EEVblog 1698 - Awesome Ebay AC Power Source: GW Instek APS-7050 - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: July 25, 2025, 12:20:03 am
content: I scored this GW Instek APS-7050 Precision AC Power Source on Ebay, let's check it out.

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: July 25, 2025, 03:58:19 am
content: So if the voltage coming in is 230v, and that's 230×1.414==325.22Vp-p how are the capacitors, which are in parallel and rated for 63v, not blowing up?!?If they were in series, that would be 189v*2=378v.

author: EEVblog
author_rank: Unknown
timestamp: July 25, 2025, 04:01:53 am
content: Quote from: ballsystemlord on July 25, 2025, 03:58:19 amThe input transformer is a step down.As an aside, with 2nd thought this is basically a big linear audio power amplifier rather than a traditional switching DC-AC inverter.

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: July 25, 2025, 04:17:33 am
content: Does it not need a PFC circuit? Or perhaps the incoming transformer works well as a passive PFC?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 25, 2025, 08:27:15 am
content: The output transformer is needed to bring up the voltage from the linear amplifier. The linear transistor amplifiers have a preferred voltage range - too high gets tricky with SOA of the transistors. I would expect the voltage steps and winding switching at the output side, not so much the input side.There is likely some injection for the fast glitches after the transformer. At least there would be the current measurement and likely also some feedback for the voltage.The 2nd PCB on top if the heat sink is just for mechanical stability with isolation (the heat sinks are likely at some +-40 V). It is a nice detail.It does not look like they have PFC. Test instruments and similar may not need it as they are usually not used very often or in high numbers. The more common case is stand-by with low power. It may make sense to have PFC in such a construction as this usually comes with a crude regulation of the raw voltage and get away with a smaller input transformer.

author: pdeal
author_rank: Contributor
timestamp: July 26, 2025, 02:39:43 pm
content: Very cool instrument Dave!I know you didn't really reverse engineer it exactly but I guess the typical power supply just turns the transistors on and so they are full on or full off but this one must modulate the transistors in a linear mode to get the desired output? Wouldn't there have to be some sort of realtime feedback of the output? Any idea how that would be done? Seems like for an instrument like this it would need to be a many times per cycle measurement of the output.I'd like to look at the output of a variable frequency motor drive and see what kind of sin wave output these generate. Seems like these would need to have to modulate the transistors in a partial on state too? Lots of heat I would think.

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: July 27, 2025, 12:51:02 am
content: Quote from: pdeal on July 26, 2025, 02:39:43 pmActually, for a VFD, you'd almost always be using IGBTs.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 27, 2025, 09:06:03 am
content: The APS7050 here very much looks like a classic linear amplifier (not that different from audio) and than the output side transformer to step up the voltage.So the transistors are not swiched, but in linear mode. The linear amplifier part is what needs the large heat sinks.I don't know if the do the feedback from before or after the transformer (likely a mix from both sides). The extra transients are quite steep and thus likely added after the transformer (the PCB below the control and thus hidden).The is some SMPS part, but it looks like this is to power the digital part and output side modulation.VFDs are usually switched mode and give a lot less clean output. These more correspond to a class D amplifier and the output may looks more like the square pulses with zero in between, but it depends on the model.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 27, 2025, 06:52:54 pm
content: Th factual errors in this video are really astounding. The big transistors are no FET's but BJT's, and the low voltage electrolytics, while dave is pointing at 115V and more transformer windings with the relays.It would have been really nice to discuss a more detailed block diagram, and then also do some measurements, for example whether the fault simulations are injected at the "audio amplifier" stage, or in that block after the transformer. But dave yapping about making general assumptions and then 5 minutes later discovering they were wrong and saying something else is a bit annoying. The APS7050 itself is an interesting piece of kit though.Quote from: Kleinstein on July 27, 2025, 09:06:03 amYes.A brother of mine bought a Chinese Class D audio amplifier and added a mains transformer, and built something very similar to this thing. First he tried it with a 230Vac transformer, but he had saturation problems and the Class-D amp triggered it's overcurrent protection. After that he bought another transformer made for 400Vac, and that apparently worked nicely.And further. If you want to build something like this. it seems logical to put in an LISN too.

author: picburner
author_rank: Frequent Contributor
timestamp: August 06, 2025, 04:09:44 pm
content: From reading the user manual it appears that the output voltage can be brought up to 600V and the frequency up to 1KHz only via two USB sticks (Opt. APS-003 and APS-004) so the hardware is already set up to operate at these values.

author: thm_w
author_rank: Super Contributor
timestamp: August 06, 2025, 09:31:37 pm
content: Quote from: picburner on August 06, 2025, 04:09:44 pmQuestion is if the APS-7050E has the hardware built into it as well? I'm guessing not.

author: picburner
author_rank: Frequent Contributor
timestamp: August 07, 2025, 03:35:06 am
content: I've read the APS-7050E user manual too, and it doesn't mention the possibility of adding these two options.This is understandable, given that the 7050E is a cut down and more affordable version of the 7050.Who knows if the reduction is just in the software or if they've also modified (reduced) the hardware.In particular, the number of windings in the output toroidal transformer.