Overvoltage protection for 115 V transformer primary against 230 V mains - Page 1
Forum section: electronics / projects

author: ampwizard
author_rank: Contributor
timestamp: January 21, 2019, 10:16:55 am
content: 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?

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2019, 10:30:15 am
content: It might be better to monitor the voltage on the DC side and switch a relay on to short the fuses on the primary side.How about designing out the problem? Switched mode power supplies with a universal 100V to 250VAC input are widely available. Noise and interference can be mitigated by careful filtering an additional linear regulators on the secondary side.

author: Ian.M
author_rank: Super Contributor
timestamp: January 21, 2019, 10:44:12 am
content: A rough calculation based on the specific heat capacity and latent heat of fusion of copper, assuming no heat is lost from the wire, (i.e. quick heating) suggests that as long as the primary wire diameter is at least 3.2 time the fuse wire diameter, the fuse will blow before the max temperature of 180 deg C rated magnet wire can be exceeded.For a more accurate answer and further advice, why not talk to your transformer manufacturer?

author: Jeroen3
author_rank: Super Contributor
timestamp: January 21, 2019, 10:45:15 am
content: How much room is there? Can you fit a voltage monitor relay?

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2019, 11:06:16 am
content: Quote from: Ian.M on January 21, 2019, 10:44:12 amJust to clarify I was talking about putting the relay contacts on the primary side, as it might not be easy to ensure the fuse blows, before the transformer.

author: ampwizard
author_rank: Contributor
timestamp: January 21, 2019, 12:16:22 pm
content: Quote from: Zero999 on January 21, 2019, 10:30:15 amThanks, that could work actually. I've ruled out a SMPS due to expense.Quote from: Jeroen3 on January 21, 2019, 10:45:15 amI'll be honest I don't know what a voltage monitor relay is. I'll have to look it up!

author: ampwizard
author_rank: Contributor
timestamp: January 21, 2019, 12:25:08 pm
content: Quote from: Jeroen3 on January 21, 2019, 10:45:15 amI just checked and these things are very expensive.

author: richard.cs
author_rank: Super Contributor
timestamp: January 21, 2019, 12:39:28 pm
content: The transformer will saturate heavily on 230 V and draw lots of primary current, are you sure that won't blow the fuse adequately quickly anyway? If you do want to use thyristors to blow the fuse then you should latch the gate drive so they're on for whole cycles after it's triggered rather than turning off at the crossing and on again as the voltage ramps up. You are going to need large thyristors for this if you want them to outlive the slow-blow fuse, do you have the current-time curve for the fuse in question?* I definitely wouldn't use relay contacts for this, a relay would protect the amp, but likely weld and need replacing along with the fuse.It would be nicer of course to switch off (or better still, not switch on to avoid having the break high current) at high input voltage, but by the time you're doing that you might as well use one of the various circuits out there for switching the primary automatically, which is another way of avoiding the problem.*In the UK a short-circuit current of between 200 and 2000 A would be typical at a normal socket, other 230 V countries may vary.

author: Jeroen3
author_rank: Super Contributor
timestamp: January 21, 2019, 12:45:43 pm
content: Then we know the instrument is not worth a $100 protection relay. This it important. Since you didn't mention a budget.

author: Ian.M
author_rank: Super Contributor
timestamp: January 21, 2019, 01:00:00 pm
content: There's no need for an expensive special voltage monitoring relay as you could do the voltage monitoring on the secondary side if you use a relay to implement auto-voltage selection for the primaries. However the relay does need to be good enough to handle a large inductive load reliably at worst-case max 230V mains, so cheap no-name ones are not a good idea.It would power up with the relay off and the primaries configured for 230V. If the secondary voltage was under 59% of the expected value, and stable for a suitable period (e.g. 1 second) the relay would activate, reconfiguring the primaries for 115V and immediately boosting the secondary voltage to its normal operating range. To cover situations like a lost neutral in the USA (which can result in excessive voltage on a 115V outlet), malfunctioning generators and idiots with variacs, a secondary side over-voltage detection circuit would be advisable to turn off the relay until power has been cycled, preferably with a sufficient capacitor reserve to keep the lockout logic state for several seconds so that a seriously unstable supply doesn't cause the relay to repeatedly pull in and drop out with a short cycle time.Depending on your application, you may also need to switch power to your load circuit so it is only powered when the secondary voltage is in its nominal range.A bicolour power indicator LED would be a nice touch if you intend to fit one - red for voltage out of range or still being configured and green for on. If you want a standby indication you could revise that to red for standby and flashing orange (red + green) for voltage out of range.

author: fsr
author_rank: Regular Contributor
timestamp: January 21, 2019, 01:14:46 pm
content: Maybe you can use a MOV or two different ones to protect the 110v/220v windings? The fuse should blow quickly when the MOV triggers. Still, the secondary side will be at higher than normal voltage until that happens, so you will need to check if your circuit can survive that, or you need another protection at the secondary side.

