Electronics > Projects, Designs, and Technical Stuff
Can someone help me review this schematic?
redgear:
--- Quote from: moffy on July 09, 2020, 01:06:12 am ---The cap C1, (C0+C1) has to be large enough that at full load i.e. max current out of 5v and all relays on, that the voltage never drops below about 8v. This is so the relays stay on, and the regulator doesn't drop out of regulation. There is a simple formula: C1 = (i*dt)/(Vmax - 8v). For 240v/50Hz, dt = 10ms.
The use of the ULN2003a is an excellent idea, just stay within the packages total current and power limitations. The built in diodes make life a bit easier also. :)
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--- Quote from: mariush on July 09, 2020, 04:43:24 am ---Probably a simpler formula to use to approximate capacitance : C = Current (Amps) / [ 2 x AC Frequency x ( Vdc peak - Vdc min) ]
So if you have a 9v AC transformer, after rectification it will be 9 x 1.414 - 2 x voltage drop on diode of bridge rectifier = 12.7- 2 x 0.8v = 11.1v peak dc voltage ... but can be higher if you use low voltage drop diodes like let's say 1n5818 (~ 0.3v at 0.1A, 0.5v at 1A) instead of a bridge rectifier.
Let's say you need 25mA for each relay, and you have 4 relays ... that's 100mA ... your micro and lcd will use another 10-20mA ... let's round it up to 125mA and let's say you're in a 60Hz mains country
So if you want at least 10v all the time : C = 0.125 / [ 2 x 60 x (11v - 10v) ] = 0.125 / 120 = 0.00104166 Farads or 1041 uF ... so I'd use at least 1200uF 16v rated capacitor.
It would make sense to use a 12v AC transformer ... after rectification and drop of around 1.6...2v (if you use a generic cheap bridge rectifier) you'll have a peak voltage of around 15v.
Most relays should be perfectly fine with this high voltage, but if you're paranoid you can simply add a couple diodes in series to drop voltage by some amount (ex 1n400x diodes will drop 0.7v per diode) and get it closer to 12v ...
What you add in cost for diodes, you potentially save in using smaller capacitor ... C = 0.125 / 120 x (15v - 10v) = 125 / 600 = ~ 208uF
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Thank you both :)
Won't the 9v transformer be a better design choice? Since I will be using a Linear Regulator, 9v would mean less energy is wasted as heat.
As for the diodes I'm considering the S1M diodes for the bridge rectifier and 1N4148 as protection diodes for the relays.
My device has a EM ballast, I'm concerned if the LCD Display will output gibberish when they are turned on, is there a way to filter it? Would filter caps be a good idea? How do I calculate the capacitance required?
Looking forward for what you guys have to suggest. :)
redgear:
--- Quote from: moffy on July 09, 2020, 06:32:10 am ---I would only say that the voltage regulation on small transformers can easily be +20%. So a no or light load your on 12v RMS can easily become 18-19v. Needs to be kept in mind, for both 5v regulator dissipation and possibly excessive voltage across the relays. Depends on a few things to do with the transformer.
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Thank you. So, it would be better to have two regulators in parallel or a vol. reg. with a higher power rating, correct?
moffy:
I would still go with my original suggestion (the relays don't need a voltage regulator), but the actual currents and transformer are important, like any electronic circuit, just be aware of it. The transformer should have a voltage regulation figure which gives you the difference between no load and full load. A 10v RMS secondary would be good, but I really haven't seen any, but 9v is more common and should do, all things considered. You just need to make an informed decision based on what is available, you can calculate the voltage readily enough. Then choose your relays and see what their minimum pull in voltage is. I looked up one:
https://docs.rs-online.com/4354/0900766b8139785f.pdf
and the minimum pull in voltage is 9v for a 12v coil and drop out voltage is 0.6v. :)
redgear:
--- Quote from: moffy on July 09, 2020, 06:59:02 am ---I would still go with my original suggestion (the relays don't need a voltage regulator), but the actual currents and transformer are important, like any electronic circuit, just be aware of it. The transformer should have a voltage regulation figure which gives you the difference between no load and full load. A 10v RMS secondary would be good, but I really haven't seen any, but 9v is more common and should do, all things considered. You just need to make an informed decision based on what is available, you can calculate the voltage readily enough. Then choose your relays and see what their minimum pull in voltage is. I looked up one:
https://docs.rs-online.com/4354/0900766b8139785f.pdf
and the minimum pull in voltage is 9v for a 12v coil and drop out voltage is 0.6v. :)
--- End quote ---
Thanks. 9,10,12v RMS is available. Have to do some readings before I choose one. Do you have any brand suggestions?
moffy:
Sorry, I don't but if you choose a reputable supplier you tend to get decent honest components. Also good components will have detailed data sheets. Not Chinese though, they can be all over the place, you often have to derate a component by at least 50%.
P.S. I don't mind buying Arduino boards from Ebay etc. because they are an open standard.
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