Switching polarity without relays - Page 1
Forum section: electronics / projects

author: raptor1956
author_rank: Frequent Contributor
timestamp: July 12, 2017, 06:02:32 am
content: I'm looking to use some Peltier modules to cool and perhaps to also heat and to do both you need to switch the polarity to the module, but I'm wondering if there is an elegant solution that avoids using relays and is cheaper than relays. The voltage is less than 15VDC but the current could be as much as 5A though 3.5A or less is more typical. So, what would you suggest?Brian

author: minion
author_rank: Contributor
timestamp: July 12, 2017, 06:04:59 am
content: What about mosfet H bridge?

author: digsys
author_rank: Unknown
timestamp: July 12, 2017, 06:16:30 am
content: Quote from: minionA very nice and elegant solution, but with that many more parts added, I doubt it'll be anywhere near cheaper. OP will need to provide more detail, especially price / room constraints.

author: Muxr
author_rank: Super Contributor
timestamp: July 12, 2017, 06:26:31 am
content: Coincidentally I just designed such a circuit last week, in order to control a mini fridge and turn it into a mini environmental chamber:For the Mosfets I used FQP47P06 (Q2 and Q4), and FDP5800 (Q3 and Q5).Mode of operation:I provide logic level inputs on the header J2, from an arduino/raspberry_pi:Either P1 and N1 are on, or P2 and N2 are on. These are the only two valid combinations. The fridge will either cool or heat based on the direction. PELTRED and PELTBLK are outputs to the peltier block.Also note P1 and P2 (PNP) inputs are inverted, they are active low.Also I should add, my cheap mini fridge has I think (not sure) a polyfuse and the original board had the circuit's ground separated by it. So I just added the F2 there to make the schematic work, but I will probably wire the original fridge's polyfuse wires to it.The F1 fuse I have there is a real fuse. Part number: RXEF375.I haven't really tested the circuit, but I can share the PCB design on OShpark once I test it.Cost is about $10-15 in components and $18.70 for OShpark to make the 3 boards.I also hacked a quick Arduino sketch which lets me control the fridge via USB-Serial from my computer or raspberry Pi:Code:/**/int n1 = 8; // D8int n2 = 9; // D9int p1 = 10; // D10int p2 = 11; // D11byte byteRead;byte status_byte;void pins_mode() {pinMode(n1, OUTPUT);pinMode(n2, OUTPUT);pinMode(p1, OUTPUT);pinMode(p2, OUTPUT);}void mode_off() {digitalWrite(LED_BUILTIN, LOW);digitalWrite(n1, LOW);digitalWrite(n2, LOW);digitalWrite(p1, HIGH);digitalWrite(p2, HIGH);status_byte = 'o';}void mode_cool() {mode_off();digitalWrite(LED_BUILTIN, HIGH);digitalWrite(p1, HIGH);digitalWrite(p2, LOW);digitalWrite(n1, HIGH);digitalWrite(n2, LOW);status_byte = 'c';}void mode_warm() {mode_off();digitalWrite(LED_BUILTIN, HIGH);digitalWrite(p1, LOW);digitalWrite(p2, HIGH);digitalWrite(n1, LOW);digitalWrite(n2, HIGH);status_byte = 'w';}// the setup function runs once when you press reset or power the boardvoid setup() {Serial.begin(9600);// initialize digital pin LED_BUILTIN as an output.pinMode(LED_BUILTIN, OUTPUT);mode_off();}void control(byte br){switch(br){case 'o':mode_off();break;case 'c':mode_cool();break;case 'w':mode_warm();break;case 's':Serial.write("status: ");Serial.write(status_byte);Serial.write("\r\n");break;}}void loop() {if (Serial.available()) {/* read the most recent byte */byteRead = Serial.read();control(byteRead);}}Very simple protocol:o - offc - coolw - warms - return statusThe temperature sensor will be inside the fridge but read separately by a raspberry pi, which is part of my existing metrology rig.

