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Old 11th Jan 2022, 4:31 am   #1
Skywave
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Default H.V. active dummy load

Scenario: you have a stand-alone high-voltage PSU on your bench or one that is integrated into a valve radio (or perhaps some other item). You need to test it: output voltage at a certain current and waveform (assuming a 'scope is available). One way of doing that is with a collection of suitable ohmic and power resistors, or a combination of them, connected to that PSU, those resistors possibly being switch-selectable, and using instruments to measure the voltage across that test load and the current through it.

And that was what I had been doing for many years, (Yow! That's hot: it's burnt my finger! ) when I came up with this idea for a high-voltage dummy load. The advantages of this approach are that the output current from the PSU under test can be adjusted to any value - within the limits of this item and the available voltage and current from the source - and it has its own integrated calibrated milliammeter and voltmeter.

HV Active dummy load – description.

Circuit description.
How this item works is mainly self-explanatory by studying the circuit diagram, but a few features require comment.
D1 is intended as protection against reverse polarity connections from the power source: a reverse polarity connection will cause fuse F1 to fail (and hopefully protect the power transistors in this item). This fuse will also fail if for any reason the current drawn by this unit is substantially greater than 300 mA approx.
Q1 & Q2 are mounted on a substantial heatsink; the load resistor (1200 ohms, 50 watts rated) is similarly mounted on a separate substantial heat-sink. Both heat-sinks are cooled by a mains-driven fan.
R1 and R2 both have calibration pre-set resistors: these have been carefully adjusted using a calibrated Fluke DMM.
The max. Vin is 500 v.d.c.; the max. I in is 300 mA.

Post-assembly assessment.

This item was connected to a regulated d.c. power supply delivering 300 v. at 200 mA for 3 hours: no problems became manifest during that ‘soak test’.
The cabinet and the inner chassis each have many holes in the metalwork. This is simply because the case of this item has been re-purposed from an item of test equipment that had become redundant and was, therefore, no longer required. Those holes do not adversely affect the functionality of this item in any manner.
One front panel meter isn't quite where it should be : that's on my 'One day I'll fix that' list!

Al.Jan. 11th.

Al., October 16th., 2021. / Updated Dec. 25, 2021
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Last edited by Skywave; 11th Jan 2022 at 4:45 am.
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Old 11th Jan 2022, 6:04 am   #2
Technomaniac
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Default Re: H.V. active dummy load

We went the other way, and pulled a power supply board out of an older Panasonic CTV, brought the tab pott voltage adjustment out on a control knob, and mounted three paralleled lampholders but with individual switching on/off in series with the output. A fourth switch bypassed the series lamp bank giving direct hard output. The whole setup fed by the original TVs power tranny. We wouldn't have available the voltage required in all cases, but that was never a problem. The lamps would have been probably 2 x 240v 60W and one 100 watt. There is also in our shop a power supply using a large old valve radio power transformer 385v aside of centretap, with a rectifier consisting of two 2A3 single triode power amplifier valves as a full wave rectifier which gave variable output voltage from about 70 to 400 volts, with a mA meter and a voltmeter. I built that while I was still in Primary School. I had a multivibrator, a grid dip oscillator and a few regenerative receivers that plugged into it for power. And it later was used occasionally for testing a particular stage of a TV, as was the other one, which would run the whole set. But you'll find that one useful, as long as you have analogue meters. Digital ones take a bit long to settle.
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Old 11th Jan 2022, 7:56 am   #3
Radio Wrangler
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Default Re: H.V. active dummy load

I built myself something similar when I spent a year developing a strange SMPS. (regulated sinewave output!)

Being able to vary the load current other than by switching resistors, is very convenient.

One thing I came across was that by using feedback (those emitter/source resistors) causes the load to try to take a constant current and the dynamic load impedance presented to the supply goes high, very high, within the bandwidth of the control loop.

Consequently, there is no damping on any dynamics of the power supply being tested and you can find undamped resonances and watch the voltage bounce dramatically.

A bank of resistors is just that and makes a load which obeys Ohm's law.

Which is right?

Well, a lot of valve circuitry is biassed to take a controlled current (class A stages) and so the constant current load is appropriate. Class B stages will take signal-dependent surges of current. Resistive loads, motors, and things whose quiescent current is set by the HT voltage, will look resistive.

So I wound up making a dog's breakfast of opamps so I could synthesise not only a load current, but also vary the slope of current versus applied voltage. I had an AC signal input for modulating the current drain, and then I had several switched bandwidths for the loop. I was doing some spec tests and in the end had to get the bandwidth to over 25kHz. This was far more extreme than needed to handle valve radio/amplifier supplies.

This AC source wound up in military test-bays and I also had to create sideband impedance curves on the AC output, but that was a separate machine!

Anyway, when you come across active loads, think about the Z they present.

Also you get to play a prank on the unknowing by suggesting a switch-mode load simulator that because it's switch mode, doesn't dissipate any heat. Amazing how few people wonder where the energy is going

David
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Old 11th Jan 2022, 7:53 pm   #4
Skywave
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Default Re: H.V. active dummy load

Quote:
Originally Posted by Radio Wrangler View Post
Also you get to play a prank on the unknowing by suggesting a switch-mode load simulator that because it's switch mode, doesn't dissipate any heat. Amazing how few people wonder where the energy is going
David
A little knowledge is a dangerous thing; a 'little knowledge' that is, not total ignorance, as in "Wassat?"
So, the "unknowing" come face-to-face with a SMPSU and, in effect, ask the Q,: "Where is the energy going"?
My response: surely they'll know the one key simple principle of how a 'switcher' works? So to enlighten them, switch the PSU off and invite them to feel the temperature of the switching device - then they'll get their answer!

Al. / Jan. 11th.
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Old 14th Jan 2022, 12:37 pm   #5
dave cox
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Default Re: H.V. active dummy load

Hi Al, that looks like a nice piece of home-brew test gear!

BTW, did you label the transistors correctly, i.e. should the 2SC4138 be the output ?

I suppose a 'switch mode' load could be made, of course you would only be able to move the dissipation into a resistor rather than dissipating in a transistor. However, the sheer number of 'switched' high power resistors needed would likely negate any benefit !!!

dc
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