If I had a decent NPN transistor that would be the most straightforward way to drive the circuit. With the virus lockdown, I can't go out and buy any parts but I may have something in the garage that would work. With a bit of hunting I located a 2N5551 and wired it up.
This is equivalent to the interface circuit in the Apollo Guidance Computer that provides the 800 Hz drive for the power supply. It is an NPN transistor through a 2K load resistor This restricts the current to a maximum of 2.5ma when tested with 5V but using the spacecraft power level of 14V it can sink up to 7ma, a tenth of a watt dissipation.
Up first was a direct test of the driver transistor delivering the appropriate 800 Hz signal at a few milliamps current, which is well below the max I want to drive the single transistor. It was breadboarded and hooked to the scope, with exactly the results I wanted.
Then I wired it to the power supply module through this driver circuit. The function generator gave me the 800 Hz square wave, then the driver transistor will sink the 14V to ground, after it passes through the primary of the first coil inside the power supply module.
RESULTS AND OBSERVATIONS
For this test, I hooked the power supply output to a segment on the electroluminescent (EL) panel to provide a proper load. I wanted to monitor the output waveform and measure the voltage using the scope. I started with the potentiometer wires open to reflect the cabin lighting pot set to max brightness. I ccould then insert a fixed resistor or short across the wires to simulate various dimmed settings.
The segments were no brighter than before - pretty dim actually. Their wasn't enough oomph to light the fixed lines and legends with the standard input voltages of +14 and +28. I had to see what the output voltage looked like on the scope. It should be 800 Hz AC with an RMS value of about 275V.
The high voltage is isolated from the input side and spacecraft ground, thus the scope has to be grounded only on one side of the output. I turned on the test bench and observed the output waveforms. It appears to produce a reasonable waveform but a bit low, just as the previous circuit did.
|800 Hz with 250V peaks, therefore below desired level|
Since there seems to be excess resistance in the ground side of the EL panel, that voltage drop might be confusing the current compensating circuit in the output, where the supply could be working just fine. To test this, I left it disconnected from the EL panel and measured the output voltage. Since the peak to peak approached 480V, it shows there is some effect but not enough to account for the low output.
At worst case, I will have to work with the El panel in a darkened room to get acceptable brightness. There are two other possibilities to brighten this up. First, I could recreate the entire circuit in discrete components, ensuring I can reach the voltage targets that I want. Second, I could find a way to 'amp up' the power supply output. The discrete component alternative is quite hard because of the specially wound transformer that is used to ensure constant brightness with a variable number of EL segments illuminated.
I can pump a bit more out of the circuit by boosting the supply voltage to the power supply module, although it introduces the risk that I will exceed the max voltages of the transistors inside causing a failure. Because of this, I can't ratchet the voltages up too much, but I should be able to go up about 25% with some safety. This was designed to operate in a spacecraft which did have some transients and high side excursions in power, thus the transistors would not have been chosen right at the margins.
Bumping up the +14V supply to 17.5 volts and the +28V supply to 35 volts would implement my 25% boost. I gave it another try, with 17.5V and 32V since my supply can't produce the full 35V. The segment was noticeably brighter even without the supply generating the full 275VAC that it should.
|Partially boosted voltages improved brightness|
|Scope leads reversed, symptoms also reversed|
I need to pull out another power supply to inject the full 35V for my original brightening plan. This is not readily at hand but the current illumination is adequate for viewing indoors in normal room conditions.
|Decent illumination level|