How to switch 2kW/12V with µC - Page 1
Forum section: electronics / general

author: DIYMicha
author_rank: Newbie
timestamp: December 26, 2016, 02:35:04 pm
content: Hello there, for my project EV conversion of a old 1989 VW Golf 2 (Rabbit for US viewers) I need to switch the heater with the control unit's µC (Arduino). The electric heater has 2kW @ 12V. I thought of a solid state relais. Possible?Also the Ardu has to PWM the revolutions of the built in air fan of the golf. (140W @ 12V) Here I thought of a MOSFET switching ground. Schematics for this are all around the net. Should'nt be a match.The reason is, that I don't want the heater to heat all the time. Only if the temperature in the car drops below a chosen temp. Also the fan does'nt have to blow all the time on full throttle. Saving energy

author: DIYMicha
author_rank: Newbie
timestamp: December 26, 2016, 02:48:24 pm
content: Shoot, I found out SSR needs AC current. Zero crossing(!). What's a big fat MOSFET to switch such loads, continuosly 160A???

author: wraper
author_rank: Unknown
timestamp: December 26, 2016, 03:00:22 pm
content: If you only going to switch it on/off once in a while, high power relay may be a good option. If often, you better put several mosfets in parallel. Also you will need a heatsink.FarnellDon't use mosfets with below 40-50V rating, car power have a lot of transients which may kill them if not overrated.Also you should take into account power filtering and possible ground loops. As well put decent mosfet driver IC to ensure fast switching, otherwise a lot of power may dissipate when switching, which may kill mosfets.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 26, 2016, 03:27:42 pm
content: If you need 2kw, you really shouldn't be using 12v

author: DIYMicha
author_rank: Newbie
timestamp: December 26, 2016, 04:07:33 pm
content: I Found a heater that is suitable for my car 2000W @ 100V. Seems to work better than the 12V heater. This way it's "only" 20A current. Now I have to find MOSFETs which match these specs. I think I'll use some in parallel.Peaks won't be a problem, EVs don't have generator or spark plugs so no peaks12V is made via DC/DC converter.

author: SeanB
author_rank: Super Contributor
timestamp: December 26, 2016, 04:39:14 pm
content: Fan motor simply keep the old heater resistor, or go hunt in the scrapyard for an E36 BMW heater controller blade, the switch and wiring loom for it. That uses a MOSFET, and it will mostly fit in there. Also is easy to repair if the motor kills it. While you have the dashboard apart take the existing VW blower motor out, take the drum fans off it and strip it and relubricate the bushings and check the brushes for wear. Use a good quality mineral oil, and heat the bushes up with hot air before dunking in the oil to allow it to soak in, and put a few drops on the felting reservoirs inside the motor.Worn bushes and brushes kill the controller pretty fast, and that resistor pack is cheap, though I have modified my one to use the much cheaper post 2002 VW one, as it is just reversed, and you just put it in the old housing. $3 for the pattern part as opposed to $30 for the genuine one. they tend to rust in humid climates.As to the heater, I really would recommend using the full EV battery voltage, whatever it is, as that will mean you do not have to use 50kg of copper bus bar to power the heater, and the control is also easier as you can use IGBT switches ( a 3 phase module with all 3 phase switches in parallel is both cheap and easy to mount to a heatsink) to switch it, and can use a low frequency to switch it as well. plus you can get a 2kW heater element made, or find one online, that will be able to fit in place of the old core, and the wiring will be thinner and easier to route. 2kW at 12V means the car battery that drives it needs to supply 200A, and that is equivalent to the starter draw, so you will need an inverter from the HV rail to supply that as well or the battery will last about a half minute before buckling all plates.2kW at 100V is a pretty standard finned air heater, you should be able to find one that fits, though please be sure to select one that has a surface temperature of under 100C when in operation, and add a thermal cutout on the inlet side of 115C as safety for failed fan, and a 130Csingle shot thermal fuse as well. You probably could fit a standard PTC element one there, giving thermal control as well, but probably you will not get 2kw in a PTC element.

