transformer basics, please help I am lost.

[KF6NFW]

I have a handfull of power transformers, 1 of which I think is to small and the other marginaly better. Either way, here is to the point.

I am building a power supply, and for some reason am getting lost in basics. I suspect because of meds and stress of the day(medical issue I deal with). I need to power and operate the HTX 10. The transformer i was thinking on is a 25.2 Vdc @ 2A. so will this work if I just cut in half and use CT for the 12.6, but will this be enough. I dont own the rig so I have no way to really build and test, and I dont have anything around here that requires so small of power besides the 11 mtr rig, but that doesnt require nearly the same power.

I have looked up the specs for the transformer, but am hesitant. otherwise, it isnt a big deal, I have some transformers that will just be about 200 times what they need to be for such use. If need be I can always rob one from the many cadavers I have collected!

Heavy Duty Power Transformer
(273-1512B) Specifications Faxback Doc. # 37410

Primary: .................................................. ....... 120 VAC

Secondary: ................................................ 25.2 VAC CT 2A
(CT = Center Tap)

DC Resistance Primary: ......................................... 11.8 Ohms
Secondary: ....................................... 0.64 Ohms

Primary current (No Load): ....................... 140 mA at Input 120 VAC
(Loaded): ................................... 760 mA Max.

Core Loss at 25 degrees C.: ...................................... 4 Watts

Temperature Rise: ................. 75 degrees C. at 25 degrees C. Ambient

Bobbin Material: .................................................. . Nylon

Primary color Code: ...................................... Black (2 Wires)

Secondary Color Code: ................................... Yellow (2 Wires)
(Black for Center Tap)

Insulation Resistance: .......................... 500 VDC @ 100 MOhms Min.

Dimensions (HWD): ........................... 2 1/4 x 2 3/4 x 2 1/4 Inches

Specifications are typical; individual units might vary. Specifications are subject to change and improvement without notice.

NOTE: Unit is UL listed under File No. E87140, UL Office, Indiana.


(EB 1/14/97)

[KA9JLM]

For a 25 Watt radio you will need a bigger transformer.

A 5 amp supply would be the minimum that I would use. 10 amp would be better.

[KF6NFW]

For a 25 Watt radio you will need a bigger transformer.

A 5 amp supply would be the minimum that I would use. 10 amp would be better.

That's about what I was thinking, I just couldn't see this transformer surviving any length of time, and would be taxed even at a medium setting of around 10 watts.

I will go pilfer one from an old stereo system I have for parts. I picked up for the power supply section anyways, i just didn't want to build it into something for someone else, i was being selfish as I had a project in mind, but this supply is going to a good friend so it isn't like I will totally lose it.

Thank you.

[W4HAY]

Assuming the transformer is rated for use with a bridge rectifier, using a full wave rectifier circuit instead of the bridge would allow a draw of 4 Amps, which is still marginal at best. Some transformers have a safety factor built into their published ratings, but I wouldn't count on it these days.

Of course you could breadboard the circuit and try the W4HAY Transformer Touch Test(TM), i.e. if you can hold your hand on the transformer for 10 seconds, it's probably OK.

[KA9JLM]

That's about what I was thinking, I just couldn't see this transformer surviving any length of time, and would be taxed even at a medium setting of around 10 watts.

I will go pilfer one from an old stereo system I have for parts. I picked up for the power supply section anyways, i just didn't want to build it into something for someone else, i was being selfish as I had a project in mind, but this supply is going to a good friend so it isn't like I will totally lose it.

Thank you.

Chances of finding something that has the power required in a stereo is slim, Because a audio amp supply is not 100 % duty cycle rated, And their transformers normally have multiple voltage taps that can't be used easily.

A cheap Walmart automotive Battery charger rated at 10 amps or more can be modified to make a good 12 V regulated supply.

I like to use PWM switching regulators because you can run things cooler, but you can use LM317Ks or the like if you have enough heat sink.

Most of the newer automotive Battery chargers use a FET output and are rated for big current and can be turned into a switching DC regulated supply fairly easily.

Some filter caps and a switching regulator are easy and cheap to get.

You can get a new Battery charger with a nice Transformer and current meter for about $20.00

The W4HAY Transformer Touch Test(TM) is a good test, or if you can smell the fish-paper then the transformer core is getting to hot.

[WB2WIK]

The 2A rated transformer might work with the HTX-100 on SSB where the duty cycle is quite small; however a transformer, rectifier and filter don't make a power supply for a rig like that.

If you used a full-wave center tap rectifier, under load you'd have about 16Vdc available for a regulator. That's a bit on the low side, considering you want 13.8Vdc out of the regulator; but it might be possible (barely).

[KA9JLM]

The 2A rated transformer might work with the HTX-100 on SSB where the duty cycle is quite small; however a transformer, rectifier and filter don't make a power supply for a rig like that.

If you used a full-wave center tap rectifier, under load you'd have about 16Vdc available for a regulator. That's a bit on the low side, considering you want 13.8Vdc out of the regulator; but it might be possible (barely).

If it is a HTX-100 then that transformer may work ok on SSB and CW as well, with light duty. I would use a regulator. Just a rectifier and a cap will not cut it. If unregulated and unloaded the voltage could pop something important.

I thought it was a HTX-10 and would be used on AM or FM, which would take a larger supply.