author: daqq
author_rank: Super Contributor
timestamp: July 12, 2017, 06:28:25 am
content: Cheaper - probably not, since you can get relays that can do this that cost 2 EUR.More elegant solution? HBridges, specialized controllers (http://www.linear.com/product/LTC1923). Maybe you could get in terms of price close to the relays with an HBridge with some MOSFETs.

author: raptor1956
author_rank: Frequent Contributor
timestamp: July 12, 2017, 05:08:03 pm
content: The cost thing is driven mostly by the fact that I'd need 4X of everything. I may have to change plans and instead of allowing each of the four circuits to be hot of cold independent of the others I could force all four to be one or the other reducing the component count.Brian

author: Muxr
author_rank: Super Contributor
timestamp: July 12, 2017, 09:00:55 pm
content: Quote from: raptor1956 on July 12, 2017, 05:08:03 pmEven at 4X an H-bridge shouldn't break the bank. I picked some more modern low RDS on Mosfets for my design ($2.30 a piece), but you can find cheaper ones.. for instance IRFZ34N N-Channel (bottom Fets on the schematic above) are only $0.85 a piece. I am sure you can find a cheap P-channel mosfets for a buck as well. Case in point:http://www.ebay.com/itm/10-x-IRF9540-IRF9540N-IR-Power-MOSFET-P-Channel-23A-100V-USA-Seller-/221324036166?hash=item3387f0c846:g:UKAAAOxygPtSvdGgand n-channel are even cheaper:http://www.ebay.com/itm/10-x-IRF530-IRF530NPBF-IR-Power-MOSFET-N-Channel-17A-100V/401031656858?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D555018%26algo%3DPL.SIM%26ao%3D2%26asc%3D44758%26meid%3Dba8ae1e394d9409e8cbc8fa188fecfa3%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D221324036166So the components for 4 boards would cost you about $15 for (8 p-channel, and 8 n-channel mosfets). And the rest of the components can't be more than $5 as they are all jelly bean.The expensive part would be the PCBs, but you could just roll your own on a protoboard or just wires. But even with PCBs you're talking $40-$45 total for all four units. You could control them from a $4 arduino clone.

author: darrellg
author_rank: Unknown
timestamp: July 12, 2017, 09:12:51 pm
content: You could use a double pole, double throw switch.

author: Nusa
author_rank: Super Contributor
timestamp: July 12, 2017, 09:20:09 pm
content: My old peltier-based refrigerator icechest has a labeled unpolarized plug on the DC input cable. You simply reverse it to change from cooling mode to warming mode. The DC fan that blows across the interior heatsink reverses direction too, but airflow is airflow, so still works.

author: jeroen79
author_rank: Frequent Contributor
timestamp: July 12, 2017, 10:41:46 pm
content: How much/fast control over the cooling/heating is required?A DPDT switch is very cheap but would be slow to operate and useful only if you only need to select max cooling, max heating and off.MOSFETs would allow very fast tristate switching for really fast control of the temperature.Relays would be somewhere in between due to being mechanical.If cost is most important, what about some cheap Chinese H bridge module?

author: NorthGuy
author_rank: Super Contributor
timestamp: July 12, 2017, 10:59:58 pm
content: Quote from: Muxr on July 12, 2017, 06:26:31 amIt's funny how Q1 and Q6 are "soldered".

author: Muxr
author_rank: Super Contributor
timestamp: July 12, 2017, 11:13:35 pm
content: Quote from: NorthGuy on July 12, 2017, 10:59:58 pmAye.. the 3D model for it is rotated (bug in that footprint which is from KiCad's standard lib), but I checked the footprints and they look right

