Low Voltage, High Current Source for Chocolate Stamping System.  0.8V 10A. - Page 1
Forum section: electronics / projects

author: rantonides
author_rank: Contributor
timestamp: March 12, 2014, 10:09:42 pm
content: Hi. I have an application where we are stamping chocolates with a stainless steel mesh to create a pattern on top. We want to heat the mesh slightly (45 deg C) to prevent chocolate buildup. I found that by passing 0.8V DC at about 8A across the mesh, we get just the right amount of heating. I'm using my adjustable bench supply. The resistance of the mesh is just over 100 milliohms.I need to make this a more elegant system. What is the best way to get down to that voltage? I thought about changing to AC and getting a stepdown transformer from 120VAC to 0.8VAC, but can't find anything off the shelf at that current.Off the shelf industrial DC power supplies generally just go down at 5V.I will want to PWM the power output to control the amount of power transferred. This will be open-loop. There's no temperature feedback.Any ideas?

author: andy1
author_rank: Contributor
timestamp: March 12, 2014, 10:18:59 pm
content: You don't really need the voltage to be 0.8V that just happens to be the voltage drop over the mesh at 8A (100mohm*8A) so any kind of constant current source should do you fine and I guess you can easily find something for high current LED driving etc.

author: rantonides
author_rank: Contributor
timestamp: March 12, 2014, 10:34:35 pm
content: I searched Digikey for a constant current LED source with a minimum of 8A. It seems that the output voltage on all matches can only go down to 9VDC in constant current mode.

author: tom66
author_rank: Super Contributor
timestamp: March 12, 2014, 10:42:41 pm
content: You could also power the mesh at approximately 8% duty cycle from a 10V DC source, using something like a 555 timer and MOSFET.Note the current through the mesh will be approximately 12.5x nominal (i.e. 100 amps) but the average current will be less (8 amps or so.)This might be too high for any reasonable power supply so you could select a lower voltage like 2.5V. Or use an L-C filter network and a high PWM frequency to limit the peak current.You could also use a resistor, e.g. for a 5V supply with 0.8V across element and 8 amps current, size resistor for 4.2V/8amps = 0.52 ohms. (0.51 ohms is a close standard value.) The resistor will dissipate I^2 * R heat = 32 watts so it will need to be a large device with a big heatsink.

author: oldway
author_rank: Super Contributor
timestamp: March 12, 2014, 10:56:13 pm
content: Variac feeding the transformer of an 12V /10A battery charger. (and a meter to measure the current)

author: ConKbot
author_rank: Super Contributor
timestamp: March 12, 2014, 11:10:54 pm
content: Just put 2 120 -> 10V transformers in series. The second transformer should have a 10A+ secondary, so its going to be a 100+ VA transformer, the first can be a tiny little 10-20 VA runt. The first transformer will divide it to 10VAC (obviously) then it will get divided by 12 again down to 0.8333 VACAnd if the first transformer has spare capacity, you can run control circuitry off of it too.edit:Just for an example, first transformer:http://www.digikey.com/product-detail/en/VPS20-1250/237-1264-ND/666150second transformer:http://www.digikey.com/product-detail/en/A41-130-20/595-1286-ND/953156$56 total (Maybe more if you throw in an enclosure for it and a fuse holder and fuse...) Not bad, imo.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 12, 2014, 11:51:44 pm
content: Take the transformer out of a cheap soldering gun and use a dimmer (or ceiling fan speed control) to fine tune the power.

author: Marco
author_rank: Super Contributor
timestamp: March 12, 2014, 11:53:57 pm
content: Quote from: rantonides on March 12, 2014, 10:09:42 pmA synchronous buck converter will do the PWM for you ... bit more elegant than throwing huge chunks of 60 Hz transformer at it and then rectifying/pwm'ing the output.PS. I just did a digikey search for board mount DC DC converters with output currents between 8A-40A and it seems to me you're spoiled for choice.Thiswas the first result (7$ for a single).

