AC - AC Step Down Transformer - Page 1
Forum section: electronics / beginners

author: iamnothim
author_rank: Regular Contributor
timestamp: March 02, 2013, 04:00:46 am
content: Hi,I am using several Hammond "Open Style" Filament transformers for a vacuum tube project. I need to provide 6.3VAC to the heater pins (4&5) of a dual triode tube. The heaters draw about 300 mA.The transformers all do the same thing. I think I know what's going on but I'm not certain. Here are the specifications:Model 166Q6 Primary: 37.8 VA 115V 60 Hz. Secondary: 6.3V C.T. 6AModel 166G6 Primary: 3.78 VA 115V 60 Hz. Secondary: 6.3V C.T. 600 mAMy voltage out of the wall is 120 V…. I think that's a problem because the Transformer wants 115V on the primary. ?Here's whats going on.My Fluke 88V measures 7.37 V across the secondaries. (Both Transformers)When I tap into my pro to-board with my scope (DS1052E) The wave is very pointed, not a sine wave at all.The scope measurements are:Vmax and Vmin are 12.2V and 7.57 Vrms when on side of the secondary is probed andVmax and Vmin are 14.2V and 8.96 Vrms when I probe the other lead.Am I taking my readings correctly? I am new to a scope.Do I need to add a resistor in series with the secondary to bring it down to 6.3V ? I calc'd 3.24 ohm but LTspice wanted 4+ ohm on the simulation.Or do I provide resistance on the primary. I got all kinds of crazy numbers for resistor that aren't common trying to reduce 5V. I was not sure if I calculate the resistor for the drop using 5V on the primary using the 10A rating on the outlet or the 37 VA on the transformer primary.Many thanks

author: amyk
author_rank: Super Contributor
timestamp: March 02, 2013, 05:25:40 am
content: 115 to 120 is a 4.3% increase. You're getting much more than that on the secondary.6.3V is "nominal". You are measuring unloaded voltages, which can be much higher. A "9V" AC adapter can output over 12V when unloaded. Give it a load of a few hundred mA with a resistor of appropriate wattage and measure the voltage across it again.How are you connecting the scope?

author: iamnothim
author_rank: Regular Contributor
timestamp: March 02, 2013, 05:37:39 am
content: I tested unloaded.The smallest resistor I have on hand is 47 ohm.Remember, I'm one step in the food chain above an idiot.I'll just hook up the heaters from a test valve. 6.3V .365 A should be 17.2 ohm of resistance.Then I'll measure.Here is the valve schematic for an E188CC heater (4&5)How am I connecting the scope?CH1, 10x, calibrated, not clipped to ground.I have a Global Specialties PB-103 board. I jumper the outside secondary leads to the voltage rail and I stick the probe into the rail.Thanks..... I think I'll wait for your reply before putting the valve into the circuit.Many thanks

author: IanB
author_rank: Super Contributor
timestamp: March 02, 2013, 06:23:18 am
content: What do you mean, "not clipped to ground"?Every circuit has two wires and two connections. You can't take a measurement with one wire only.

author: iamnothim
author_rank: Regular Contributor
timestamp: March 02, 2013, 06:39:12 am
content: I'm very new to scopes.Off the probe is a short alligator clip that can be fastened to the contact with a ground icon, in the lower right corner of the scope.As far as I know that's it. It's about 1/8th the length of the probe leads. I haven't done my homework on it's use yet.I doesn't make sense to me to connect the channel 2 probe because, well, it's channel 2.If I use both probes I'm going to get 2 different traces on the scope.From my photo you can see the traces using one probe only. It has an AC wave and it's measuring voltage.As such, I assumed I was using it correctly. Please let me know if I'm mistaken.A multimeter, no problem. Red and Black.

author: IanB
author_rank: Super Contributor
timestamp: March 02, 2013, 06:59:04 am
content: I'm struggling to understand what you are doing. The wave on the scope doesn't look right and I'm wondering why.The scope probe has two connections on the end. The center of the probe is the "red" lead, and the short wire with the alligator clip is the "black" lead. Just like you do with a multimeter, you connect red and black on each side of the voltage you want to measure.But unlike a multimeter the scope is not isolated. You must only probe circuits powered by batteries or circuits on the secondary side of a transformer. Also if you happen to use both probes to take two different measurements at the same time you must connect both of the ground clips to the same point.

