Electronics > Projects, Designs, and Technical Stuff
Overvoltage protection for 115 V transformer primary against 230 V mains
Ian.M:
--- Quote from: ampwizard on January 21, 2019, 01:50:19 pm ---
--- Quote from: Ian.M on January 21, 2019, 01:00:00 pm ---There's no need for an expensive special voltage monitoring relay as you could do the voltage monitoring on the secondary side ...
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... for voltage out of range.
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OK I like this idea. I'm going to have a crack at putting together a design based on this idea. Thanks!
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Although it could all be done with discretes and simple chips*, getting the lockout circuit right so it wont reconfigure for 120V then trigger on a 230V brownout isn't so simple. Also, the wildly varying secondary voltage as it operates has implications for the relay.
It would be much easier with a low pin count MCU, which can monitor the secondary voltage via a potential divider and its ADC, handling the under/overvoltage sensing, implement all the timing and logic, and the relay coil via a MOSFET with PWM in inverse proportion to the secondary voltage to maintain the same operating current as the supply varies. Use a low current linear regulator for the MCU Vdd supply. It could also handle any power/status LED and control switching the unregulated DC bus to the rest of the amplifier for under/overvoltage lockout, standby, or if any other protection circuit needs to cut power.
Once you have the code finalised, consider getting the MCUs preprogrammed by the manufacturer (e.g. MicrochipDirect's programming service) so it doesn't require any special handling on the production line.
N.B. When its finished, do make sure the ratings on the nameplate are appropriate. "100 V - 250 V" is *NOT* correct as its not a universal PSU. "100 - 130 V, 210 - 240 V, 50 - 60 Hz AC ONLY" would be more appropriate.
* I'm pretty sure most of the logic could be handled by a NE556 dual timer, one half for the 1 second delay and the other for the latching over/undervoltage detection.
Zero999:
Voltage auto-selection is a good idea. The microcontroller could be powered from a cheap universal input 5V switched mode power supply, which is only used for the voltage monitor/selector and nothing else. The MCU will power up first, with the transformer in 240V mode, monitor the mains voltage and decide whether it should be set to 120V or remain in 240V mode. If the mains voltage is something weird such as 170V, it could switch off the transformer and/or flash an LED or beep a buzzer, indicating the supply voltage is incorrect. Once powered, the circuit could continuously monitor the circuit and disconnect it, if the power supply voltage is wrong.
GeorgeOfTheJungle:
--- Quote from: ampwizard on January 21, 2019, 10:16:55 am ---[...]
The primary fuses are time-delay (slow blow) types: 1.6 A per primary.
[...]
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The fuse, even if slow blow, will (very likely) fuse much faster than the primary, so the primary will be safe.
And the secondary won't output twice the voltage when the primary is fed with twice the voltage, it will be a little more but not anywhere near twice as much because when the core enters saturation the output V not only stops increasing... it drops.
Ian.M:
There's no need for a switching supply for the MCU - just power up with the transformer primary set for 230V, and there will be enough secondary voltage to feed a 5V regulator. The MCU should only need a few mA, so all it needs is a linear regulator. By PWMing the relay coil it can be powered from a secondary side unregulated DC rail.
However if instead it was decided to put the MCU and voltage sensing on the primary side, one would use a capacitive dropper shunt regulated supply for it, and it may be worth including a TRIAC so the voltage selection relay doesn't have to switch under load.
Wolfgang:
--- Quote from: ampwizard on January 21, 2019, 10:16:55 am ---I am designing a portable musical instrument amplifier for a touring musician. It has a dual primary 2 x 0 - 115 V mains transformer with a switch to go between 115 V and 230 V operation. Now, I would like to include some kind of protection against having the switch set to 115 V and plugging the unit into 230 V which would be very destructive. The primary fuses are time-delay (slow blow) types: 1.6 A per primary. Would I be able to use a pair of thyristors (one per primary) to blow the fuses or even trip the venue's circuit breaker?
Since the thyristor resets on every half cycle of the 50 Hz mains, I'm not sure the average AC current will be high enough to blow the fuse fast enough to offer any protection. Has anyone had any experience with this or is there a better way of protecting against this scenario?
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I use a part called a SIOV from TDK, like here:
https://en.tdk.eu/inf/70/db/var/SIOV_Leaded_StandarD.pdf
They will reliable blow a fuse, must faster than the overcurrent caused by a saturated core.
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