author: Niklas
author_rank: Frequent Contributor
timestamp: March 13, 2014, 12:06:04 am
content: We did a low voltage, variable high AC current stimuli application a couple of years ago that was pretty simple. A variac followed by a 15 VA toroid transformer with a new secondary winding made of ordinary cable. In our case the output current was measured and any adjustment was made to the variac. I think we used 7 turns of 0.5 mm^2 wound on the outside of the toroid and fixed with some tape to get up to 6 Amps. The wire cross section and number of turns must be matched and trimmed with all the connector and load resistances. Dedicated high current connectors or soldered connections are essential as crappy connectors can ruin the whole setup by adding milliohms. Also consider line voltage variations and effects of heating in copper wire and connectors.

author: rantonides
author_rank: Contributor
timestamp: March 13, 2014, 12:11:50 am
content: Quote from: NiHaoMike on March 12, 2014, 11:51:44 pmJust opened up my cheap Yihua soldering/hot air rework station. Soldering heater element appears to be about 16 ohm. Transformer has 10.6 VAC, 34V and 26V output taps. Looks like much higher voltage than what I need. That would have been a great option, because it already regulates the temperature with PWM, but this load is 0.1 ohm instead of 16 ohm.

author: digsys
author_rank: Unknown
timestamp: March 13, 2014, 12:16:59 am
content: No need to mess around with transformers, just pick up something like this -www.mean-well.com.au/shop/rs-50-50w-single-output-switching-power-supply/28--rs-50-33.html38 bucks, approved, stable DC voltage. Adding a linear constant current to the end of it is straight foward, andthere'll be minimal heat. There's no advantage to going lower than 3.0/3.3V as 10A is really not a big deal.There are zillions of similar off-the-shelf alternatives as well, some EVEN have CC modes built in !!PS: If you need any more help, I'd need a HUGE box of choclate to sample to errr get the parameters right :-)

author: rantonides
author_rank: Contributor
timestamp: March 13, 2014, 12:17:30 am
content: Quote from: Marco on March 12, 2014, 11:53:57 pmThat looks like the solution. Can easily get industrial power supply at 5V-15V. It would be nice if they had it in an industrial DIN Rail version, but looks good. Thanks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 13, 2014, 03:10:04 am
content: Second the transformer solution. Industrial reliability, at industrial simplicity: you can't beat a transformer. A little pricey (you could spare a few amps on the second transformer I think), but it will literally never fail. I hope your business lasts a hundred years, because the transformers will in that kind of service!Tim

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 13, 2014, 03:36:45 am
content: Quote from: rantonides on March 13, 2014, 12:11:50 amSomething like one of those:http://www.harborfreight.com/180-watt-industrial-soldering-gun-61170.html#.UyEngVYVtb4

author: Richard Head
author_rank: Frequent Contributor
timestamp: March 13, 2014, 05:34:54 am
content: I vote for a single 15VA custom transformer with a 1V output and use phase control (light dimmer) on the input. You may need quite a heavy pre-load on the dimmer so it behaves.It should be super reliable due to its simplicity.Dick

author: oldway
author_rank: Super Contributor
timestamp: March 13, 2014, 08:11:52 am
content: I wrote about choosing a battery charger transformer and there is a good reason for that.They have a high short circuit voltage to help regulating the secundary current.The primary and secondary are side by side and not one upon the other to maximize the inductive leakage.Short circuit voltage is an important caracteristics of a transformer: this show how much is the value of the virtual serial inductance.In a battery charger transformer, short circuit voltage is between 15 and 20 % of nominal voltage.In a toroidal transformer, this value is far lower...between 5 and 8 %.How can we measure this short circuit voltage ?You short circuits the secondary, feed the primary with a low voltage from a variac and increase this voltage to reach the secondary nominal current.The primary voltage is a % of the nominal primary voltage: that's the short circuit voltage of the transformer.Why is this important for a battery charger or for this application ?With a 15% short circuit voltage transformer 110V/10A, you will need a little more than 16.5V to have 10A in the secundary.This reduce secundary current variations when secundary resistance vary or when primary voltage does vary.So it work a little as a current regulation ... this is why its used in battery charger so charging current does not vary too much.NB: using 2 cascaded transformers is also a good solution because the resulting short circuit voltage is the sum of the short circuit voltages of the two transformers.Don't use toroîdal transformers, that's the worst choice.