author: amyk
author_rank: Super Contributor
timestamp: March 02, 2013, 07:01:06 am
content: Keep in mind that voltage is always a RELATIVE quantity measured between 2 points. The tip of the probe is one point, the ground clip the 2nd point. Since you are measuring a transformer output the circuit is isolated so you may safely put the ground clip there.See the advice from Dave here:http://www.eevblog.com/2012/05/18/eevblog-279-how-not-to-blow-up-your-oscilloscope/

author: iamnothim
author_rank: Regular Contributor
timestamp: March 02, 2013, 07:26:53 am
content: Thank you very much for the tutorial.I wasn't getting it...It makes sense now.

author: iamnothim
author_rank: Regular Contributor
timestamp: March 02, 2013, 09:16:24 am
content: Another rookie mistake.I purchased one transformer with a secondary that is 6.3 VAC and 6 A (37.8W) I only need 300 mA. a 16 ohm resister that is rated for 40W is...... not feasible.Can I build a circuit to reduce the current another way?

author: AlfBaz
author_rank: Super Contributor
timestamp: March 02, 2013, 11:22:12 am
content: Quote from: iamnothim on March 02, 2013, 09:16:24 amThe tranny is rated at 6A, the amount of current that comes out is dependant on the load. Presumably you used ohms law to determine the resitor value you needed so the current drawn will be I=E/R.The power rating required for that resistor, if you were to place directly accross the secondary can be calculated by P=E^2/R which is about 2.5W.What you also have to take into account is the resistance of the heater element, thatplusyour current limiting resistor have to add to become a resistance that at 6.3V will give you 300mA.This is all based on the premiss that the heater element is purely resistive

author: IanB
author_rank: Super Contributor
timestamp: March 02, 2013, 03:45:44 pm
content: Quote from: iamnothim on March 02, 2013, 09:16:24 amYour vacuum tube needs 300 mA. The transformer has 6 A. If you connect the tube to the transformer it will draw 300 mA and leave 5.7 A sitting in the transformer unused.It's like if you have $600 sitting in your bank account and you need $30 to buy something. You don't have to figure out a way to waste the other $570. That would be daft.

author: IanB
author_rank: Super Contributor
timestamp: March 02, 2013, 03:46:38 pm
content: Quote from: AlfBaz on March 02, 2013, 11:22:12 amI kinda think there is a joke going on here that I am not included in. Are you guys trolling?

author: iamnothim
author_rank: Regular Contributor
timestamp: March 02, 2013, 05:46:59 pm
content: Iana & Alf,I know my questions are very elementary. This is my first time implementing this. I've used the textbook from "AllBaoutCircuits.comand I "thought" I had a handle on all things Ohm's Law. Divider circuits Series, Parallel, and combined. I worked through all of the example equations. I also ran the examples through LTspice and the results cam out as expected.Here's my confusion, and I'm not entirely up on tranny calc's, but I don't think it matters. So….Very elementary circuit. Secondary potential is 6.3V, 37.3 W at 6A. Heater voltage is 6.3V (check) Only uses .365 A (check) to bring the current to .365 A I put a 17.26 ohm resister.I ran it through LTspice. Power source is 6.3V and a single 17. ohm resister in the loop. The simulation show + 6.3 V going into the resister. Set a reference probe on the left + and you get -6.3V on the right side. As expected Zero. The current across the 17.26 resister measures .365 A. perfect. LTspice shows the power across the resister is measured at 2.2995WEverything makes sense to me.I go to the breadboard hook up the secondaries and place a 47 ohm resister rated 0.5W and it smokes. Now I'm confused, and I think ok, the tranny is 6A and 37.8W that's why it smoked. As such, I figured the 6.3 V drawing .365 A 2.2995 W….. as shown in the sim would smoke too.I know it's simple, you probably already answered it. What am I missing? What am I doing wrong? I didn't think it mattered if I put a 47 ohm resister across the potential.Thanks for your assistance.

author: iamnothim
author_rank: Regular Contributor
timestamp: March 02, 2013, 06:01:37 pm
content: I think I'm getting it. It's the power rating.I looked at the tube / valve spec sheets and they don't express the power "rating" in Watts. Rather they have a set of "Maximum" values. The heater pins, 4 & 5, are designated "f" "k" is the cathode, pins 3 &8. The heater heats the cathode to release electrons. So.... A Telefunken E88CC spec shows a max "U sub f/k+" of 150V and a "U sub f/k-" of 100V. Ohm's law 150V going in pulling .365 A current equals 54.75 W.Is this telling me the heater is "rated" at 54.75 W, thus I don't have a problem with the secondary 37.8W ? That said, the U sub f/k- at 100V falls short at 36.5 W.