author: rob77
author_rank: Super Contributor
timestamp: December 26, 2016, 05:44:39 pm
content: what's your battery voltage ? you should really go for a heating element suitable for your battery voltage. 2kW @ 12V is pure nonsense unless you plan to heating with the wiring harness as well

author: DIYMicha
author_rank: Newbie
timestamp: December 27, 2016, 06:27:48 pm
content: Thanks Sean. I will check your hints out.The battery pack has 100V. As I mentioned earlier, I will now use a 2kW / 100V heater, which means the current will be around 16A. 2kW@12V was a stupid idea.The heater doesn't need to be pwm controlled. If the temperature drops under a chosen temp, it will switch on, if it exceeds max temp. it will be switched off.

author: rob77
author_rank: Super Contributor
timestamp: December 27, 2016, 08:47:50 pm
content: Quote from: DIYMicha on December 27, 2016, 06:27:48 pmin that case i think a plain relay or contactor is the best way to go - and you can easily drive the contactor's coil with a regular mosfet. you just need to introduce some hysteresis to avoid rapid on/off/on/off switching at the threshold temperature (but you would need that with solid state switching as well).

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 28, 2016, 05:30:17 pm
content: It's so easy to PWM a resistive load that there's very little reason not to. A few hundred Hz would work nicely.I would suggest getting some heated seats as that would be a lot more efficient.

author: DIYMicha
author_rank: Newbie
timestamp: December 28, 2016, 10:17:16 pm
content: I found out that the PTC Elements 2kW are merged together from six separate heating elements. Everyone with its own connector. That cuts current down to 3.3 amps per heating element. For this I will surely find the correct MOSFETS. But thank you guys for giving me some good hints (overheat protection etc.)

author: johansen
author_rank: Super Contributor
timestamp: December 29, 2016, 06:30:06 am
content: why don't you install heated seats and forget about heating the cabin air temperature (with the exception for defrosting the windshield)

author: DIYMicha
author_rank: Newbie
timestamp: December 30, 2016, 09:11:53 am
content: Hi there, you gave the answer already. In my area it's pretty cold in wintertime. Because of that I want to install an arduino driven heater timer, when the car is on the charger it should already preheat the inside, a few minutes before I have to go to work. In this way I don't have to heat the car from -10°C to +15°C from the driving batteries. The heater then only has to keep the temperature plus minus a little hysteresis. This will also be arduino driven.So the plan is, that the ardu measures inside temp, preselected temp, time, and if the car charges to fire up preheat. In drive mode (w/o charger) the ardu keeps the preselected heat. Therefore it has to switch on and off the heater when the inside temp is off the limits, and also PWM the speed of the blower. For the blower I want to install a button to switch between automatic and manual mode.

author: DBecker
author_rank: Frequent Contributor
timestamp: January 06, 2017, 07:27:31 am
content: Quote from: rob77 on December 27, 2016, 08:47:50 pm100 VDC is difficult to switch with a relay. You'll need an expensive one with a magnetic blow-out. EVs typically use a Kilovac EV200, which are about $80.If you already have traction-side circuits of your own, use a 170V-200V MOSFET connected to the motor side of the existing safety contactor. Use an oversized part to minimize the heatsink needed -- you can likely just use the circuit board. A DC-DC isolator can supply the gate voltage. I've used Silicon Labs isolating gate drivers, but there are now multiple other options. I've never used an ADuM5230, but it might be a good single-part solution since your switching rate (and thus gate drive power) will be so low.

author: tszaboo
author_rank: Super Contributor
timestamp: January 06, 2017, 05:51:23 pm
content: Quote from: johansen on December 29, 2016, 06:30:06 amI have heated seats. I call them the balls cooker. The steering wheel is 0 degrees, and the seat is soo warm, that you cannot sit in it anymore.OP, if you want a high current transistor, check the SOT223 package. The good thing, no PCB needed for the high current, just crimp a cable with ring terminal.

author: james_s
author_rank: Super Contributor
timestamp: January 23, 2017, 05:49:26 am
content: If the 100V heater only pulls 22A then that's easy to switch with a mosfet. You can get some now that are such low resistance that you'd barely even need a heatsink for a load that size.