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 12, 2017, 11:42:06 pm
content: You can buy h-bridge ic if you want more compact solution.. maybe thishttp://www.ebay.com.my/itm/2p-TLE7209-2R-SOP-20-IC-7A-H-Bridge-DC-Motor-Applications-LI-/142441362243?hash=item212a2ab343%3Ag%3AMAYAAOxyrxZRyqln&_trkparms=pageci%253A0e51668f-675b-11e7-b779-74dbd180dcb5%257Cparentrq%253A392a0e2f15d0ab4da47b2bbcffe7e069%257Ciid%253A3

author: ejeffrey
author_rank: Super Contributor
timestamp: July 13, 2017, 03:22:25 am
content: An H-bridge is certainly the conventional solution. It doesn't have to be that expensive. For instance, here is a $4 h-bridge controller with integrated switches and a 5 amp current limit:https://www.digikey.com/product-detail/en/infineon-technologies/TLE5205-2G/TLE5205-2GINCT-ND/1557940?WT.z_cid=ref_findchips_standardThe Rds On is fairly high at 200 mOhm, so it will dissipate quite a bit of heat. Discrete MOSFETs would do better, but not as simple, and possibly more expensive.That said: you said you want to use this to drive a peltier. It is not really recommended to drive a peltier with either unfiltered PWM or thermostat style on-off regulation.The problem with PWM (especially in cooling mode) is that the cooling power is proportional to I, but the parasitic heating is proportional to I^2. So with PWM, you get the worst case resistive heating which you then have to cool.The problem with slow on-off control is that the frequent thermal cycling causes stress from thermal expansion.You really want to drive a PWM with DC if you want good performance and longevity. Fortunately, this is relatively easy to do with a H bridge PWM driver and a simple LC filter. This basically turns it into a buck converter.

author: Muxr
author_rank: Super Contributor
timestamp: July 13, 2017, 04:25:41 am
content: @ejeffrey you bring up a good point.In my particular case, I will be turning the peltier on and off every 10 minutes or more. The board I am replacing was doing the same thing except using manual switches. They have some sort of a varistor limiting current and I will keep that in place.I haven't thought about maintaining a certain temperature/current with PWM, but the consensus is, as long as the resulting pulse is longer than the 0.001 seconds, the peltier should be fine even without filtering.http://www.ferrotec-nord.com/support/thermoelectric-module-feedingThe efficiency will suffer of course, but I can live with that.The LC filter may not be as trivial due to the nature of reverse polarity at play.

author: cellularmitosis
author_rank: Unknown
timestamp: July 13, 2017, 06:20:16 am
content: Quote from: Muxr on July 12, 2017, 06:26:31 amAwesome, thanks for posting about this! I have a mini-fridge with the same idea in mind but haven't made much progress yet.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 13, 2017, 06:49:23 am
content: last time i check, you need several layer of peltier element to make it useful as a fridge, and each layer will consume few to 10 of A, overall we are talking about 500W and above of power consumption. i dismissed my intention to source peltier elements due to this... iirc..

author: BravoV
author_rank: Super Contributor
timestamp: July 13, 2017, 08:25:59 am
content: Quote from: darrellg on July 12, 2017, 09:12:51 pm+1 , why complicate things when it can be so simple ? Cmiiw.Randomly googled, just replace the motor with the peltier element.Good quality switch like this (randomly googled too) can cover your current requirement.

author: Muxr
author_rank: Super Contributor
timestamp: July 13, 2017, 01:43:35 pm
content: Quote from: Mechatrommer on July 13, 2017, 06:49:23 amI am modifying an existing Peltier mini fridge.. this model in particular:https://www.amazon.com/Gourmia-GMF600-Portable-Fridge-Cooler/dp/B00DQT2ZGE/ref=sr_1_1?ie=UTF8&qid=1499953202&sr=8-1&keywords=gourmia+mini+fridgeIt draws about 3-4A of current (12V) and has a very simple control board like the schematic linked above me. I am just replacing it with an h-bridge in order to control it programmatically for TempCo characterisations.It's a very small and simple unit.Quote from: cellularmitosis on July 13, 2017, 06:20:16 amNo problem. I will probably make a full post on the metrology section once it's done and share the project on github.

