Running Rigol scope off rectified AC from inverter, i.e. DC. - Page 1
Forum section: electronics / testgear

author: Circlotron
author_rank: Super Contributor
timestamp: June 06, 2015, 04:14:16 am
content: I have a DS2072 and the other day I powered it from a 7Ah SLA battery and this 150 watt inverter ->http://www.jaycar.com.au/Power-Products-Electrical/Power-Conversion-%26-Transformation/DC-AC-Inverters/150W-Can-Sized-Power-Inverter-with-2-1A-USB-Output/p/MI5127Even though the scope pulls I think about 30 watts on normal mains, after about 10 minutes the inverter tripped out on over temp. You could smell it was getting warm inside. I'm thinking that it doesn't like the 100nF or so X cap across the mains inside the scope. The stepped voltage waveform coming out of the inverter causes momentary high currents to flow through the cap on the zero to +/- 340V transitions, unlike when the scope is running off normal sinewave AC mains.Anyway, I was thinking, to get around this problem is there any downside to putting a bridge rectifier between the inverter output and the scope SMPS AC input? The only thing I can think of is that the AC line trigger function would not work. I realise that an even better method would be to go right inside the inverter and tap onto the HVDC line before it reaches the mosfet H-bridge and gets chopped up into a psuedo sinewave but I want to leave the inverter undisturbed.Any opinions?

author: SeanB
author_rank: Super Contributor
timestamp: June 06, 2015, 04:39:44 am
content: Series choke in the AC line will work as well, as it will reduce those transitions and make the voltage to the scope look more like a sine wave. Old microwave transformer primary winding ( removed in one piece from the core and used as an air core coil) would probably work there.

author: TMM
author_rank: Frequent Contributor
timestamp: June 06, 2015, 05:52:58 am
content: My gut feeling is that the 100nF X cap isn't large enough to bother a modified sine inverter. Imho it's more likely that the power factor of the SMPS itself is poor enough that although it is only using 30Watts, it's still exceeding the VA / rms current capability of the inverter.A series choke as suggested may work to improve the power factor of the load. Some SMPS with passive power factor correction (e.g. older computer powersupplies) have large cored inductors around 40mH or so which would be ideal.

author: Circlotron
author_rank: Super Contributor
timestamp: June 06, 2015, 10:20:05 am
content: Actually one of my first thoughts was in fact to use a small series inductor as suggested, but when you think about it, when the scope is switched off so there is no load but the input to the SMPS is still connected to the inverter waveform, this inductor in combination with two of the diodes in the bridge rectifier of the scope SMPS would work to a certain extent as a voltage doubler boost converter. With the scope switched off and so no load on the power supply the input electro might get pumped up to way too high a voltage.-----------------As for power factor, a flat topped waveform as produced by the inverter is in fact an excellent waveshape for feeding a SMPS, at least one that doesn't have active PFC. Current is supplied for the whole length of the flat portion of the wave, not just the very tip as in a sine wave. Did a measurement with a Voltech power analyser years ago, sinewave was 0.6 PF, rectangular UPS waveform was 0.9 PF.

author: Circlotron
author_rank: Super Contributor
timestamp: June 06, 2015, 01:01:45 pm
content: Okay, tried it with a bridge rectifier and the scope ran just fine. Just a small amount of HF noise pickup from the inverter but not a concern for 1V/div or greater. Actually, that was with nothing plugged into the probe sockets so with probes it may well be fine. Anyhoo, this time it ran for 25 minutes instead of 10, but it still got quite warm. Even though the inverter is rated at 150 watts and the scope only takes about 30 watts. I have another inverter I will try ->http://www.jaycar.com.au/Power-Products-Electrical/Power-Conversion-%26-Transformation/DC-AC-Inverters/150W-%28450W-Surge%29-12VDC-to-230VAC-Electrically-Isolated-Inverter/p/MI5102This one is way better contructed inside - double sided plate though pcb, all mosfets properly clamped and heatsunk to metal case. The smaller one is a bit of a toy. I'll bring the bigger one home from work and try it on Wednesday. But overall, the bridge rectifier seems to have been a worthwhile idea.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 06, 2015, 01:14:33 pm
content: Since the Rigol supports 100-240 V, 45-440 Hz you can get really creative. Too bad it doesn't run on DC directly.You can try to get a better powerfactor by adding some resistive load, such as a filament light bulb, because capacitive power factor is bad.You can also try a 220-110 transformer in front. Or 110-220 depending on the inverter. If the inverter can handle that, and you accept the losses.We have a box with a motor driver that has three phase transformer to get from ugly pwm to smooth sine for testing purposes.

author: Circlotron
author_rank: Super Contributor
timestamp: June 06, 2015, 01:40:23 pm
content: Quote from: Jeroen3 on June 06, 2015, 01:14:33 pmIt does run okay directly on DC! The bridge rectifier I put between the inverter and the scope power inlet proved that. Maybe I should simply get 141.4 volts worth of batteries and be done with itEdit -> The 45Hz lower frequency limit is probably not so much that it wouldn't run on DC (it does) but rather as you get lower and lower in frequency there are longer and longer intervals without power so at 1Hz ac for example the scope will just sit there and blink on-off-on-off.Quote from: Jeroen3 on June 06, 2015, 01:14:33 pmDo you have capacitors across the secondary side? I have seen sinewave UPSs that use the same method. They rely on the leakage reactance of the transformer rather than use an extra inductor for filtering the HF PWM.

author: nitro2k01
author_rank: Frequent Contributor
timestamp: June 06, 2015, 01:58:58 pm
content: A more subtle consideration about running it on DC is that only half of the rectifier bridge (one pair of diodes) is being used, which means the power isn't distributed. I doubt it will actually be a problem on a well designed PSU with margins but worth keeping in mind. I've also heard about (but not seen) some PFC circuits that don't like DC on the input side and let out the magic smoke.

author: djQUAN
author_rank: Frequent Contributor
timestamp: June 06, 2015, 03:27:58 pm
content: I have run a Behringer DCX2496 in my car using a modified cheap inverter since 2010 and it still runs today.The inverter is modified to put out about 300VDC (yanked the entire H bridge, drivers and 60Hz oscillators out) and fed the HVDC directly to the Behringer AC input connector.I think it should be the same as running the Rigol on DC as it also has a SMPS.

author: Circlotron
author_rank: Super Contributor
timestamp: June 10, 2015, 07:16:04 am
content: Update - I tried out the second (better quality) inverter and much better results. Again with a bridge rectifier on the inverter output because it reduces battery drain by about 300mA. With a 7Ah battery, and inverter shutting off at 10.26 volts (maybe a whisker low) I got 1 hr 37 minutes run time.

author: BradC
author_rank: Super Contributor
timestamp: June 10, 2015, 08:08:53 am
content: If it runs on DC why don't you just tap off the DC rail in the cheap inverter that feeds the H bridge? Should be about 340VDC. Do away with all the switching and rectification and get it straight from the source.

author: Circlotron
author_rank: Super Contributor
timestamp: June 10, 2015, 12:46:55 pm
content: ^^ The cheap inverter has the two mosfets that drive the transformer primary just standing up in mid air, no heatsinking at all. It overheats at only 30 watts load so it's a dead end. Yeah, straight from DC would indeed be better but I wanted to leave the inverter untouched.