Soft turn on cicuit for toroidal transformers - Page 1
Forum section: electronics / projects

author: vargasmongo3435
author_rank: Newbie
timestamp: April 18, 2017, 07:04:36 pm
content: Hello this is my first post and I need some help because I'm designing a new PCB for a soft turn on circuit for a 800VA toroidal transformer that is going to be use for an audio amplifier I search online I fund one circuit but I'm not sure if this is correct the PCB I'm designing it will have the option for 120V USA mains and 230V Europe mains so I have the schematic and I notice that it use 1M? resistor and another 220? on both circuits that I found online I'm guessing that this after main swith turn on it will then pass to the 1M? resistor and the 220 to charge C1 1000uF so relay will be activated any help I will really appreciated

author: vargasmongo3435
author_rank: Newbie
timestamp: April 18, 2017, 07:06:16 pm
content: this is the PCB so far

author: james_s
author_rank: Super Contributor
timestamp: April 18, 2017, 07:22:54 pm
content: An even simpler approach you might try is one of those inrush thermistors, they heat up as current passes through them and the resistance drops.

author: Karel
author_rank: Super Contributor
timestamp: April 18, 2017, 07:25:51 pm
content: Why so complicated? Just connect this in series with the primary of the transformer:http://www.newark.com/epcos/b57464s100m/ntc-thermistor-10-ohm-20/dp/29M3208

author: calexanian
author_rank: Super Contributor
timestamp: April 18, 2017, 07:29:22 pm
content: Actually the relay is the preferred method. You have to introduce a small time delay though. Traditionally what is done here is a lower voltage relay coil with a cap across it, and then a series resistance feeding it. You can also feed it half wave rectified to get your average dissipation down ad well. Also your series resistance to the toroid appears to be a little high.

author: calexanian
author_rank: Super Contributor
timestamp: April 18, 2017, 07:32:37 pm
content: Oh, and I also forgot. power it from the secondary side of the transformer. There should be enough excitation current from your primary shunt to wake up the secondary side. I have seen this done without a timing circuit as well. I will try to draw something up later.

author: dimkasta
author_rank: Regular Contributor
timestamp: April 18, 2017, 07:37:54 pm
content: +1 for the thermistor approach.Check the F5 and F6 articles on the first watt site to see how N Pass uses them.

author: james_s
author_rank: Super Contributor
timestamp: April 18, 2017, 07:57:39 pm
content: A relay is certainly another valid approach, powering the relay coil from the secondary of the transformer ought to work, you might not even need to add a RC delay. There are multiple different ways to accomplish what you want, each with advantages and disadvantages.

author: julian1
author_rank: Frequent Contributor
timestamp: April 18, 2017, 10:30:39 pm
content: C4 330nF works as a capacitive voltage dropper. The 1M is a bleeder resistor.The issue with cap droppers is the fixed current. Any current not drawn by the relay coil/associated components, has to be shunted by the zener D7 to maintain the DC voltage level. That will be wasteful/inelegant if you wanted the circuit to operate for different line voltages. The dropper will need to be rated to supply enough current at the lower line voltage, and then rely on the zener to bleed off excess current as heat for the higher line voltage.Also, the issue with a simple in-series NTC is that it stays hot and takes minutes to cool and reset. So there is no inrush protection for short duration line drop-outs.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 19, 2017, 12:00:51 am
content: The combination of your inrush limiting resistance and the caps on the secondary side should be enough to provide the delay.

author: Zero999
author_rank: Super Contributor
timestamp: April 19, 2017, 08:04:58 am
content: Quote from: calexanian on April 18, 2017, 07:29:22 pmYes, a 555 timer and a small relay would work.https://www.eevblog.com/forum/chat/adj-60v-to-70v-dc-power-supply/msg33609/#msg33609Alternatively buy a timer relay.http://uk.rs-online.com/web/p/products/8700720/?grossPrice=Y&cm_mmc=UK-PLA-_-google-_-PLA_UK_EN_Safety_And_Security_And_ESD_Control_And_Clean_Room-_-Security_Alarms_And_Detectors_And_Accessories&mkwid=sTN51O3kc_dc|pcrid|88057044123|pkw||pmt||prd|8700720&gclid=CJW-svWHsNMCFY8y0wodXaMKTwQuote from: calexanian on April 18, 2017, 07:32:37 pmI agree, the relay can be run off the same transformer. The schematic above shows a separate transformer because it was designed for a power supply with a much higher voltage transformer.