9 volt low current power supply circuit wanted. - Page 1
Forum section: electronics / projects

author: Jim0000
author_rank: Contributor
timestamp: May 03, 2019, 01:59:01 am
content: I am looking for a circuit for a 240v mains supplied power supply that will deliver around 9 volts at 50ma max.The requirements are that this device be a small as possible.What it comes down to, is the question of finding a very small 240/12 volt transformer. I am able to design a full bridge rectifier, zender diode, cap and resistorregulated system.If someone can direct me to a source I would appreciate it.Jim.

author: MarkF
author_rank: Super Contributor
timestamp: May 03, 2019, 02:08:08 am
content: I saw these for some else yesterday. I do not know how noisy these might. They seem simple enough with a 12V Wall Wart. They say they accept either AC or DC inputs.https://www.ebay.com/itm/AC-DC-Power-Supply-Buck-Converter-Step-Down-Module-3-3V-5V-9V-12V-15V-24V-36V-US/122832954547?hash=item1c996a24b3:m:mNop_RHwBUxr99MJ7MTU6Cg

author: Jim0000
author_rank: Contributor
timestamp: May 03, 2019, 05:21:12 am
content: That would most likely do the job nicely. Is that a LM7809 regulator? I assume that it is.Thanks!

author: Jwillis
author_rank: Super Contributor
timestamp: May 03, 2019, 08:43:14 am
content: Both examples from MarkF and blueskull are buck convertors .They are in a class of switch mode power supply (SMPS). The LM7809 is a linear regulator that would require a much larger package to facilitate a transformer and rectifier.But I'm sure either would be fine for your purposes.

author: mariush
author_rank: Super Contributor
timestamp: May 03, 2019, 09:33:30 am
content: if you want to fully avoid switching power supplies, tme.eu sells some tiny pcb mounted transformersHere's a link with 7.5v..12v ac , 0.5va.. 2.5va :long linkIn theory a 7.5v transformer could be used if you don't need a stable 9v, like for ex. you just want a 9v battery replacement.you use a bridge rectifier which converts 7.5v ac to dc with peak of 7.5v x 1.414 - 2 diode drops ... so ~10.5v - 2 x 04..0.6v (low drop at 50mA) so around 8.v..9.5vYou'd only need a capacitor to smooth the output .. let's say a 1000uF 16v cap would be enough ... c= i/[2 x Fac x (Vpeak-Vmin)] = 0.05/2x50x(9.5v-9v) =0.05/100x0.5 = 0.001 FaradsProblem is when your circuit only consumes 5-10mA instead of 50mA.. at such low voltage and current transformers output much higher voltage at low load.. ex at 5mA a 7.5v ac transf. may output up to 25% more, let's say 9v ac.. which rectified could mean 10..11v dcSo it may be better to go with 9v AC, rectify with bridge rectifier to ~11v, use a smaller electrolytic to guarantee a minimum of let's say 9.5v and then use a LDO to get 9vA 7809 or 1117 will need ~0.7v even at 50mA but there are ldos with 100..300mV drop at low currents.ex.1.69$ each : 12v 1.9VA (158mA) 32.5 x 27.5 x 23.8mm 81g :https://www.tme.eu/en/details/bvei3032032/pcb-transformers/hahn/bv-ei-303-2032/1.95$ 12v 1.5va (125mA) more sqare-ish and taller 70g :https://www.tme.eu/en/details/tez1.5_d_12v/pcb-transformers/breve-tufvassons/tez1-5-d230-12v/1.95$ 9v 1.5va (166ma) same size/weight :https://www.tme.eu/en/details/tez1.5_d_9v/pcb-transformers/breve-tufvassons/tez1-5-d230-9v/2.00$ : 9v 1.8va (200mA) :https://www.tme.eu/en/details/bvei3022001/pcb-transformers/hahn/bv-ei-302-2001/for ldo theres plentymic5225 :https://www.digikey.com/product-detail/en/microchip-technology/MIC5225YM5-TR/576-2998-1-ND/1834780ap2204k :https://www.digikey.com/product-detail/en/diodes-incorporated/AP2204K-ADJTRG1/AP2204K-ADJTRG1DICT-ND/4505290lp2951 (100mA max) :https://www.digikey.com/product-detail/en/on-semiconductor/LP2951CDR2G/LP2951CDR2GOSCT-ND/2120680etc...

author: ebastler
author_rank: Super Contributor
timestamp: May 03, 2019, 11:04:30 am
content: @Jim0000: Is this for tinkering, or for use in a product? What are the safety and compliance needs -- CE, UL etc.?

author: Tek Tech
author_rank: Contributor
timestamp: May 04, 2019, 05:43:30 am
content: @ Jim0000, you can also use a super small and simple circuit of a series capacitor, bridge rectifier and a Zener under two conditions: (1) Your project is drawing a relatively constant current or a small current in relation to the Zener's max current, and (2) It does not need to be isolated from the mains. (If your project does need to be isolated from the mains then a small transformer or a switched mode PS are the options.)You can use one of the online calculators to estimate the value of the series cap in circuit below. Note that 50Hz and 60Hz mains will require slightly different C values. E.g. a 470 uF cap will have 6.8 kOhm reactance at 50 Hz - which will allow you to draw up to 30 mA. A 680 uF will let you draw about 48 mA, which is very close to what you need. 820 uF will get you up to 57 mA @50 Hz, etc.http://www.sengpielaudio.com/calculator-RC.htm