Regulated 9 volt dc power supply from 115 volt AC - Page 1
Forum section: electronics / beginners

author: fixit7
author_rank: Regular Contributor
timestamp: May 22, 2019, 10:07:09 pm
content: I would like to build a regulated 9 volt D.C. power supply that uses 115 v AC to supply low current circuits.The plan I found uses a power transformer, voltage regulator, zener diode, transistor, and some capacitors.My wall transformer puts out .6 amp which is more than I need. There is no way to take it apart without destroying it.It weighs 44 grams( or about 2 ounces) so I am wondering if it uses a power transformer?

author: fixit7
author_rank: Regular Contributor
timestamp: May 22, 2019, 10:57:47 pm
content: I checked my scale. A 500 g calibration weight weighed 499.76 g.https://www.dropbox.com/s/6f6y2ibp9as6bwt/9VoltPowerSupply.png?dl=0

author: golden_labels
author_rank: Super Contributor
timestamp: May 22, 2019, 11:32:06 pm
content: It”s most likely an SMPS. Nowadays it is unlikely to have transformer→rectifier→regulator scheme found in common consumer goods. Too bulky, too expensive, not really needed.You may get those in an integrated, potted form for mounting on PCBs. SeeMYRRA 47201as an example: 9V / 830mA / 7.5W / 80% eff / 100–240VAC input / 56g weight.

author: MarkF
author_rank: Super Contributor
timestamp: May 22, 2019, 11:56:33 pm
content: I would build the basic linear supply with a 9V three terminal regulator. As a bench supply.I would stay away from a switching boost/buck regulator to eliminate the noise from the switching frequency.(Unless you need to drop a lot of voltage across the regulator.)

author: fixit7
author_rank: Regular Contributor
timestamp: May 23, 2019, 12:05:49 am
content: I just need to supply less than 250 ma.The weight of the circuit is not an issue.

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2019, 08:27:40 am
content: That circuit will work, but is not as good as the one with the MC7809.My advice is just buy a 9V mains adaptor. They're not expensive. Here's one from my local distributor, which works anywhere in the world.https://cpc.farnell.com/powerpax/sw4463-b/ac-dc-power-supply-9v-1-67a-intl/dp/PW04101?mckv=sCXXnkGAt_dc|pcrid|224680288544|kword||match||plid||slid||product|PW04101|pgrid|48556615353|ptaid|pla-458134591184|&CMP=KNC-GUK-CPC-SHOPPING&gclid=EAIaIQobChMInruasZ2x4gIVL7HtCh3-VwpXEAQYAiABEgL8bvD_BwE

author: Brumby
author_rank: Unknown
timestamp: May 23, 2019, 11:34:08 am
content: I agree. Using the 7809 circuit would be my first choice if I were to make my own. It is a better choice, IMHO.As well as the benefits you get using the 7809 itself, there are a couple of protection elements about that circuit I like.

author: Brumby
author_rank: Unknown
timestamp: May 23, 2019, 11:35:34 am
content: Buying a pre-made unit is always an option, but it does take away from the experience of building something yourself.

author: fixit7
author_rank: Regular Contributor
timestamp: May 23, 2019, 12:32:41 pm
content: Quote from: Brumby on May 23, 2019, 11:35:34 amYes, it does.I built a power supply using a power transformer over 20 years ago.

author: MarkF
author_rank: Super Contributor
timestamp: May 23, 2019, 01:52:17 pm
content: I did a PCB some time ago if you like

author: fixit7
author_rank: Regular Contributor
timestamp: May 23, 2019, 03:04:10 pm
content: Do you have a circuit showing more of the specific parts?I see the MC78xx chip and the power transformer.

author: MarkF
author_rank: Super Contributor
timestamp: May 23, 2019, 03:54:39 pm
content: Quote from: fixit7 on May 23, 2019, 03:04:10 pmThe circuit for the PCB is the one I posted earlier in reply #5.The large capacitor has a 10mm lead spacing and the small one has a 2mm spacing.Edit- Do you want actual part numbers?

