Circuit to identify the voltage on the dc bus - Page 1
Forum section: electronics / projects

author: aless2056
author_rank: Contributor
timestamp: September 11, 2023, 09:39:43 pm
content: Hey guys,I have equipment that has a DC bus of around 800Vdc. How can I make a circuit to read the voltage value and when there is no voltage on the bus, activate a relay.I thought about measuring the voltage on the dc bus with an op amp and then using a voltage comparator to activate the relay.What would be your suggestions? Any examples?

author: TimFox
author_rank: Super Contributor
timestamp: September 11, 2023, 09:48:09 pm
content: One can obtain electromechanical relays that only need a few mA to activate.(We used to call them "plate relays", since they were used in series with the plate of a vacuum tube.)Example:https://www.sherburn.com/sensitive-relays.htmlAll you need is a suitable resistor in series with the coil to hold the relay closed at your normal operating bus voltage, opening at a lower voltage, and connect the contacts to your alarm.The advantage of a good relay is the galvanic isolation between the coil and the contacts.Similarly, you could use an opto-coupler with sufficient insulation between the light and the sensor.Do you really need to "read" the voltage, or is it sufficient to sense that it is way too low?

author: aless2056
author_rank: Contributor
timestamp: September 11, 2023, 10:46:07 pm
content: It would be a safety feature, the relay contact would prevent the door from opening if there was still power on the dc bus.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 12, 2023, 01:02:20 am
content: Quote from: aless2056 on September 11, 2023, 09:39:43 pmYou need a safe voltage divider, to get things to the realm of ordinary electronics.You also need to decide what trip voltage you need, and any hysteresis on that.You can get opamps with voltage references inbuilt, or you can buy higher voltage Voltage Monitor / reset generators (eg TL7712).Note if you already have a regulated voltage for the relay, you can use that to compare against.With such interlocks, it is useful to think about fail safe, and other voltage indicators.

author: Infraviolet
author_rank: Super Contributor
timestamp: September 13, 2023, 03:30:19 am
content: If you've got 800V dc somewhere you are probably not in a situation where you are trying to save every last milliwatt...So what about putting a resistor for an appropriate sizing and voltage rating, making sure it can handle the appropriate current too given the I=V/R involved, fron the 800V rail to whatever ground it is relative to. Monitor the current through this resistor using a non-contact current measurement technique, this lets you isolate your low voltage stuff away from the 800V.Because you are deliberately placing a resistor fro the power rail to ground, parallel to the thing your 800V rail is supplying power to, you'll always have current through it when the DC rail is at a high voltage, this is not like measuring the current in whatever the rail is powering which could vary depending on what the load is even while the voltage remains the same.Even if you have capacitance effects between the 800V rail and your ground then when the rail gets turned off they'll discharge over some time period, and your current measurement won't say that the rail is at low voltage until the capacitors have drained for sufficiently long that it actually is at low voltage.

author: Zero999
author_rank: Super Contributor
timestamp: September 13, 2023, 02:24:07 pm
content: Since this is safety critical, one needs to consider what will happen, if it goes wrong and fails to work, for whatever reason.I would use an opto-coupler with a TLVH431 in comparator mode.The example circuit has a threshold of 600V and no hysteresis. I would suggest a lower threshold of something like 100V, which can give a bit of a shock, but is unlikely to be lethal. Hysteresis can be included with the addition of a PNP transistor.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 14, 2023, 12:59:29 am
content: Quote from: Zero999 on September 13, 2023, 02:24:07 pmYes, and even some means to detect a cut/open cable to the 800V may be considered important.I've also seen end of line specific resistors used, as a simple step up from open/short attacks.

author: Smokey
author_rank: Super Contributor
timestamp: September 14, 2023, 01:59:51 am
content: How about a big physical lock on the cabinet door. Problem solved?