author: muvideo
author_rank: Frequent Contributor
timestamp: March 13, 2014, 03:54:52 pm
content: I would grab a toroidal transformer, wind manuallya low voltage secondary with thick wire, few turnsof 2x2.5mm2pvc coated stranded wire(the one used for electrical wiring) are enough for 0.8V 10A.Cheap 35 to 50VA toroidals can be found in halogenlamps, the "heavy" ones with 12V lamp inside.For power control slap a triac dimmer for inductive loadson the primary.

author: rantonides
author_rank: Contributor
timestamp: March 13, 2014, 04:24:47 pm
content: Quote from: Marco on March 12, 2014, 11:53:57 pmThose DC-DC converters have a remote on/off feature. I have a query to their tech support to find out if I could use that to pulse the supply every 2 seconds or so to control heat. I'll keep you posted. There's not really that much choice at that low a voltage (0.8VDC) and the efficiency really goes to shi*.

author: muvideo
author_rank: Frequent Contributor
timestamp: March 13, 2014, 06:21:28 pm
content: Quote from: muvideo on March 13, 2014, 03:54:52 pmAlso, if you want to build the partialization circuityourself, you can use burst mode control, using anormal triac and an optotriac with zero-crossingdetector like a moc3032.The pulse train will be a slow PWM, you can useany circuit to generate it, since will be isolated bythe optotriac.The efficiency will be reasonably high.

author: Marco
author_rank: Super Contributor
timestamp: March 13, 2014, 07:14:20 pm
content: Quote from: rantonides on March 13, 2014, 04:24:47 pmCan't you just use a pot to set the output voltage? At 0.59 V you have half the power than at 0.8V ... doesn't that give you enough range of control? (You can probably use an opamp in the feedback loop to get the supply to go below 0.59V too, but you seem to want a mostly turn key solution.)

author: dumle29
author_rank: Regular Contributor
timestamp: March 13, 2014, 08:52:07 pm
content: 0.8v and 10 amps is only 8w. Not really a lot of power. Any current limiting circuit (with that amp capability) should do, and it should drop the voltage low enough to keep the amps? If you can't find anything that will handle 10A without being ridiculously over speced, you should be able to build a simple chopper circuit with a sense resistor, dual op-amp, where the second op-amp is configured as a comparator. You amplify the small voltage over the sense resistor (this needs to be a SMALL resistor, as you only have ~0.8v), and feed the amplified signal into the non-inverting input of the "comparator" and the center tap of a pot in to the inverting input. You can then set the max current (the voltage that will be present over the sense resistor x the gain of the op-amp) and the output of the "comparator" should then run the gate of a mosfet. An IRLB3813 is a high amp mosfet I can think off off of the top of my head. (N-channel, mount it on the low side of the load) It's logic level too.Now EEV community. Bash everything I'm wrong about. My brain says this should work. Am I spewing garbage?

author: dumle29
author_rank: Regular Contributor
timestamp: March 14, 2014, 10:34:30 pm
content: @rantonidesSo I wanted to do a board, and came up with this schematic in eagle, and laid it out on a small 7.35$ for three boards layout.I have not tested this, and I have not done anything like it before, but my brain is telling me it should work. Brains are dumb sometimes thoIt uses one SO-8 4081 and gate (kinda a waste, I'm only using one) and one LM324 (using two of those internal OP-amps)Some support components, and I think it should work.For R2-5 I'd say go for 1/2W .1% resistors. As per the schematic, 0.1RIf you go for it, I'll MSPaint a hookup diagram.Schematic:OSHPark upload:http://oshpark.com/shared_projects/i2vadt6ZSo yeah, any questions?EDIT: TerminalJack505 played with the circuit in a sim, and caught the second op-amp being backwards, and pointed out the need for a cap. See next page with the updated links

author: dumle29
author_rank: Regular Contributor
timestamp: March 19, 2014, 01:00:31 am
content: Rantonides: Did you solve the issue? You got me interested

author: David_AVD
author_rank: Super Contributor
timestamp: March 19, 2014, 02:27:16 am
content: Quote from: dumle29 on March 14, 2014, 10:34:30 pmCan you explain how this circuit works please?

