltage to skyrocket when i reach certain frequency....2Vpp max.

What did i do wrong?
Coil is not open, it shows some DC resistance, why is the output voltage so low?
 

Thread Starter

bushrat

Joined Nov 29, 2014
209
Started from 10 Hz, kept going up until it peaked at 58 khz and then the amplitude kept going lower. at 58kHz I only got 2 Vpp.
 

crutschow

Joined Mar 14, 2008
34,285
Unfortunately a bolt makes a poor transformer since the magnetic loop is not closed.
Instead it has a 7 inch air gap, which gives the observed low inductance, and also poor coupling between primary and secondary.
A transformer needs a closed magnetic circuit for proper operation.
Putting the bolt inside an iron pipe with end caps touching the ends of the bolt would greatly help.
You could cut a slot in the pipe for the wires.
 

Thread Starter

bushrat

Joined Nov 29, 2014
209
Damn... 2 hrs of carefully guiding thin wire wasted....

Edit:
Wouldn't this be an air core transformer?
Even as just plain inductor, woundn't the inductance be a bit higher? Iron core, thousands of turns....

Do the overlaping wires (going up and down) have anything to do with it?
 
Last edited:

Kjeldgaard

Joined Apr 7, 2016
476
In addition to Crutschow wise words, the iron in the bolt is probably the least suitable as a transformer core and as a massive rod there will also be a high eddy current loss.
 

Thread Starter

bushrat

Joined Nov 29, 2014
209
In addition to Crutschow wise words, the iron in the bolt is probably the least suitable as a transformer core and as a massive rod there will also be a high eddy current loss.
I am not disagreeing with Crutschow, I'm simply trying to understand where did I do wrong?
What can i replace the bolt with? I used it because it was easy to attach it to power drill..
 

Kjeldgaard

Joined Apr 7, 2016
476
If it is low-frequency (Up to a few Kilo Hz), then it may be a closed core of laminated soft iron.

At higher frequencies, it may be ferrite. As ferrite is a bad electrical conductor, it may be a massive closed core or a rod for certain purposes.
 

kubeek

Joined Sep 20, 2005
5,794
You could also try simply closing the loop with two plates on the end of your coil and another bolt. Not as good as the pipe idea (no pun intended), but should get you a better coupling than just the air gap you have now.
 

KL7AJ

Joined Nov 4, 2008
2,229
Ok, so I got a hold of some transformer wire (coated), and decided to make my self a HV transformer.

I took a 7" iron bolt, put 2 large washers at the end to keep the coils in place without spilling over, and i used a power drill to turn the bolt for me while I guided the wire neatly. I used few hundred feet of wire, wrapped without any gaps, made some 30-50 layers onto a bolt. One I reached the top, i continued to wrap the wire down in same direction (always left hand rule). The whole process came out rather nicely.
I measured the inductance (rough estimate, not precise), and it shows about 400 mH (rather low), but ok. DC resistance came out to about 38-40 ohms
Let this be my secondary coil.

I wrapped some regular wire on top of the first coil, about 10 turns, and I connected sig generator to it, 1 Vpp. I connected o'scope to the secondary coil (the one with thousands of turns) and I was expecting voltage to skyrocket when i reach certain frequency....2Vpp max.

What did i do wrong?
Coil is not open, it shows some DC resistance, why is the output voltage so low?
Steel bolts are HARD iron, which is the last thing you want for a transformer core. You need a soft iron with low hysteresis.
 

SLK001

Joined Nov 29, 2011
1,549
That doesn't look like magnet wire - it looks more like wire-wrapping wire. Also, you'll have to really drive your primary quite hard with that few of turns. You are probably seriously loading your sig gen - what you think is 1Vp-p is probably closer to 0.01Vp-p (it basically looks like a short to the sig gen). Even with a poor quality core, you should be getting more out of the secondary.
 

crutschow

Joined Mar 14, 2008
34,285
You are probably seriously loading your sig gen
Yes, I missed that. :oops:
The few turns of your secondary likely have a very low inductance and resistance, so the generator is driving a near short-circuit.
Measure the generator voltage when connected and you'll see what we mean.
 

Kshishtoff

Joined Dec 2, 2017
12
You should to rewrap your primary coil (green wire) Make its resistance 4 ohms. Connect your sig generator to the input of an sound amplifire and connect you primary coil to the output of your amplifire. That design will work much better.
 

Thread Starter

bushrat

Joined Nov 29, 2014
209
This is not for any project, or for any purpose, I am simply killing time.
if it doesn't work as transformer or inductor, at least i have neatly wrapped coil of wire...
 

Thread Starter

bushrat

Joined Nov 29, 2014
209
You've chosen an excellent weapon. :rolleyes:
Another one is in the works. Took apart a an E and I frame transformer, and re-wrapping it with huge primary to secondary ratio. Will attempt to re-assemble the frame back together, take inductance measurements, and will report on my update.
 
Last edited:

Thread Starter

bushrat

Joined Nov 29, 2014
209
Update:
Took apart large transformer, making sure to save the iron core (E and I frame).
On primary winding, I wrapped wire around 30-40 times, counter clockwise going up. Primary wire is on the bottom.
On secondary winding, i kept wrapping the wire until I had no room left, wire going same direction as primary. When i reached the top, I started going down, same direction. I tried to keep the wires as neatly as I could. Secondary wire is on top.
I re-assembled the core.
Here are the readings that i am getting:
Primary side: DC resistance - 1.2 ohms, 1.7276 mH
Secodary side: DC resistance - 11.3 ohms - 3.052 H
IMG_20171204_210112812[1].jpg

I know there is more that 1:10 turn ratio. It should be somewhere between 1:50 to 1:100.
...What did I miss this time? What did i do wrong? not enough primary turns?
Is there a formula to calculate the core for specific inductance?
 

kubeek

Joined Sep 20, 2005
5,794
The winding ratio is the square root of the ratio of inductance, so your ratio is about 42:1. At 4.8kHz the primary should have 50ohm impedance which would be ideal for the signal generator, but probably the core probably won´t cooperate much at that frequency. Try and see what you get out of it.
 
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