author: Zero999
author_rank: Super Contributor
timestamp: July 14, 2017, 07:42:30 am
content: I've never liked Peltier fridges. They're unreliable, the element has a tendency to fail short circuit, releasing nasty smoke and are less efficient than mechanical fridges.Quote from: BravoV on July 13, 2017, 08:25:59 amThat's good, unless you want to control it via an electrical signal.

author: darrellg
author_rank: Unknown
timestamp: July 14, 2017, 03:06:13 pm
content: Quote from: Hero999 on July 14, 2017, 07:42:30 amThat was not mentioned as a requirement.

author: mikerj
author_rank: Super Contributor
timestamp: July 14, 2017, 04:46:04 pm
content: Quote from: Hero999 on July 14, 2017, 07:42:30 amPerhaps a solenoid could be hooked up to the switch for electrical operation

author: cellularmitosis
author_rank: Unknown
timestamp: July 14, 2017, 05:11:55 pm
content: I see where you guys are going with this, and I'm way ahead of you...

author: BravoV
author_rank: Super Contributor
timestamp: July 14, 2017, 05:22:50 pm
content: Quote from: darrellg on July 14, 2017, 03:06:13 pm+1 , here quoting the OP and highlighted below, current requirement is covered if using quality switch, and still cheaper than relay.Quote from: raptor1956 on July 12, 2017, 06:02:32 am

author: jeroen79
author_rank: Frequent Contributor
timestamp: July 14, 2017, 10:22:52 pm
content: Quote from: mikerj on July 14, 2017, 04:46:04 pmInteresting.And if you put the solenoid and switch into one housing it can be used as a single module for nany purposes..

author: hermit
author_rank: Frequent Contributor
timestamp: July 14, 2017, 11:03:33 pm
content: Quote from: Muxr on July 12, 2017, 11:13:35 pmE for edit footprint in pcbnew and go to 3d properties. You can rotate it from there if you want.

author: Muxr
author_rank: Super Contributor
timestamp: July 15, 2017, 02:17:51 am
content: Quote from: hermit on July 14, 2017, 11:03:33 pmThanks for that tip. Didn't really bother me but now that I know it can be easily fixed might as well fix it. Cheers!

author: Andreas
author_rank: Super Contributor
timestamp: July 15, 2017, 05:59:23 am
content: Hello,there are some design considerations with peltiers:On heating cooling boxes there is usually the hint that when switchingfrom heating to cooling you will need a cool down (off) phase of 5 minutes.The reason is that a peltier also acts as generator (battery) when there is a temperature difference.So when immediately reversing polarity there is a large current flow (both voltage sources in series).So I use dedicated heater foils (similar to car mirror heaters) together with the peltier to control the temperature.https://www.reichelt.de/Heating-Foils/THF-77110/3/index.html?ACTION=3&GROUPID=7777&ARTICLE=108462&OFFSET=16&(unfortunately pricing has gone up by a factor 3-4 recently).With best regardsAndreas

author: Zero999
author_rank: Super Contributor
timestamp: July 15, 2017, 04:07:43 pm
content: Quote from: Andreas on July 15, 2017, 05:59:23 amThat's interesting. I wonder if short circuiting the Peltier module, rather than leaving it open circuit, will improve the thermal transfer?

author: raptor1956
author_rank: Frequent Contributor
timestamp: July 16, 2017, 01:08:39 am
content: Quote from: ejeffrey on July 13, 2017, 03:22:25 amThanks, appreciate your input. I think I'm going to proto with H-bridge but select units with lower RdsON. I also planned to filter the PWM output to reduce the peak-to-peak down to the average using caps or LC if needed.In a past life (IBM) we used peltier coolers to cool chemicals used in the semiconductor industry to dope the silicon and getter electrical contaminants. Back in the day we used BBr3 and POCl as well as oxide cleaner TCA and these needed to be maintained at between 5C and 20C. The PS for these chillers were, as I remember, 6A.Brian

