when stepping up do u have to supply a resistor on the primary? - Page 1
Forum section: electronics / beginners

author: robowaffe
author_rank: Regular Contributor
timestamp: July 06, 2023, 10:37:21 pm
content: So if I supply the resistor on the secondary with the high voltage, I cant seem to add a resistor on the primary or it doesnt work. (edit... in the sim i mean!)What happens if I dont put a resistor there and I let the constant current activate instead when its shorting through the coil, does this work? Or does it supply less power on the secondary as well, when the current limit is reached?

author: RJSV
author_rank: Super Contributor
timestamp: July 07, 2023, 05:06:41 am
content: (I'm kind of left, to piece this together...with my own supplied assumptions).Is it a lower voltage project, or involving dangerous voltage levels ? Meaning that if you start out with, say, 120 VAC (volts of alternating current), that's already a dangerous voltage, before you even increase it with your transformer. That would be, likely 50 hz or 60 hz power.The term 'constant current' hasn't any relevance.Are you, perhaps, wanting to control the output voltage, with a resistor ? Not sure that would make sense.You CAN get a kind of alteration of a resistor image, via a transformer, if my recollection of circuit theory is right. Like, for example if transformer is stepping up some audio, by 1:2 one to two ratio, then I believe a 1 kohms resistor would appear as 2 kohms equivalent...(but this feels awkward).Simulator might not be enough, for learning basic circuit features.Any additional details would help, to understand your question. Thanks.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 07, 2023, 06:19:02 am
content: Transformers don't need a resistor in the primary.Are you trying to simulate a circuit. Is this a problem with setting up the model parameters?Draw a picture of your circuit.I'm not sure if this applies to your problem but see this page on how series resistance in the primary is "reflected" into the secondary.www.yourelectricalguide.com/2017/08/equivalent-resistance-of-transformer.html?expand_article=1

author: wasedadoc
author_rank: Super Contributor
timestamp: July 07, 2023, 07:14:38 am
content: Quote from: RJHayward on July 07, 2023, 05:06:41 amReflected impedance depends on the square of the turns ratio. A 1k resistor on the secondary of a 1:2 step up will appear to be 250 Ohms to whatever is driving the primary.

author: robowaffe
author_rank: Regular Contributor
timestamp: July 07, 2023, 01:43:03 pm
content: You need a resistor on the secondary cause thats where the load is going to be! =)Thank-you for the help, I've been hard at work today for a long time in Falstad again, getting absolutely nowhere as usual, I usually spend a few days solid trying things and always leave depressed and miserable and with nothing then go back to programming, to do something I actually get something out of, cause everything in electronics doesn't work for me, very tricky hard to understand.How to resist a transformer correctly seems to be very confusing! The most confusing thing I've ever tried to do, thanks for the tip on leaving the primary bare ended, I just wasn't sure if it were right or not, and the rest of the tips thanks a lot.I don't understand still, but I'm getting a bit better at reading the numbers on the sim and I've noticed you can tell by the amp draw on the power what the resistor is supposed to be sometimes, but having it hard nailed not yet, Seems to be loss involved, you need to supply a million times more amps on the input that appear on the output when stepping up volts, I didn't guess it was going to be so much!I can see they have these items on the internet->https://www.aliexpress.com/item/33053175612.html?spm=a2g0o.ppclist.product.36.2433ViZ0ViZ0t9&pdp_npi=2%40dis%21AUD%21AU%241.24%21AU%240.92%21%21%21%21%21%402101c5c316887370973194530e23ce%2167434358446%21btf&_t=pvid:40addcfc-e859-4dbb-b25d-666ab4fe43fcThese things could help you out a lot when it comes to nasty shorts that stop your circuits from operating correctly, good galvanic isolation.Yep, and to think I spent about 10 - 11 years in electronics and I never looked at them! I was under the impression that you couldn't print them on PCB's, so they went down in favour to me, but I looked further and found out you can, as long as they are high frequency enough! They might make things happen for me, don't know yet, have to wait and see but quite hopeful.btw, of that pic I posted, could someone tell me what the henry's of that little isolation transformer would be?

