Transformer Amperage/Wattage Calculation - Page 1
Forum section: electronics / beginners

author: nathon
author_rank: Contributor
timestamp: December 06, 2015, 06:27:51 pm
content: I know this is probably very basic stuff. I have a second hand Veris Industries X240DAA 120 to 24V 240VA transformer. I'm trying to determine the amperage it's capable of.http://www.veris.com/Item/X240DAA.aspxAll of the online calculators ask for the voltage, but they don't specify whether that's the input or output voltage. Given the transformer is wired with 18 AWG wire, I'm assuming they mean the input voltage since that would give me 2A and 18 gauge wire is rated at 2.3A for power transmission (from a table I saw).Is this correct?

author: SeanB
author_rank: Super Contributor
timestamp: December 06, 2015, 06:37:06 pm
content: 240VA at 24VAC means it can supply 10A AC, but if you use a bridge rectifier and smoothing capacitor you will find the current you can draw is less, around 7A DC. 24VAC rectified will be around 32 VDC, and 240W at 32VDC is around 7A. At full load from the 120VAC mains it will draw 2A. Power rating of a transformer is always what it can supply, the input power will always be higher because of losses in the transformer.

author: mariush
author_rank: Super Contributor
timestamp: December 06, 2015, 07:31:24 pm
content: When you convert the AC voltage on the secondary side to DC using a bridge rectifier, you're going to have a DC output with lots of peaks and valleys and the peak DC voltage will be equal to 1.414 x Vac, minus the voltage drop on the two diodes that are always working in the bridge rectifier:Vdc peak = sqrt(2) x Vac - 2 x Vdiode = 1.414 x 24v - ~1.4v = ~ 32.5vThe maximum current can be approximated using the formula Idc = 0.62 x 10A = 6.2AThis 0.62 is a sort of constant that's not ideal but gives a very decent approximation for transformers of around 100VA. With your 240VA transformer, the number may be on the low side.If you multiply the current and voltage, you have power = 32.5 x 6.2 = ~ 200VA which gets us close to the original 240VA rating.Note that the 32.5v value is PEAK DC... the voltage goes up and down about 120 times a second (twice the mains frequency in your country, which is 60 as you say you're from US).To keep the voltage above a minimum value all the time, you need to use a capacitor after the bridge rectifier, which charges with energy when the output of the transformer is close to peak and provides this energy to devices when the transformer output is on the low side.You can figure out the optimum capacitor size with this formula which approximates capacitance C = Maximum Current / [ 2 x Mains Frequency x (V dc peak - V dc minimum) ]If we assume a maximum current of 6 A and that we want at least 28v all the time, then we need C = 6A / 2 x 60Hz x (32.5v-28v) = 6/540 = 0.01111 or 11111 uF ... 2 x 5600uF in parallel would do the job.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 06, 2015, 08:52:41 pm
content: The factor 0.62 is an approximation and depends one on the resitance / inductance and the size of the charging capacitor. The factor gets smaller for larger capacitors and lower resistance (e.g. larger transfromers). For smallm high loss transformer the constant can go to about 0.8, for large toroid transformers it may get less than 0.5.Its a good idea to have a fuse between the transformer and rectifier, as rectifiers may fail with a short. The added resistance of the fuse helps to get a higher current, though less voltage.

author: nathon
author_rank: Contributor
timestamp: December 07, 2015, 04:35:59 am
content: Thank you everyone! I appreciate the detailed responses! I'm also quite pleased that the amperage is more than 2A. I'm considering using this transformer in a bench top variable voltage, constant current arrangement. I understand the basics of power supply design and the losses, riple, capacitance, etc considerations. However, these details give me a lot more to consider.Thanks again!