Author Topic: Can Most Programmable Electronic Loads (Array or Maynuo)Handle Negative Current?  (Read 7365 times)

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Offline K1JOS

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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
 
 

Offline evb149

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I have no idea about those brands / models.  I've never heard of them.
But if it is a load it should say if the applied voltage (current) at the load terminals should have a preferred polarity or not.  If there is no preferred polarity, it should be able to handle applied positive or applied negative voltages (and consequently currents).  If there is a preferred polarity and it doesn't say it can handle negative voltages/currents elsewhere, it probably cannot.
It might be specified as being one quadrant (positive voltage & positive current only) or two quadrant (including also negative voltage and negative current) in operation.  You can always ask on the manufacturer's sales / technical support contact area.
 

Offline johansen

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chances are no, unless they are four quadrant loads/supplies
 

Offline H.O

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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 says Over current, over voltage, over power, over heating and polarity reversal protection and the manual says that if you connect it "backwards" it'll beeb and the display will show REVERSE. (I've never verified it on mine).
 

Online Jeroen3

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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.
 

Offline retemitlum

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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 !
 

Offline albert22

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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.?

.

 

Offline synapsis

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For reference, the Maynuo 9812 doesn't do negative current either.
 

Offline rx8pilot

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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.
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Offline H.O

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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?
 

Offline nctnico

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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?
My Array 3710A DC load has reverse polarity protection diodes.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline rx8pilot

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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 expectations  :) I 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.
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Offline free_electron

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Nope. you need an AC load for that. Chroma and Kikusui make that.
Professional Electron Wrangler.
Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).
 

Offline rx8pilot

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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?
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Offline K1JOS

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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.

thanks 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.
 

Offline Kevin.D

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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
 

Online Jeroen3

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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?
 

Online Zucca

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My Array 3710A DC load has reverse polarity protection diodes.

Are they not the parasitic reverse body diodes inside the bis ass MOSFETs used there?

Can't know what you don't love. St. Augustine
Can't love what you don't know. Zucca
 

Offline H.O

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Quote
But... how is the mains-earth referencing in these things?
The negative load terminal on the front of the Maynuo M97 series is not connected 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 an isolated TTL<->RS232 cable then "you" will connect the negative load terminal to mains earth thru this cable and the PC power supply.
 

Offline rx8pilot

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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

I 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.
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