author: Brumby
author_rank: Unknown
timestamp: May 23, 2019, 04:11:47 pm
content: A variation on the PCB to keep it neat and single sided.I've also added notations for the components:D1, D2, D3, D4, D5, D61N5395 (note polarity)C16800uF voltage to suit output of rectifier (note polarity)C210uF 16V or higher (note polarity)R1Current limit resistor - say 1K0LED1LED for power indicator - almost any simple LED (note polarity)Heatsink as appropriate for output power draw and input/output voltage differential.Edit: Polarity of power indicator LED corrected.

author: MarkF
author_rank: Super Contributor
timestamp: May 23, 2019, 04:46:11 pm
content: @Brumby- Your LED polarity is reversed.And just to take things over the top.

author: fixit7
author_rank: Regular Contributor
timestamp: May 23, 2019, 04:59:50 pm
content: Nice pic. That's quite a large heat sink.What is "Mercedes" device?

author: mariush
author_rank: Super Contributor
timestamp: May 23, 2019, 05:29:52 pm
content: Bridge rectifiers are so cheap these days that everyone should have a bunch in their inventory.ex.https://www.digikey.com/product-detail/en/smc-diode-solutions/UG3KB60GTB/1655-1838-ND/7244866Added bonus... some have hole in the package allowing for easy heatsinking :https://www.digikey.com/product-detail/en/micro-commercial-co/GBU4J-BP/GBU4J-BPMS-ND/1140592So 2nd bonus, being glass passivated, they can be heatsinked on same heatsink with the regulator ... which could be the metal case itself.And while you're there, you could easily replace the fixed voltage regulator with an adjustable one and either use a switch to fix between some preset voltages, or use a potentiometer.ex LM317, 1117 regulators, 1085 or 1084 (lm1085, ams1084, whatever) etc etcand depending on input voltage you may not need such a big capacitor.ex for 9v output with 7805, you need a minimum of around 10.5v at input to get 9v at 1A at output .If you have a 12v ac transformer , after rectification you have a Vdc peak of 1.414 x Vac - 2x voltage drop on diodes, so 12v x 1.414 - 1.5v = ... let's say 15vSo you can approximate capacitance required with formula C = Current / [ 2 x AC freq x ( Vdc peak - Vdc min) ]For 1A and 10.5v min voltage, you have C = 1A / 120 x (15-10.5) = 1/540 = 0.00185185 Farads or 1850 uF ... so basically for 1A, a 2200uF 25v capacitor would work in this scenario.Doesn't hurt to add a power on led, if only to be a minimum load and help the regulator give a better output.Also wouldn't hurt to add some decoupling capacitors .. add a ceramic 0.1 uF capacitor as close to the input voltage pin as possible. Maybe add a 1uF ceramic on the output, besides the regular electrolytic capacitor.

author: MarkF
author_rank: Super Contributor
timestamp: May 23, 2019, 06:15:08 pm
content: Quote from: fixit7 on May 23, 2019, 04:59:50 pmIf you mean the heatsink: EA-T220-51EThere are three heights: 38mm, 51mm and 64mm

author: Brumby
author_rank: Unknown
timestamp: May 24, 2019, 12:04:17 am
content: Quote from: MarkF on May 23, 2019, 04:46:11 pmI know. I saw that after I had posted - but it was after 2am here and I'd run out of steam. **I will fix it up and add a note.Edit: ** Done.

author: rust_collector
author_rank: Newbie
timestamp: May 24, 2019, 11:34:56 am
content: Quote from: fixit7 on May 23, 2019, 04:59:50 pmIf it is the thing I believe you are asking about, it is the top of an electrolytic capacitor

author: MarkF
author_rank: Super Contributor
timestamp: May 24, 2019, 01:54:25 pm
content: Quote from: rust_collector on May 24, 2019, 11:34:56 amDah...I get it now.

author: Brumby
author_rank: Unknown
timestamp: May 24, 2019, 02:21:13 pm
content: Quote from: MarkF on May 24, 2019, 01:54:25 pmI was a step behind you on that one. All caught up now.