Can Most Programmable Electronic Loads (Array or Maynuo)Handle Negative Current? - Page 1
Forum section: electronics / testgear

author: K1JOS
author_rank: Regular Contributor
timestamp: December 09, 2014, 05:04:37 am
content: Maybe a dumb question but I want to use an electronic load such as the Array 3711A for testing a DC Motor PWM H-bridge controller. Testing the H-bridge controller will at some point reverse motor current. I cannot find anything in the Array, Maynuo or BK load manuals that gives the specs on a reverse voltage/current rating to the inputs. Are these generally protected or susceptible to negative currents?thanks

author: johansen
author_rank: Super Contributor
timestamp: December 09, 2014, 05:24:15 am
content: chances are no, unless they are four quadrant loads/supplies

author: H.O
author_rank: Frequent Contributor
timestamp: December 09, 2014, 06:30:26 am
content: Hi,The Maynuo (M97 series) can only sink current from the positive terminal to the negative (single quadrant). I can't speak for the Array but it's most likely the same. If you look at the specifications for the Maynuo it saysOver current, over voltage, over power, over heating andpolarity reversal protectionand the manual says that if you connect it "backwards" it'll beeb and the display will showREVERSE. (I've never verified it on mine).

author: Jeroen3
author_rank: Super Contributor
timestamp: December 09, 2014, 06:50:03 am
content: Depending on the motor voltage, you could use an diode bridge rectifier to make to load polarity free.This doesn't simulate a motor winding, but neither does an DC load.

author: retemitlum
author_rank: Newbie
timestamp: December 09, 2014, 08:12:15 am
content: I have verified the reverse protection on my Maynuo M9712.It has diode protection across the terminals. (measured approx. 0,5V)So the protection protects the load, by shorting the source !

author: albert22
author_rank: Regular Contributor
timestamp: December 09, 2014, 05:54:25 pm
content: A teardown of the M9711 reveals that it is based on IRFP250 MOSFETS Which should almost short the terminals if a reverse voltage is applied (due to the intrinsic diodes). Explaining what retemitlum measured.However the manual states that if the polarity is reversed a buzzer will sound and the display will show REVERSE.Does this means that while the buzzer sounds, a BIG SHORT CIRCUIT is in effect or maybe they have a smarter reversal protection which does not blow the equipment under test.?.

author: synapsis
author_rank: Regular Contributor
timestamp: December 09, 2014, 06:10:32 pm
content: For reference, the Maynuo 9812 doesn't do negative current either.

author: rx8pilot
author_rank: Super Contributor
timestamp: December 09, 2014, 06:25:40 pm
content: Run it through a full wave bridge to ensure the negative swings never get to the load. Not sure how accurate you need to be, but you could calculate the loss in the bridge and offset that with what the load says.Cheap solution possibility.

author: H.O
author_rank: Frequent Contributor
timestamp: December 09, 2014, 07:20:54 pm
content: I just tried it on my M9712.Connecting a 24V supply, current limited to 250mA, in reverse to the M9712 does short the supply out but does not make any buzzer sound, nor does it show REVERSED in the display....If the load is ON there's basically a dead short (MOSFETs conduting in reverse while "on"), display show UNREG, no buzzer.If the load is OFF there's the ~0.6V drop across the intrinsic diode(s), display shows OFF, no buzzer.Can anyone else verify?

author: nctnico
author_rank: Super Contributor
timestamp: December 09, 2014, 08:14:01 pm
content: Quote from: K1JOS on December 09, 2014, 05:04:37 amMy Array 3710A DC load has reverse polarity protection diodes.