author: jmole
author_rank: Regular Contributor
timestamp: March 19, 2014, 06:02:43 am
content: I've been using these modules for an upcoming project. Super cheap, and very easy to use.http://www.digikey.com/catalog/en/partgroup/micro-dlynx-series/563

author: AhhhChoo
author_rank: Newbie
timestamp: March 19, 2014, 06:46:33 am
content: I'd always go with simplicity as well.Take just about any toroidal mains transformer (10 Watt or more) and just wind a couple or so turns onto it yourself using a suitable thickness enabled copper wire. Their you have it!Obviously the number of turns will depend on the transformer, but for 0.8V RMS I doubt it'll be much more than 2 or 3 turns (if that).

author: dumle29
author_rank: Regular Contributor
timestamp: March 19, 2014, 09:18:13 am
content: Quote from: David_AVD on March 19, 2014, 02:27:16 amWell it has a sense resistor consisting of 4 0R1 resistors, so that's 0R025 resistor. This should at 10A give you 0.25V over the sense. I then have the first LM324 op-amp in the quad package, set to a non inverting amplifier, with a gain of 10 (set by R6 and R7) The output of that opamp goes into another opamp, which is used open loop as a comparator (open loop/ no feedback = very very high gain) the inverting input on the second opamp has a pot as a voltage divider on it, which is used to set the current trip point.When the current trip point is reached (so when the voltage over the sense-resistor, and the then amplified voltage of that is the same or more than the voltage on the non inverting input from the pot) The output of the second opamp goes high, and that output is fed to an and gate. The second input on that andgate is then connected to a µC, and it has to be pulled high to run at all.you then connect this on the low side of the load, and provide logic power through the 3pin header, and an enable signal, that you could PWM if you so chooseAgain: I have not tested this, but I've done something sorta similar before, and I belive this should work.

author: David_AVD
author_rank: Super Contributor
timestamp: March 19, 2014, 09:52:26 am
content: I can see how it would oscillate once you get over the set threshold, but without some filtering how will it regulate?

author: Psi
author_rank: Super Contributor
timestamp: March 19, 2014, 10:05:33 am
content: A semi-modern but unwanted computer motherboard (Core2duo) would provide some good parts to build a 0.8-1.3V psu up to 80A, since that's what most CPUs use.You could re-purpose the fets+diodes+inductors and they would all be exactly the right value for this sort of psu.Then all you would need is a control IC. The one on the motherboard would likely be a bit hard to drive, if you could even find the datasheet.You could even probe the motherboad running beforehand to check what the switching frequency is.

author: muvideo
author_rank: Frequent Contributor
timestamp: March 19, 2014, 10:10:57 am
content: Quote from: Psi on March 19, 2014, 10:05:33 amA multiphase syncronous buck like that is not that easy to build and run,the pcb is an active part together with the L's C's and mosfets, not countingall the shoot-trough paths

author: dumle29
author_rank: Regular Contributor
timestamp: March 19, 2014, 10:28:20 am
content: Quote from: David_AVD on March 19, 2014, 09:52:26 amYeah it will oscillate, but hopefully around the set current? I might just try it out on a breadboard, and see what it does.

author: Psi
author_rank: Super Contributor
timestamp: March 19, 2014, 11:03:25 am
content: Quote from: muvideo on March 19, 2014, 10:10:57 amYeah, id forgotten about the boards using multiphase buck.Pretty sure some old boards have a traditional buck converter which would be suitable.

