Summary and context:
Several years ago I bought a linear power supply from Pyramid, model PS-3KX.
- I would like to understand in detail how the overload protection really works, as said by Pyramid manufacturer (red box in the Specification part).
- It seems the PS-3KX is capable of 2 A in continuous service (blue underline) but triggers the protection above 3 A (maximum current - purple line).
- An AC component of the secondary transformer seems to be "detected" by the components marked in red (D5, R3&R4, D6&D7, C3, and Q1), which triggers Q2 through C4 and D8 - marked in purple.
- Severe current overload reducing the stabilized Output voltage, or a short-circuit in the output, would lower the polarization voltage in Q1 (green makings) that would raise the voltage at C4, conducting Q2 and shutting down output through Q3 and Q4.
- Voltage feedback regulation is obtained by the circuitry marked in blue, but I have concerns if the ripple detection (red and purple circuitries) would not worsen voltage stabilization or even induce a higher ripple.
UPDATE#2:
After studying the replies and simulation from recently accepted answer and the results of my "Update#1" (Waveforms in a PS transformer), I will comment with UP#2, certain paragraphs in my original post, to avoid misguiding future readers in EESE here.
TLDR: The simulated original circuit does not work as expected and the manufacturer's performance figures for protection could not be confirmed.
UPDATE #1:
I made a collage of the PS-3KX I have, to share the general view of the internals:
== Resuming Original Post ==
The user manual, which can be obtained here, shows:
Specification Summary, with some highlights for my PS model.
Circuit Diagram, with circuitry features color-marked:
Doubts and questions:
Based on the presented information, and assuming the actual PS-3KX unit is not available here for further testing, please help me to solve the following doubts and questions.
If I understood something inadequate or simply wrong or missed any hidden feature, please share your views:
{1} Short-circuit & Overload Protection #1 - Seems to be effective only if the output voltage is lower than 4.1 V. What happens if a higher current is required at higher voltages?
{1A} For example: Let's say, a high-powered LED array starts to conduct at 8 V and requires 4 A @ 12 V? The voltage could be high enough to not trigger green and Q1 circuitry.
{1B} Did I miss something for this circuitry feature?
{2} Overload of the Transformer - Protection #2 - An apparent ingenious circuitry, marked in red and purple, seems to detect higher frequency (harmonics) if the transformer core is overloaded.
{2A} These overload, magnetization, and higher frequency are the most intriguing features for me. I would like to understand properly/completely how this happens.
UP#2: Scoped waveforms in Update#1 and some simulation results (answer), made my mind that the red and purple-marked circuit, as it is, is NOT reliable overload protection. The overload protection circuit could be improved using Antonio51's answer and associated discussions as a guide and starting point.
{3} Effectiveness of the Transformer Protection #2 - This feature could be a feature we could use on an unknown power-supply transformer. Curiously, I have not seen this kind of circuit being used elsewhere.
UP#2: As mentioned above, the existing protection is not reliable/effective to protect either the electronics or any overload on the transformer.
{3A} Is it too good to be true? (UP#2: It is "Not True"...). What would be the hidden trade-offs?
{3B} Would it be problematic, eventually lowering the output voltage even when the current is within the continuous 2 A limit?
Please observe the ripple at this current is specified as 150 mV RMS (in the blue box).
{3C} It is specified the overload protection is +10% ~ 15% with auto-reset (see red box in spec).
I assume this would be above 3 A = maximum stated current.
How precise could this be as Pyramid's stated, if there is no current being directly measured?
{4} Semiconductor Protection - How safe is this schematic to protect the series-pass transistor and rectifier diodes from overcurrent?
{4A} Transformer Protection - How safe is it to avoid severe overload of transformer?
UP#2: Original circuit does not provide the specified protection.
A R_shunt and 1 or 2 BJTs working as a classic current limiting circuit, or working as hiccup protection (see answer) are necessary additions for PS protection.
I'm sure my description was a bit repetitive (or recursive); sorry about that, but see it as a shared brainstorming to understand the circuit and its potential problems.
UPDATE#1: Waveform in Linear Power Supply Transformer
After seeing simulation work made by Antonio51 (Thanks!) I decided to check how a Transformer inside a smaller power supply I made 40 years ago (CV = 1-25 V; CC = 0.13-1 A) would behave from:
- No/Minimum Loading = 76.5 Vpp @ 0 A & 71.0 Vpp @ 0.13 A.
- Average Loading = 58.5 Vpp @ 0.5 A
- Light Overloading = 46.5 Vpp @ 0.94 A.
- Severe Overloading = 11.5 Vpp @ 2~3 A.
The photo collage shows the tests and waveforms.
This test was made with a different linear PS, made with a repurposed car slot transformer (12+12 VAC, 2 A) ~ about the same VA magnitude of the PS-3KX. So I believe the transformer in the PS-3KX would behave similarly.
I did not see any unexpected spikes, just the sinusoid AC wave being flattened when able to charge the capacitor and be drained under constant current.
Even when the transformer is severely overloaded, the waveform is qualitatively similar to regular loading.
Conclusions (by UP#1 & UP#2):
Unless it is missed something, as my hobbyist Scope does not do FFT (it would be great to see), I did not find any transformer behavior in real life (not as model/simulation) that would justify the manufacturer's protection method to be effective or safe.
If any of you has a different view or had another experience, please let us know!
