Dummy Load, Input Protection - Page 1
Forum section: electronics / projects

author: baoshi
author_rank: Regular Contributor
timestamp: March 08, 2013, 01:54:24 am
content: Hi,I'm designing my over-engineered electronic dummy load based on Dave's and tons of other designs on the forum.One thing I found often neglected is the reverse input protection, i.e., if the input source is reversed, current will follow through shunt resistor and the body diode of main pass MOSFET and quickly smoke it.Blackdog's design uses a protection diode and a fuse to solve the problem. However since my design also measures input voltage, I do not want the negative voltage to be fed into ADC, even 1ms.So I sketched out my design using relays. And to overcome the contact resistance of relay, I put in a second relay to make a "pseudo 4-wire" voltage measurement.I added another true 4-wire terminal as well.My sketch is attached. Any suggestions?And one more question on the rating of relays: If the contact current rating is 2A for a relay and the relay is DPDT, does it mean if I parallel the 2 contacts, I can pass 4A current?ThanksBob

author: krish2487
author_rank: Frequent Contributor
timestamp: March 08, 2013, 05:04:11 am
content: Not saying what you did is a bad idea.I did not even look at what you drew to make a comment on it, but if I were in your shoesI d trade the relay solution for a overvoltage crowbar but inreverse polarity modei.e trip the protection circuit if the input is applied in reverse.You can google for those circuits and adapt as required.

author: c4757p
author_rank: Super Contributor
timestamp: March 08, 2013, 05:20:06 am
content: Quote from: baoshi on March 08, 2013, 01:54:24 amWell, you probably shouldn't be feeding an external connector directly into an ADC anyway. I'd be feeding it through a current-limiting resistor, then to a pair of fast clamping diodes, and maybe a buffer if the resistor is too large relative to the ADC input impedance.

author: baoshi
author_rank: Regular Contributor
timestamp: March 08, 2013, 01:37:07 pm
content: Thanks for the suggestions.Hi Krish2487,I did some investigation about crowbar. But I found it seems quite hard to make it trigger at low voltage, say, -0.3v. Am I right?Hi c4757p,I did a reversion according to your suggestion. I'm now using a diode for reverse protection and a pair of Schottky barrier diodes for clamping before voltage follower. BTW the ADC of choice is AD7714 (overkill but I had several 2nd hand ones). The input rating is between -0.3V ~ AVDD+0.3V max.

author: RjSa
author_rank: Contributor
timestamp: March 11, 2013, 01:06:23 pm
content: Exactly what I was looking for ... I´ll "steal" your idea and apply it on my own dummy load circuit.

author: baoshi
author_rank: Regular Contributor
timestamp: March 11, 2013, 02:03:45 pm
content: Quote from: RjSa on March 11, 2013, 01:06:23 pmNope RjSa.The circuit has serious problem. If input voltage (VBUS) is higher than opamp supply (V+), the top diode will be forward biased. It implies the voltage measurement range will be limited by V+.I still incline to relay based design.

author: baoshi
author_rank: Regular Contributor
timestamp: March 13, 2013, 02:19:01 pm
content: I have come out this circuit, based on MC33161 voltage monitor. LTSpice simulation also attached.Basically it verifies the input voltage is between 0V ~ 28V. The outputs are !OV and !UV. Can be "ANDed" to drive relay.Thanks for viewing and comments please.

author: 2nOrderEDO
author_rank: Regular Contributor
timestamp: March 13, 2013, 03:25:25 pm
content: What about using a simple diode bridge? You use the AC terminals for your current load input, and the "+" and "-" terminals connected properly to the current source. This has the disadvantage of having two diode drops in your load, but it makes impossible to connect your load the wrong way.Edited: Take a look at the attached PDF!

author: baoshi
author_rank: Regular Contributor
timestamp: March 13, 2013, 03:42:53 pm
content: Quote from: 2nOrderEDO on March 13, 2013, 03:25:25 pmYes but it makes voltage measurement impossible. Almost all the commercial electronic loads have voltage measurement and hence constant power / constant resistance mode.

author: 2nOrderEDO
author_rank: Regular Contributor
timestamp: March 13, 2013, 04:02:08 pm
content: Quote from: baoshi on March 13, 2013, 03:42:53 pmIf you are using a microcontroller for controlling this active load, I'm sure that you will have two or three free ADC. You can use them to look at the power consumed at just two of the diodes (one for each polarity) and, because the current through both diodes is the same, and assuming that you are using an encapsulated diode bridge (all diodes are equals and are tracked in temperature), you can add to your calculations all the info you need.You are right that it's a bit tricky but is just an Idea, not a critic to your design :')

author: RjSa
author_rank: Contributor
timestamp: March 14, 2013, 08:06:49 pm
content: Baoshi,Take a look at this vidI simulated that input protection circuit + dummy load and it appears to do the trick without consuming too much current that could affect the dummy load settings.I haven´t had the time to build the circuit yet but it looks promising. Let us know the results if you actually test it ... I´ll do the same as soon as a I get some spare time.