"Medical" Transformer Suitable for Power Supply?

Discussion in 'Homebrew and Kit Projects' started by NQ4T, Sep 2, 2018.

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  1. NQ4T

    NQ4T Ham Member QRZ Page


    Someone was showing me a little 50 watt HF linear kit they were assembling and it got me wondering about the more powerful ones out there; I wound up stumbling on some LDMOS kW HF Linear kits. As I got to looking at them I decided I wanted to maybe build one...but I would need a power supply as it requires 50VDC@35A.

    While looking around for one I came across a "medical" toroidal transformer. Datasheet says it supplies up to 50VDC @ 50A.

    I've been trying to figure out why a medical grade transformer wouldn't be the proper type...but to me a power transformer is a power transformer. I found a place that has one for under $120...legit distributor, not a chinese clone off eBay. They apparently have them on sale. It's normally a $400 transformer so I'm very tempted to pull the trigger.

    Can anyone think of a reason this might not work? I checked to see if that was a pulsed rating...but the data sheet does not indicate it...it just says it's a 50VAC @ 50A or 100 VCT @ 25A.

  2. W9GB

    W9GB Ham Member QRZ Page

    48 VDC was (and still is) used in the Telecommunications industry (Plain Old Telephone Service — Central Office batteries)
    IF you look at modern data centers they will frequently have 120 VAC and -48VDC power sources.
    In the Medical industry, 48/50 VDC power supplies are generally found with Medical imaging, or CT scanning equipment. Since 1977, Medical industry equipment must meet IEC 60601 standards.

    RF transistors (LDMOS) are most frequently found as 48/50 Volt or 24 Volt devices.
    Power supplies from those industries (telco, military, medical) do appear via surplus.
    Last edited: Sep 2, 2018
  3. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

  4. NQ4T

    NQ4T Ham Member QRZ Page

    9 times out of 10 this is the case. I couldn't think of any reason that wouldnt apply this time.

    I just have to look in to how I will handle current regulation and decide if this is a good route.
  5. KB0MNM

    KB0MNM Ham Member QRZ Page

    There are at least 2 reasons why it might not work with the transformer you mention: 1. Specifications- I have found that some transformers are rated for a given voltage at a given current- and while this is the best for many applications- they sometimes fail to specify what the open circuit voltage or heavily loaded voltage would be. In some cases, there would be no problem. In others, I have found that a higher voltage due to lighter loading ( less current ) would cause an inexpensive regulator circuit to produce undue heat*. Very few circuits seem to care about the internal capacitance of the windings, however, some might rely on the total inductance as part of the current limiting circuit. Transformer makers like Triad ( this is just one example, and I have no financial interest in them ) will often put graphs of the output characteristics of their products either online or packed with the product. They also sometimes specify what the expected insulation voltage breakdown would be between the windings. 2. Dimensions- Were you expecting it to fit in a certain size case? The medical-grade transformer that you seek might or might not fit in the same dimensional area as a 'Filament' transformer, due to the shape.*The overvoltage problem can be 'relocated' with additional resistance, yet that may involve special resistor mountings and/or cooling fans. Dimensional issues can also affect where the heat is concentrated. You might wish to consider what sort of rectification and voltage regulator would be used. The transformer would product AC ( You knew that ). The rectifiers could boost or drop the effective DC voltage. 73
  6. WA9SVD

    WA9SVD Ham Member QRZ Page

    I'd be suspicious. The "medical" rating, especially. It MAY indicate a very short period of current draw for a short period, and then an"off" period for a long period of time. I don't think that is what you want.
  7. NQ4T

    NQ4T Ham Member QRZ Page

    It actually is a Triad Magnetics transformer. VPM100-25000. The datasheet doesn't indicate loaded or unloaded voltages.

    It also doesn't mention if there is a dutycycle for the current.

    Size isn't an issue. This was going to be a 100% custom thing from the ground up.

    But I got to playing around with simulations for the current and voltage regulation and the BOM and complexity almost make buying a surplus switching supply of the same voltage and current a "better" option.

    I also got to lookong at everything required to build this amp....and in the long run I can probably find a second hand solid state linear.

    This is still something I'm going to kick around in the back of my head...but Newark had the transformer on sale for $116. So it put a sense of "maybe I should buy it for future use" in my head.
  8. KB0MNM

    KB0MNM Ham Member QRZ Page

    I have built a few different projects from parts, and can attest: 1. There is great satisfaction in completion of a 'did it yourself' project, especially if the design was either original or a modification. 2. Obtaining the parts and building from 'scratch' can be more expensive than 'second hand', by far. That said, 'second hand' means that you may need to repair something after a short period of time. That can depend on how 'old' the equipment is, yet some of the older solid-state equipment and tube equipment has been shown to be 'built like a tank', as in for reliability. 3. If you employ a switch-mode power supply, and subsequently have noise issues, folks will guess that the electrolytic capacitors have 'dried out' ( lost their ability to filter due to the dielectric chemical hardening or breaking down due to heat or age ).
    This is known to occur with linear supplies as well. Both also have rectifiers, which if damaged, would contribute to noise issues and faster breakdown. -and-
    Copper for wire and other uses is subject to price fluctuations. For that reason, some manufacturers are now using more alternate metals. Reduction of Hazardous Materials ( RoHS ) is an international agreement product. When/if you consider buying solder, it ( RoHS ) is now of concern.
  9. W9GB

    W9GB Ham Member QRZ Page

    ECONOMICS ... is always a consideration. You have to decide IF it is the most important Criteria.
    The Power Supply is a major component, providing proper voltage, current, & regulation.

    DIY Building is often drive by “What is Available?”
    Current production parts, readily available surplus, your “junk box” of harvested surplus parts or components, and avoiding Obsolete or End-of-Life (EOL) components.

    The transformer is adequate for your project: Solid-State HF Amplifier, 1000 watts RF Output.
    HOWEVER, it is NOT a complete DC Power Supply. The additional parts for DC rectification, smoothing, thermal dissipation (heat sink), voltage regulation, and Control (OVP, Short-Circuit Protection) can add up quickly.

    Some builders and restorers, use the Meanwell SMPS, such as:
    Meanwell RPS-3000-48 (48 VDC, 62.5 Amp) or
    Meanwell RPS-2400-48 (48 VDC, 50 Amp) at $499.00

    Others use the “off-shelf” Ameritron/MFJ ALS-1300SPS switch-mode PS, for < $1000.
    Last edited: Sep 4, 2018

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