Ray-

I would endorse using old 400 Hz transformers, I build a couple of inverters with them and they work amazingly well, plus it vindicated me- I caught a lot of crap for years from people questioning why I would save them.  The also make great modulation transformers, if you can find  the proper ratio and power level.  BTW, I have run them as high as 2500 Hz, the efficiency usually peaks well north of 400 Hz when driving with a square wave. 

Scott

From: [email protected] <[email protected]> On Behalf Of Ray Fantini
Sent: Tuesday, April 2, 2024 08:12
To: [email protected]; [email protected]; [email protected]
Subject: [MRCA] Parts Box Inverter

I wanted to throw together something that would provide between 90 to 135 volts for operating radios in the field along with using junk from around the shop and not buy anything.

Thought about doing a high frequency inverter but with all the issues with RFI and having to use high frequency transformers decided to go with low frequency. The power supply may be bigger and not suited for a small radio but with the WS, TBX or GRC stuff I am playing around with don’t care about size and weight.

The inverter is built around a 555 developing a square wave output that is directly coupled to a Mosfet driving a power transformer in reverse. The 10 K pot allows you to adjust the pulse width of the oscillator and affects the output voltage. Combinations of the 0.33 and 12 K resistor set the frequency of the oscillator and figure its running around seventy or eighty Hz.

The RFP50N05 50V 50A 120A Pulsed N-Channel Power Mosfets is from an old UPS system and is magic, unlike a transistor they have almost no internal resistance when turned on and develop almost no heat.

The transformer is a 36 volt and although it is not happy about being driven by a square wave it works and provided a good range of output. Thru trial and error found this one to be best. Transformers work both ways so you can use the primary as the secondary and they don’t know the difference. The trick is to not exceed the insulation value.  Cheap Radio Shack transformers tended to get hot or buzz.

The output side of the transformer, the old primary feeds a full wave bridge and conventional filter network.

Couple things that I would have liked to do different is maybe use a 400 cycle transformer because that way can use a smaller transformer and filters. The Mosfet I am using runs all day long under load and never gets warm so will keep it simple for now.

Ray F/KA3EKH