| Electronics > Projects, Designs, and Technical Stuff |
| My first power supply |
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| queennikki1972:
The Capacitor across the fan stopped the fan noise almost completely. |
| MarkF:
--- Quote from: queennikki1972 on September 25, 2019, 12:01:55 pm --- --- Quote from: MarkF on September 23, 2019, 02:00:03 am ---Also since you are driving both regulators, the current capabilities of both the transformer and the rectifier bridge must be large enough for both (i.e. greater than 2A). The capacitance of the filter cap must be large enough for both also. You might actually be better off with two before the regulators. --- End quote --- I've taken notes and saved the schematics MarkF. Thank you. I get it now, just a little slow learner at times. Bridge Rectifier is 6A and the one large cap is 6800uf . When I redo the new board Ill put 2x2200uf cap 35v per side. --- End quote --- You don't have sides unless you are going to use a CT transformer as in my second circuit. In your original circuit, all I was trying to say is you might want to add a 2200uF to the 6800uF since you are at the minimum as shown in @mariush's calculation. I don't think it is necessary for you to disassemble what you have. If you don't expect to pull maximum current, you could stay with just the 6800uF with larger heatsinks. If you want four 2200uF capacitors in parallel, that's okay too. Oh, you don't need LEDs either since your meters will provide the minimum load. You might want to review the calculation for the ripple capacitor value. --- Quote from: mariush on September 24, 2019, 11:49:56 am ---For capacitance after the bridge rectifier you can sort of estimate how much would be needed based on your minimum desired voltage. Capacitance = Maximum Current / [2 x Mains AC Frequency x (Vdc peak - Vdc minimum desired)] So you're saying you have a 12v AC 1.8A transformer and you're in US where mains frequency is 60Hz... that means your peak DC current will be approximately 0.62 x Iac = 1.16A but let's round it up to 1.2A Let's say you want minimum 14v dc voltage, because the 7812 regulator will have approximately 2v drop at 1A of current. "The LM7812 dropout voltage is 2V." When your bridge rectifier converts 12v AC to DC you get a peak voltage of sqrt(2) x Vac - 2 x voltage drop on rectifier diode. So your Vdc peak is 1.414 x 12 - 2x 0.8v = ~15.37 v ... let's round it to 15.5v So you can put the numbers in the formula and the minimum capacitance you need to have minimum 14v would be: C = 1.2A / [ 2 x 60 Hz x (15.5v - 14v) ] = 1.2 / 180 = 0.0066666 Farads ... so you should have at least a 6800uF 25v rated capacitor there to get your minimum 14v. --- End quote --- With some visual aids. See also: https://www.electronics-tutorials.ws/diode/diode_6.html The minimum capacitor is C = ILOAD / (2 * fLINE * VMAX_RIPPLE) |
| MarkF:
--- Quote from: queennikki1972 on September 21, 2019, 11:07:01 am ---So, I did go back and add the protection diodes, green banana jacks tied to PE, larger heat-sinks and a larger transformer of 12v 1.8 amps. I did not use the adjustable pots as the LM7812 and LM7805 are not variable. I measured the ripple and it looks good. I am purchasing a electronic dc load after December so Ill be able to test the load. I've tested about .25 amp load and seems to run nice and cool. I've since built a variable supply with LM317T and working on a dual adjustable supply of 1.5-60vdc. With each type I learn more. After I fully understand the linear supplies Ill try some switch-mode supplies or some built using IC's and Transistors. --- End quote --- BTW If you would like to build a DC Load, here is a link to the one I built: https://www.eevblog.com/forum/beginners/struggling-with-dc-load/msg2698044/#msg2698044 |
| queennikki1972:
I did build a different power supply using a positive +12 and Negative -12 with a common in the middle and a center tap transformer. I used the protection diodes, the led indicators and larger heat sinks. Yes I am very interested in building the load. I was actually going to buy one but this sounds fun. |
| MarkF:
--- Quote from: queennikki1972 on September 28, 2019, 03:49:51 pm ---I did build a different power supply using a positive +12 and Negative -12 with a common in the middle and a center tap transformer. I used the protection diodes, the led indicators and larger heat sinks. Yes I am very interested in building the load. I was actually going to buy one but this sounds fun. --- End quote --- You will want a large heatsink for the MOSFET. I used: https://www.digikey.com/products/en?keywords=RA-T2X-64E It will definitely will need a fan also. The MOSFET I used was an IRFP064. I reworked the PCB and going to use an IRFP250 that's cheaper. I suggest something in a TO-247 package. Twice the size of a TO-220 for better heat dissipation. Design guidelines: - The (+) op-amp input (and hence the sense resistor) voltage range is 0V to 0.5V. You set the maximum current capability by selecting a sense resistor knowing its max voltage will be 0.5V. In the circuit the sense resistor is 0.200 Ω which results in a 2.5A maximum load current setting. I recommend you select a sense resistor in the range of 0.100 to 0.500 Ω (i.e. 5A to 1A respectively). |
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