author: rx8pilot
author_rank: Super Contributor
timestamp: December 09, 2014, 08:47:35 pm
content: Reverse polarity protection diodes protect the load by potentially destroying the source. True polarity protection with detection requires quite a bit more thought. I simple ideal diode could easily serve the purpose of blocking reverse polarity but would not offer the ability to buzz/warn the user about why they do not see anything. The combination of a full wave bridge for sensing and an ideal diode to pass good voltage through works - but Array and Maynuo are in the make it as cheap as possible or copy-cat markets so a simple diode will have to do. PS, I have an ARRAY unit arriving tomorrow - my expectations are low but at least it will meet my expectationsI could not afford the high-end versions, especially since I need 6 of them.For the OP, you could easily put together a full wave bridge with Schottky diodes to go between the source and the load. It will show up on the load side appropriately polarized. On the input side - it does not matter how you connect it. Then you can use the load you have with no worries.

author: free_electron
author_rank: Super Contributor
timestamp: December 09, 2014, 09:46:21 pm
content: Nope. you need an AC load for that. Chroma and Kikusui make that.

author: rx8pilot
author_rank: Super Contributor
timestamp: December 09, 2014, 10:08:46 pm
content: Do those measure the negative swing or do they just rectify it and measure the resulting DC? Maybe they rectify it and have a sensor on the AC side to keep track of when incoming power is on the + or - side of the cycle.Anyway...do you think it is a problem to rectify an AC source like the OP described so that it can be sent to an inexpensive load?

author: K1JOS
author_rank: Regular Contributor
timestamp: December 10, 2014, 04:01:07 am
content: Quote from: rx8pilot on December 09, 2014, 08:47:35 pmthanks this sounds like a simple compromise to at least be able to test the H-bridge with the electronic resistive load while not risking blowing the H-bridge from a protection short from the electronic load.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: December 10, 2014, 10:21:44 am
content: A Bridge will a work but you'l lose ~ 2 Volts or so across it ,also the motor is inductive load and your DC load is resitive load so you wont get same Current PWM response.It's because of the Voltage drop that a series diode would cause in a DC load that they don't normally use then for reverse polarity protection (So Restricting Minimum input Voltages) .I haven't done an extensive search but I haven't found any DC Loads (other than the one I designed and posted here) that have polarity protection for the DUT .The polarity protection commercial models refer to in the specs is usually just the body mosfet diode(or a parrallel one) to protect the DC load itself which effectivley shorts the DUT and has a light/indicator that kindly tells you it shorted your DUT!! .Regards

author: Jeroen3
author_rank: Super Contributor
timestamp: December 10, 2014, 11:52:55 am
content: You can use two loads, with a schottky to prevent the idiot circuitry to blow the fuse.You'll have some fancy behavior between -0.5 and +0.5 Volts though.But... how is the mains-earth referencing in these things?

author: Zucca
author_rank: Unknown
timestamp: December 10, 2014, 02:41:29 pm
content: Quote from: nctnico on December 09, 2014, 08:14:01 pmAre they not the parasitic reverse body diodes inside the bis ass MOSFETs used there?

author: H.O
author_rank: Frequent Contributor
timestamp: December 10, 2014, 04:26:53 pm
content: QuoteThe negative load terminal on the front of the Maynuo M97 series isnotconnected to mains earth. I can't, at the moment, find any specification on what the maximum allowed voltage at the negative terminal is relative to mains earth though.But, you still need to be careful. The common or GND (pin 5) of the serial port interface at the back is connected to the negative load terminal so IF you've connected the load to a PC and you're NOT using anisolatedTTL<->RS232 cable then "you" will connect the negative load terminal to mains earth thru this cable and the PC power supply.

author: rx8pilot
author_rank: Super Contributor
timestamp: December 10, 2014, 04:39:15 pm
content: Quote from: Kevin.D on December 10, 2014, 10:21:44 amI figured in the compromises, but the alternative is to have no test at all. At least with the bridge, you can characterize the interaction and losses and offset the readings accordingly. Maybe not perfect, but better than nothing.It would be a good experiment to try an ideal diode bridge that would have far less voltage loss but a slower switching speed. If that worked, it could be a reasonable way to allow these low-cost electronic loads to work with AC and H-bridge drivers.