relay's versus power mosfets for "contact" resistance

[Thunderchild]

I'm controlling 20-30 amps (at 12 V), I need to simply switch this current on or off. Now i could do it with relays or i could do it with mosfets. now I assumed that relays will dissipate less heat as they are a swith where as a mosfet will have a minimum resistance.

However after a search for the sort of thing I'd be using I have become dubious:
**broken link removed**
This relay has a reistance of less that 50mohm the specs say, they do not say how much lower so I must assume 50 mohm in a worst case scenario, now that's starting to look a bit ugly in the face of the fact that you can get mosfets with very low full on resistances like 5 mohms ? so maybe I really should be looking to mosfets instead of relays ? just confirm I'm on the right wavelength with this

 

[Nigel Goodwin]

Relays are simple, relays are cheap, relays are rugged, relays are reliable - I'd far sooner use a relay than a MOSFET for switching in a car.

 

[MikeMl]

Don't forget how much power it takes to pull-in the relay. Most 12V automotive types have a coil resistance like 85Ω, so the coil alone produces 1.7W of heat.

 

[Thunderchild]

Don't forget how much power it takes to pull-in the relay. Most 12V automotive types have a coil resistance like 85Ω, so the coil alone produces 1.7W of heat.

I was thinking of that too

 

[Hero999]

But it's run from a huge battery and alternator which can produce 100s of amps so who cares?

The only time it's an issue is when it's going to be powered with the ignition switched off.

 

[Ubergeek63]

But it's run from a huge battery and alternator which can produce 100s of amps so who cares?

The only time it's an issue is when it's going to be powered with the ignition switched off.

in an electric it can be a very draining experience...

FETs can easily stand up to it at higher currents with longer life than relays that tend to arc the contacts at high currents...

relays meant for high currents are often called contactors since they have huge contacts on the armatures to handle the current...

we use little relays that we pay $1.50-$2.00 each for... they are rated for 30A at 250VAC but only 28VDC since the AC zero crossing extinguishes the arc and the DC does not...or at least would not at higher voltages.

in a FET you can get 200A in a TO220 package for a couple dollars... ever priced a 200A relay?

dan

 

[carmusic]

i often use "intelligent" mosfet switch instead of relay since they have a built-in over current shutdown and over temp protection, these are very common on new cars. and they take about 1/5 of the size of the relay. ive also made huge switches 100A 100V AC or DC with mosfets (about 12 mosfet 150V 6 parallel back to back)!

 

[Thunderchild]

But it's run from a huge battery and alternator which can produce 100s of amps so who cares?

The only time it's an issue is when it's going to be powered with the ignition switched off.

my concern is heat, I'm aiming to keep it dissipating as little heat as possible

 

[Ubergeek63]

https://www.electro-tech-online.com/custompdfs/2010/04/irfs3004-7ppbf.pdf for <1.5W @40V

https://www.electro-tech-online.com/custompdfs/2010/04/irfp4310zpbf.pdf x4 for 100V

 

[Hero999]

What's the point in saying 400A (silicon limited)?

The real limit is 240A and even then you'd need to mount it to a piece of bus bar to get it to carry that current without overheating.

 

[olly_k]

Don't forget how much power it takes to pull-in the relay. Most 12V automotive types have a coil resistance like 85Ω, so the coil alone produces 1.7W of heat.

add a capacitor / transistor couple of resistors you can make a small pull in circuit so there is a surge of current to energise the relay which drops down to a holding current of a couple milliamps. Simple enough and effective.

 

[Ubergeek63]

What's the point in saying 400A (silicon limited)?

The real limit is 240A and even then you'd need to mount it to a piece of bus bar to get it to carry that current without overheating.

it is called specmanship: how much can i claim it will pass safely ... FINE PRINT: on an infinite heat sink (case temperature = 25C) at the junction temperature limit.

in real life you need to look at max ambient, heat sink and package thermals as well as max junction temp, RdsON, and switching losses (max current number based on steady on)

dan

 

[Nigel Goodwin]

my concern is heat, I'm aiming to keep it dissipating as little heat as possible

I presume this is to do with your dynamo regulator project? - I know which is likely to be by far the most reliable!

 

[Thunderchild]

I presume this is to do with your dynamo regulator project? - I know which is likely to be by far the most reliable!