author: mariush
author_rank: Super Contributor
timestamp: March 02, 2013, 06:08:40 pm
content: The power dissipated in a resistor is calculated with the formula V x V / R ... so your 47 ohm resistor across the 6.3v dissipated 0.844 watts , which is more than the resistor's 0.5 watt rating.If I may suggest, download the "Electronics Assistant" application fromhttp://www.electronics2000.co.uk/it will help you with some of these basic formulas and has calculators along with the formulas explainedThere's alsohttp://www.falstad.com/circuit/which has a circuit simulator as a Java applet - you can probe, put resistors, capacitors etc in the circuit and so on... and there's a link to a zip file, so you can use the applet offline as long as you have Java installed.You say the transformer is rated for about 36 watts. Most if not all transformers don't have a "fixed" voltage, if there's little load on them the voltage will be higher than the one specified, I think 6.3v in your case. As you increase the load on it, the voltage will come closer and closer to the 6.3v.If you connect something that only uses a few watts, that's OK, whatever is left just doesn't get used.If a device you connect to the transformer is designed to need more power than the transformer can handle, well.. in some cases the transformer may be capable of doing more than its rated power but it could potentially overheat its core and get damaged.

author: iamnothim
author_rank: Regular Contributor
timestamp: March 02, 2013, 06:25:21 pm
content: excellent stuff...especially the power dissipated by a resister formula. I did not get to, or retain, that one in my reading.Intuitively, which is all I have, versus knowledge, that the low rating of the resister caused it to smoke. My I couldn't get a grasp of that via Ohm's law. Which with Kirshoff is all I have right now.I have several foo iPad and iPhone applications (I'm a Mac OS/X guy) that I like EE ToolKit, and Electronic Toolbox. I like Linear Technology LTspice and I think it's widely used..... But count on me looking into Electronics Assistant and Falstad.Another member clued me into Voltage output of a transformer under load v open. I'm the direction I was going. Put a load across the secondary. Thankfully I was also schooled on the use of my oscilloscope and the video on how not to blow it up. I probably would have if secondaries aren't floating.Thank you.

author: iamnothim
author_rank: Regular Contributor
timestamp: March 02, 2013, 11:35:39 pm
content: Unbelievable. I am still failing with this simple circuit.I have the two AC / AC transformers shown in the graphic. They both do the same thing...With a 500 ohm 5W resister across the potential, I take a voltage measurement Outside Lead to C.T. I get an expected value (3.7 VAC). However, if I bypass the CT and measure Outside lead to Outside lead the meter indicates Overload.The primary does not go to mains ground.It does the same thing without a resister, just open leads.I measured the resistance of the secondary leads (w/o power ) the sum of resistance of the outside leads to the center tap equaled the resistance of the outside leads. 0.6 ohm x 2 = 1.2 ohm I have continuity.

author: IanB
author_rank: Super Contributor
timestamp: March 02, 2013, 11:42:46 pm
content: Stop trolling us.But hey, I'm going to respond because it's fun.Set your meter to a voltage range that doesn't go off scale when trying to measure 7.4 V AC.

author: iamnothim
author_rank: Regular Contributor
timestamp: March 02, 2013, 11:50:13 pm
content: Thanks Ian.I'm an old guy. What does "trolling" mean.I think I've been accused in the past.ThanksLuke

author: IanB
author_rank: Super Contributor
timestamp: March 03, 2013, 12:01:46 am
content: Trolling means pretending to be less competent than you truly are and asking questions designed to elicit a response without really needing to know the answer. Otherwise known as "leading people on" or "winding people up".

author: mariush
author_rank: Super Contributor
timestamp: March 03, 2013, 12:24:03 am
content: Yeah, set the multimeter to AC and if it's not autorange, set it to a higher range (100v ac or something).Only other thing I can think of ... if there are 4 wires on the secondary side, if you want to measure the output of all the secondary side you have to short the two wires in the middles to get a single secondary coil.If you leave the center wires separate, you'll have 2 separate coils.

author: AlfBaz
author_rank: Super Contributor
timestamp: March 03, 2013, 12:47:34 am
content: Quote from: IanB on March 03, 2013, 12:01:46 amhmm... I was gonna take the op's side and ask you to stop being so obtuse, but if the op is smart enough to turn auto ranging off on his 87V then he clearly knows that the range button exists! Hear the sounf of flushing? The chain is being yanked