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2019, 01:23:14 pm
content: Quote from: ampwizard on January 21, 2019, 12:16:22 pmWhere are you buying parts from?What output voltage/current do you need?A switched mode power supply is often cheaper than a transformer, rectifier and linear regulator.

author: ampwizard
author_rank: Contributor
timestamp: January 21, 2019, 01:50:19 pm
content: Quote from: richard.cs on January 21, 2019, 12:39:28 pmI'm not sure if the transformer will draw enough current to blow the fuse and I don't know how to work it out. The fuse I'm using needs 10x the rated current to open within 150 ms (worst case).QuoteI think this is the nicer option. This is what I need to do. I don't want to rely on the user to flick the switch.Quote from: Ian.M on January 21, 2019, 01:00:00 pmOK I like this idea. I'm going to have a crack at putting together a design based on this idea. Thanks!

author: ampwizard
author_rank: Contributor
timestamp: January 21, 2019, 01:55:05 pm
content: Quote from: Zero999 on January 21, 2019, 01:23:14 pmI've actually been down this road and it really isn't an option for me. I have fairly specific requirements for secondary voltages/currents including a high voltage supply for valves. I also have a relationship with a transformer manufacturer which means I can build a linear supply to my exact spec for a good price.

author: richard.cs
author_rank: Super Contributor
timestamp: January 21, 2019, 02:03:37 pm
content: Quote from: ampwizard on January 21, 2019, 01:50:19 pmIt is calculable, but probably easier just to test it.Quote from: ampwizard on January 21, 2019, 01:50:19 pmDo you have the curves? If you want a semiconductor to survive it's far more important to know how many milliseconds it will open in at 100x or 1000x. In terms of the heat capacity of a tiny bit of silicon 150 ms might as well be forever. Thyristors are way easier to damage than transformers.Quote from: ampwizard on January 21, 2019, 01:50:19 pmAgreed, it's both a nicer user experience and more idiot proof.

author: Ian.M
author_rank: Super Contributor
timestamp: January 21, 2019, 02:57:34 pm
content: Quote from: ampwizard on January 21, 2019, 01:50:19 pmAlthough 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.

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2019, 03:31:30 pm
content: 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.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: January 21, 2019, 04:00:38 pm
content: Quote from: ampwizard on January 21, 2019, 10:16:55 amThe 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.

author: Ian.M
author_rank: Super Contributor
timestamp: January 21, 2019, 04:38:39 pm
content: 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.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 21, 2019, 06:17:11 pm
content: Quote from: ampwizard on January 21, 2019, 10:16:55 amI use a part called a SIOV from TDK, like here:https://en.tdk.eu/inf/70/db/var/SIOV_Leaded_StandarD.pdfThey will reliable blow a fuse, must faster than the overcurrent caused by a saturated core.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 21, 2019, 06:47:20 pm
content: Another approach : use a relay, which in its NC position selects 230V. Have a circuit on the secondary, which, after a delay to allow settling only turns on the relay when the supply is below a fixed threshold.One caveat for using relays to do series-parallel voltage switching though, you can get issues with arcing during switching, which effectively links both the NC and NO contacts together briefly during switching. At the very least this will cause rapid contact damage, if not blow a fuse.The solution is to use two relays, one for 110v and 0ne for 230v, configured in a way that only one can be on at a time, both using only NO contacts.Another approach could be to have a "soft" power switch, with a zero-volt dropout, so when plugged in, a button needs to be pushed to turn it on. Some logic can be added to the button-to-relay drive to prevent it turning on if the wrong voltage is selected.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: January 21, 2019, 07:07:46 pm
content: Can also put a beefy 140V varistor after the fuse.

author: ampwizard
author_rank: Contributor
timestamp: January 21, 2019, 09:32:45 pm
content: One detail I've omitted is I actually have two dual primary transformers. I was thinking of switching them both between 115 and 230 V with a single DPDT switch (or relay) which looks OK on paper.Quote from: Wolfgang on January 21, 2019, 06:17:11 pmI'll take a look at that, thanks.

author: ampwizard
author_rank: Contributor
timestamp: January 21, 2019, 09:36:11 pm
content: The more I think about relays the less keen I am on using them since they must presumably be rated for the transformer inrush currents (which are high).