author: muvideo
author_rank: Frequent Contributor
timestamp: March 19, 2014, 12:34:00 pm
content: Quote from: Psi on March 19, 2014, 11:03:25 amI don't know if there are non multiphase designs around for Vcore,but at 0.8V the problem is that you need syncronous rectification,otherwise Vf of the recirculation diode will kill the efficiency, giventhe low duty and low Vout. With that kind of inductors, probably fewuH, switching frequency will go up. Gate drive of top and bottommosfets will be not easy, too high delay and you heat up bottommosfets, too low and you short the rails, a fun design, but not for starters.

author: TerminalJack505
author_rank: Super Contributor
timestamp: March 20, 2014, 02:27:36 am
content: Quote from: dumle29 on March 19, 2014, 10:28:20 amI played around with your circuit in the simulator. It looks like you have the two inputs of second op amp (the comparator) backwards. Once that was fixed it oscillated between about 5A and 15A--so it averaged out to about 10A.I got the oscillations to go away by putting a 1nF cap across R6.

author: dumle29
author_rank: Regular Contributor
timestamp: March 20, 2014, 08:53:31 am
content: Sounds good. What did you sim it in, because LTSpice is giving me trouble, and I'ts rather anoying.Also, yes I see, the second opamp is indeed backwards

author: dumle29
author_rank: Regular Contributor
timestamp: March 20, 2014, 09:19:32 am
content: Okay, here's the updated board, and schematic:Schematic:Board:Here

author: dumle29
author_rank: Regular Contributor
timestamp: March 20, 2014, 10:09:30 am
content: I see that OP haven't actually been online since his last post, so let's see if he comes around to checkAnyways, @rantonides: If I find this board works. It should easily fit inside a 1x DIN case, as it's only 24.77 by 38.43 mm (Or 0.97 by 1.51 inches if you want)

author: TerminalJack505
author_rank: Super Contributor
timestamp: March 20, 2014, 11:33:37 am
content: I usedTina-TIfor the simulation. I left the AND gate out and used a different MOSFET.

author: rantonides
author_rank: Contributor
timestamp: March 29, 2014, 07:39:48 am
content: Awesome stuff. Thanks guys. This is a bit over my head as I tend to deal on the system level, but I'll get the boards made and see what happens.Cheers

author: dumle29
author_rank: Regular Contributor
timestamp: April 21, 2014, 02:00:28 pm
content: Quote from: rantonides on March 29, 2014, 07:39:48 amI just got the boards today, so by wednesday I should have the parts, and see if it works

author: dumle29
author_rank: Regular Contributor
timestamp: May 04, 2014, 12:16:36 pm
content: Soo good news is that it works. Bad news is not at a gate voltage of 3,7vyeah I did a derp. So to drive this, 5v logic should be used if possible

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 04, 2014, 01:15:13 pm
content: Low voltage, high current is welding territory. There are off-the-shelf spot welding products from Harbor Freight, et.al.There are also folk who tear down microwave oven transformers (removing the secondary winding) and putting one or two turns of heavy wire to achive this kind of high-current, low voltage power. There are also DIY torroidial power transformers that come with the mains primary winding in place, and plenty of room to wind your own secondary over the top. A super low-voltage, high current winding is literally trivial, a very few turns of heavy wire.Maybe its just me, but I would much prefer using a "passive" transformer solution than some fancy switching regulator, etc.

author: dumle29
author_rank: Regular Contributor
timestamp: May 07, 2014, 01:56:29 pm
content: Quote from: Richard Crowley on May 04, 2014, 01:15:13 pmIf it's cheaper I would too, but the iron required is quite expensive, compared to a cheap 12v PSU, and then a current chopper. That said, your suggestion would have worked, with close to literally no parts that could really fail.@rantonidesSo I did some thinking on the test I ran with the boards, and the FET died. I drove the gate at 4v (almost fully charged lipo) and that might have been a tad too low, however I also suspect the frequency the analog circuit switches the fet at are waay too high. For now I'd say do one or more of the following:Use a supply voltage for the logic of 12vPut a heatsink on the FETPut an RC circuit in there to lower the frequency