Well done nigel yes, I'm currently using Diodes but have not done a full test so am considring alternatives when I may get into trouble, less heat = more efficient etc

 

[Nigel Goodwin]

Well done nigel yes, I'm currently using Diodes but have not done a full test so am considring alternatives when I may get into trouble, less heat = more efficient etc

Efficiency isn't that big a concern in a car electrical system - reliability is FAR more important.

 

[Thunderchild]

well I'm going for the best in both worlds, i assume that the more current through a diode the higher the voltage drop and the higher the temperature the higher the voltage drop ? I'm using 2 double diodes in parallel so as to spread out the thermal "load" and hopefully have lover drop voltages because I'm not working at full load

 

[Thunderchild]

Is it me or is it very hard to find p channel mosfets with low on resistance ? I am rather appalled with both RS components and Farnell, Farnell seems to stock more that RS but both are terrible at stocking specific products

 

[Hero999]

P-channel MOSFETs always have a higher on resistance than N MOSFETs. It's something to do with the charge carriers, I think electrons are better than holes, I can't remember in great detail, someone else will probably fill in the gaps.

You can use a high side driver but you'll need to bootstrap, which only works for AC, for DC you'll need a higher voltage source for the gate, which is a pain.

 

[k7elp60]

What I have learned about replacing 12VDC relays with power mosfets.

I first approached this idea about 11 years ago. I do not remember what the N channel that had a low rds at that time,(may have been IRFZ40, rds of 28milliohms) but I knew that no matter what the rds was that there would be a voltage drop and power dissipation because of the rds. I did some test on a Potter and Brumfield 12V relay that not only had a dc coil but the contacts were rated at 28VDC and 25A. I also knew that the power dissipated by the relay was two parts. 1. The contact resistance and the coil power. I also knew if the relay would remain energized at 1/2 the nominal coil voltage the power dissipated by the coil would then be 1/4 the power at full voltage.
The results of the test on just the contacts were: With the coil voltage of 12V and a 25A load the voltage drop across the contacts was 86.7mV(or 3.46 milliohms) With a coil voltage of 6V and the same 25A load the voltage drop across the contacts was 88.6mV(3.54 milliohms)

I wanted to test the contact resistance at half the coil voltage as I knew that the pressure on the contact could be related the strength of the magnetic field of the coil.

With these facts in mind I designed a built a relay coil drive circuit that energized the coil at nominal full voltage and then a short time later reduced the coil voltage to 1/2 the nominal voltage. The results at that time were that the total relay power(contact dissipation + coil power) was less than the current mosfet rds power dissipation. I also went ahead a designed a low voltage cutoff if the battery became discharged. At this time I was using batteries charged by solar panels and the relay circuit operated some 12V equipment.

If a IRFZ40 was used at that time the power dissipated at 25A would be 25 X 25 X .028, or 17.5 watts.
With the 12V relay with a coil resistance of 110 ohms, and the coil at 1/2 normal the voltage the power would be: 0.327 Watts + (25A x 25A x 3.54millohms)=2.54W, way less than the power mosfet. Even if the relay was at full voltage the power of 3.8 watts was still less than the mosfet.

I recently did some more research for the same thing. I looked for P channel mosfets as the drive for high side switches is easy. I found IRF4905 with a rds of 20 milliohms. Doing the math again with a 25A load we have 25A x 25A x .02 ohms or 12.5 watts.

A N channel mosfet IRF1404, rds of 4 millohms seems like the one to use as the power dissipated would be 25A x 25A x .004 ohms or 2.5 watts. For a high side DC switch then I would need about 12V +10V(gate drive) or 22 volts. My solution is to use a LT1054cp voltage pump to double the input voltage to drive the gate.

I don't know what the nominal contact resistance of the small auto relays is, but with a coil resistance of 85 ohms the power for the coil is 1.7 watts, as pointed out by MikeMl

At this point to me using the relay is still more convenient, and as Nigel said that relays are reliable.
I am still using solar panels to charge batteries and I am finding more ways to use the stored power at night. I plan to use more for some patio lighting with a timer and a control relay.

 

[Thunderchild]

so basically you say that relay contact resistance is far lower than wht they often tell you ? I was hoping to use a relay as it is very robust but I also want a solution that makes as little heat as possible, I expect experimentation is the only way of find out a few facts, although won't the contact resistance increase with age ?