author: raptor1956
author_rank: Frequent Contributor
timestamp: July 16, 2017, 01:12:07 am
content: Quote from: Andreas on July 15, 2017, 05:59:23 amThe deep space probes that use Pu as the energy source (RTG) generate electricity using thermocouples using the Seebeck effect -- quite similar to the way peltier modules work.Brian

author: Andreas
author_rank: Super Contributor
timestamp: July 16, 2017, 03:39:22 am
content: Quote from: raptor1956 on July 16, 2017, 01:08:39 amCaps alone will be not sufficient to filter the current ripple to below 10% of the average current (requirement for peltiers).So you will have to use a relative high PWM frequency (and fast gate drivers for the FETs) if you want to avoid very bulky inductors. (6A).And for the fans you will need the (nearly) full supply voltage.with best regardsAndreas

author: raptor1956
author_rank: Frequent Contributor
timestamp: July 17, 2017, 06:13:10 pm
content: Quote from: Andreas on July 16, 2017, 03:39:22 amThe other thing is that since the polarity switches the caps would need to be tolerant of either polarity. That won't be an issue for inductive components, but electrolytic caps could be a problem.Brian

author: Zero999
author_rank: Super Contributor
timestamp: July 17, 2017, 09:56:38 pm
content: Quote from: raptor1956 on July 17, 2017, 06:13:10 pmIf you want to filter PWM, you need inductors. A capacitor alone will increase the power dissipation in the switch, as there will be nothing to limit the charging current.

author: Muxr
author_rank: Super Contributor
timestamp: July 18, 2017, 01:48:50 am
content: I put my little h-bridge together.. will be testing it this weekend.That polyfuse is huge so I mounted it in the back.. (note to self, always check the dimensions).

author: Zero999
author_rank: Super Contributor
timestamp: July 18, 2017, 07:45:44 am
content: Quote from: Muxr on July 18, 2017, 01:48:50 amI doubt it will do anything to protect the transistors though.

author: jbb
author_rank: Super Contributor
timestamp: July 18, 2017, 09:30:20 am
content: If you're doing PWM already, you might as well go the whole hog and do H-bridge PWM. You can get PWM control and polarity switching, plus smooth control all the way from +ve output to -ve output (and vice versa).One thing to be aware of: when you are transitioning the system from warming to cooling (and vice versa), the Peltier can actually operate as a thermopile generator, and feed energy (via the power converter) back into the DC link.

author: raptor1956
author_rank: Frequent Contributor
timestamp: July 25, 2017, 02:40:08 am
content: OK, so I'm going to prototype the H-bridge and have ordered some parts from Digikey and Amazon. I first ordered some SMD MOSFET's but the first batch I ordered were too small, I mean like 3.3mm on a side. Amazingly though, the the constant current rating is -11A to -15A with pulse to -46A. The RdsON for the P-channel NTTFS3A08PZTWG is 6.7mOhm at -4.5Vgs. I ordered a similar N-channel Si7108DN which has RdsOn values of 6.1mOhm at 4.5Vgs and 4.9mOhm at 10Vgs. Again, the power and current handling capabilities of MOSFET's barely more than an 1/8" on a side is amazing.But, I realized my mistake in ordering parts that small and then ordered some SOP8 sized parts that should be more workable to prototype. The P-channel version is a Vishay Si7141DP that has RdsON values of 3.0mOhm at -4.5Vgs and 1.9mOhm at -10Vgs with constant current well above the 5A I need. The N-channel one, also SOP8 sized, actually has a bit higher RdsON values of 3.2mOhm at 4.5Vgs and 2.6mOhm at 10Vgs. These two are 1.27mm pitch versus the 0.65mm pitch of the first two and the SOP8 proto boards (DFRobot FIT0290) will work with the latter two parts.I am just amazed at where we've come with power devices and with such low RdsON values and current capacities of more than 20A continuous in devices that are just about a quarter the size of a finger nail -- wow...Brian