author: iamnothim
author_rank: Regular Contributor
timestamp: March 03, 2013, 12:57:27 am
content: I am very grateful for your help. Without the direction on Oscilloscopes, mine would be toast. Ditto this stuffI'm not sure being a Troll isn't a compliment.I do know a lot of stuff. I'm hands on more than book.Owned a networking company, but never understand board / component level. I'm retired now and want to learn.I genuinely have some learning "issues". Memory is a big one. More than being 56.I promise I won't ask a question that I can't dig into myself. I am in the weeds with electronics.... but I dove there.AND.Not pushing the "Auto Ranging Button" has bitten me before.I was so timid and concerned I would cook something...Ok, don't answer this...... I'm going to see if Auto Range can default ON.... Going to the manual.I just ordered a Fluke 287. Maybe that has one.... I do like toyz.One of the "Why does that Moron have one of those?" "He can barley turn it on."I'll try to keep out of your hair for a while.... I'm burning through my chits

author: iamnothim
author_rank: Regular Contributor
timestamp: March 03, 2013, 01:53:27 am
content: It worked....!!!A nice sine wave on my scope. I'm even using ground on the probe.I just need to get the voltage on the secondary fixed and I'm good for a while. (I can do that now)Next lesson Capacitors. I know what they do, I don't know why you put one in a circuit...

author: mariush
author_rank: Super Contributor
timestamp: March 03, 2013, 02:23:14 am
content: Capacitors (among other things) store energy and give it back to the circuit when there's a fluctuation... they smooth the output.The transformer will output AC voltage on the secondary side.If you want to transform this into DC voltage, which is used by most electronics, you have to rectify it and then smooth out the voltage... and that's a basic example of where capacitors are useful.See this video, it explains it in very simple terms, which also goes in detail about diodes, which are very useful to know about - probably more useful to know compared to capacitors:

author: iamnothim
author_rank: Regular Contributor
timestamp: March 03, 2013, 03:03:49 am
content: Thanks I'll watch it. I need all I can get.I just cooked the tiny probe attachments on my FLuke 15 minutes ago... I need to slow down, think about what I'm doing, and be methodical.I don't want to abuse asking for help. I watched Daves video's a month ago.THen I discovered the blog, saw beginners, and purged 10 things I had in my mind.Back on topic...I won't need the rectifier because the circuit is for old valve, dual triode testing.They are all AC.I am rebuilding an old valve tester that I ripped apart. (I have the schematic)The whole thing has 5 resisters, 3 pots, 4 caps, an reference valve "6BN8" and the valve under test.

author: mariush
author_rank: Super Contributor
timestamp: March 03, 2013, 03:08:37 am
content: That's Ok if you don't need it, but watch the video anyway, because it helps you understand some concepts about electronics .. voltage, current, all that.The video is done by a guy that has a really good series of tutorials with the basics, you should really watch it, i guarantee it you won't be disappointed and won't waste your timeHere's the link for his electronics related tutorials playlist:https://www.youtube.com/playlist?list=PLC8A1AD995AB64055

author: iamnothim
author_rank: Regular Contributor
timestamp: March 03, 2013, 03:17:58 am
content: Martin is great.I've watch quite a few already and they've helped to bring me along.I'm 3/4 through the DC book on AllAboutCircuits.

author: G7PSK
author_rank: Super Contributor
timestamp: March 05, 2013, 12:12:43 pm
content: One reason your voltage readings may be off compared to the transformer ratings is your Fluke 88V is meant for automotive use and from reading the specs on Fluke's web site I am not sure that it is true RMS. on the AC range's.

author: iamnothim
author_rank: Regular Contributor
timestamp: March 05, 2013, 06:47:44 pm
content: @G7PSK,More great info. I did not know my Fluke couldn't handle RMS. I purchased it years back when I had a hot rod. I thought it might be sketchy for circuits, so guess what, the UPS man will drop off a Fluke 287 today. I should be banked. I also have a buyer for the 88V.That said the measure function on my DS1052E shows a Vrms of 7.35 on the secondary. Close to the 88V.I know it's Root Mean Squared, correct? But I haven't grasped it's importance in circuit design.I posted the stuff below on my thread with PCB software questions. Sorry to be redundant but I wanted to give an update here, should anyone care...... If not....bail now.I want to thank y'all for your patience with the resistance calc and power ratings. I hammered for several hours, almost ordered the wrong parts, but realized they were wrong because Mouser and Digikey didn't stock anything close. I went back worked through the power calc's and got it right. Sure enough they were stocked parts. LTspice double checked everything. I have 3 different resistance settings for 3 different tube types. I'll put them on a rotary sw. and each will have the 6.3VAC and the required current. Sounds small, but this was huge for me, and that's the easiest part of the design.Should anyone care here are the results of my calculations for the (4 & 5) heater circuit and the schematic I'm working from. Thanks again for the help and patience.