Opinions on this inverter transformer for prototyping NIMOs

[Dylan Distasio]

Hi all-

I noticed that electronics goldmine is selling compact inverter transformers cheap, so I just ordered a few:

https://www.goldmine-elec-products.com/prodinfo.asp?number=G22744

Can on of my betters on the list advise on whether they see any issues with using this to test a NIMO tube using a low voltage DC input?

I'm still a beginner on some of the theory side, so wanted to make sure I was not missing a problem here.

Also, it doesn't come with a pinout, and I had heard that inverter transformer inputs are typically on a diagonal instead of one side.  Can anyone confirm that this is standard, and suggest a way to figure out the pinout using a multimeter only (not sure if that is possible but I figured I'd ask).

Best,

Dylan

[John Rehwinkel]

> Can on of my betters on the list advise on whether they see any issues with using this to test a NIMO tube using a low voltage DC input?

It seems to me it should work. The main problems I'd watch out for are overdriving it (an arc in a winding will ruin it), and regulating the voltage. You may or may not want a doubler on the output.

> Also, it doesn't come with a pinout, and I had heard that inverter transformer inputs are typically on a diagonal instead of one side. Can anyone confirm that this is standard, and suggest a way to figure out the pinout using a multimeter only (not sure if that is possible but I figured I'd ask).

It's easy with a multimeter, just check for resistance. You should get low resistance on the primary (watch out for getting zapped on the secondary while doing this), and moderate resistance on the secondary.

Regards,
John

[alb.001 alb.001]

Turns ratio 260...   1 Volt AC in gives about 260 Volts AC out - without rectifyers etc.   Need to be very low on the AC in or you will get excessive HV out    states it is for devices using much higher voltage. 

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[Tomasz Kowalczyk]

This is not the turn ratio. The person who wrote that text measured DC resistance... so 260 is resistance ratio, which is a pretty useless parameter. Both windings are most likely wound with different wires (thicker for low voltage side, the thinnest possible on secondary). 

What you need to do to determine turns ratio is to plug one side of the transformer to a known AC source and measure secondary.

Due to unknown inductance I would not recommend using mains voltage, as people can do when identifying turns ratio of an output transformer for valve amplifiers.

However, you might not need to know the turns ratio at all. I would use some low voltage, low duty cycle PWM at about 50-100kHz (practically any microcontroller is capable of delivering that) and just look how much voltage there is on the output, which would give a starting point in designing the supply.

[Dylan Distasio]

Thanks to all for the answers including the clarification on turns ratio.  One question though, this reads as an inverter in the listing, so I was under the impression I could feed it DC in and get AC out.  They reference using a 9V battery as input in the description.  Is my impression correct?

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[John Rehwinkel]

Thanks to all for the answers including the clarification on turns ratio.  One question though, this reads as an inverter in the listing, so I was under the impression I could feed it DC in and get AC out.  They reference using a 9V battery as input in the description.  Is my impression correct?

It's not a complete inverter, it's just a transformer for an inverter.  Transformers only work on AC signals.  I'm guessing they used a small transistor oscillator powered by a 9V battery to drive the primary to test it.  You would need to do essentially the same thing to use it.

- John

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[gregebert]

You have to make a decision based on how this transformer tests-out. 

Option 1: Step-up transformer to rectifier+capacitor. This is by far the simplest solution if the turns-ratio is in the right ballpark, and it doesn't overheat under load when running on 60Hz. As mentioned above, try applying a small AC-voltage to the primary winding. You can identify the primary by measuring the resistance with an ohmmeter; primary resistance for a step-up transformer will be lower. Now measure volts-in and volts-out to get the ratio. Once you have that, you need to select from your available AC sources, such as 6.3V filament. The optimum voltage for a NIMO tube from the datasheet is 1850V, but I would target a higher voltage to allow for losses; you can always add an anode resistor if necessary.

Once you have the target voltage correct, run it under load (180uA for 6 NIMOs). A 10Meg resistor is about right for that. Let it run for several days to make sure it doesn't overheat, etc.

Option 2: If the voltage is not high enough, you can use a voltage doubler, tripler, etc. But you wont know until you have the turns ratio.

Option 3: Flyback converter. This uses pulsed DC, and will require voltage feedback to prevent the output from going too high. The output voltage of a flyback converter is load-dependent. Let's wait until you have more info about the turns ratio before going this route.

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Generally, I dont build anything unless I can simulate it thoroughly in SPICE and Verilog. But this is a chance to make a simple HV supply with minimal time/money so go for it. The worst thing that can happen is you will burn-up a $2 transformer, or a get nasty shock.

[Tomasz Kowalczyk]

Worth noting: 
1. Your typical voltmeter is rated for 600V or 1000V. You'll need to use a voltage divider (for example a 10 MEG : 1 MEG). Depending on your voltmeter internal resistance, you might need to adjust the result to take it into account - check its datasheet for more information about input resistance.

2. Such small high voltage supplies are very limited when it comes to output current. Even this 11 MEG probe will be a load, which in low voltage circuits you would be able to safely ignore.

[redrok]

Ditto on the turns ratio thing.
If the impedance specs are, in fact, correct the turns ratio would be 260^.5 or about 16 to 1.
Also Electronics Goldmine also has a high voltage divider resistor:
http://www.goldmine-elec-products.com/prodinfo.asp?number=G17753
Could be useful in a nice "Flyback" circuit to regulate the voltage.
An "Inverter" circuit would not produce high enough voltage.
redrok