author: wasedadoc
author_rank: Super Contributor
timestamp: July 07, 2023, 04:52:28 pm
content: An ideal transformer is lossless, meaning the power on the two sides is equal. So if the voltage on the secondary side is n times that on the primary side, the current on the primary side must be n times that on the secondary side.

author: RJSV
author_rank: Super Contributor
timestamp: July 07, 2023, 11:20:58 pm
content: Wasedadoc mentions that transformers convert a load resistor 'image', as related to the SQUARE of turns ratio, and I think I can explain that aspect;Power, is always conserved, as is energy, so that parameter remains constant, across the ideal transformer, where current in the primary creates the magnetic fields to drive secondary voltage. So, although the turns ratio DOES magnify the voltage, such as in a 1:2 or one to two relation. The example could be having 40 turns on primary, in magnetic interface with 80 turns of secondary section wire.Although the voltage gets magnified, or stepped up, by 2X the preservation of energy concept calls for the current (I) to be half of what the primary is. That way, you've neither created nor destroyed any energy (just so we don't piss-off Einstein, lol...)However, (by ohm's law) resistance or the 'appearance' of resistance, is the simple ratio; Voltage divided by current. So, in the example, the primary side current will be twice the secondary, and THAT number goes into the divisor portion, of R = V / I which gives a partial factor of 1/2, seeing as it's on the bottom of the division equation. So then that is compounded by the top factor, of 1/2 (of sec. Voltage).All together the two factors make the secondary resistor value be 'APPEARING' as 1/4 of it's physical value.Notice no mention of any actual resistor value, but you could use a (physical) value of 1 kohms, connected as a secondary load for example. Then, the primary would 'see', looking into the input, would 'see' a value at 1/4 that, or 250 ohms. That example would, with a 1 volt ac supply, cause a supply current of 1/250, or 4 mA (milliamp).If you think about it, that 250 ohm 'image' will be what the transformer circuit presents, to a AC power source driving the input, exactly same as an ACTUAL physical resistor would be causing, and controlling what input current happens.Some 'ideal' assumptions like this are going to be off a little bit, as there are some losses, but still maybe 97% when ideal would behave at 100 % conversion efficiency.You still need to account for a load resistor, but as a secondary circuit LOAD resistor,...not as any SERIES connected limiter.In theory, omitting any resistor should cause the transformer to just sit there, not drawing any (practical) current at 'no load'. Voltage out would be double, in the above example, but not connected to anything.(Don't expect 100 % ideal).

author: robowaffe
author_rank: Regular Contributor
timestamp: July 08, 2023, 03:28:47 pm
content: Ok thanx. so if I have 4 milliamps on the output of a 1:10 transformer I would have 400 milliamps on the primary side? (that would be square loss)And, I have one more question, what happens with reverse driving a transformer with a resistor on the primary, would the primary side resistance put a "virtual resistor" on the secondary side if I were to pass through the shorted coil on the secondary - or would it just short through untouched.

author: TimFox
author_rank: Super Contributor
timestamp: July 19, 2023, 04:28:59 pm
content: This is covered in all electronics textbooks.For an ideal transformer with primary:secondary turns ratio =N:1,VPRI/VSEC=NIPRI/ISEC= 1/NTherefore,PPRI=PSEC

author: Terry Bites
author_rank: Super Contributor
timestamp: July 19, 2023, 04:46:00 pm
content: NopeVoltage and currentallstep and step down with the ratio. eg, 10:1 turns changes current or voltage by a factor of 10Power in = Power outOnly Resistance or impedances in general change with the square of the ratio.Have a go at the sesimulationshttps://www.falstad.com/circuit/e-transformerup.htmlhttps://www.falstad.com/circuit/e-transformerdown.html

author: TimFox
author_rank: Super Contributor
timestamp: July 19, 2023, 05:38:22 pm
content: Like I wrote, current steps up and voltage steps down, so the power is the same.Therefore, the resistance seen at the primary scales asN2from the resistance connected to the secondary."The bronco jumped up and the cowboy came downThey met at the old saddlehornIt made a deep impression, you could say it changed his lifeAnd that's how the yodel was born"--Riders in the Sky