Hi,
Im looking at making 24v " solid state relay" which will be controlled from a pic.
The current will be around 30-50A depending on the temp of the glowplugs.
I'm looking for a really robust mosfet for this application?
The plugs will only be on for nomore then 60 secs.
I was planning on driving the gate with a tlp627 as i have many of them.
I have also lots of izf44 fets about 200 I accedently bought. could these be parrelleled through one opticoupler?
Would pulsing be a better option for the fet? something I've not looked into?would this require a quicker way of charging/discarging the gate?
Any input would be greatly appreciated
Best regards

 

Since you'd only need 3 or 4 of them to have a decent safety factor, paralleling them is reasonable. (I assume you meant IRZF44?)

The transistor should be ok. You'll want an opto that can drive at least 5mA in order to take full advantage of the 5A and 1000x gain of the transistor, but that shouldn't be a problem.

I don't see any reason to pulse the plug unless you need to regulate down the (average) current.

Robustness will depend in part on how well your circuit is protected from the elements and from under-hood conditions. You'll need to consider the cables and connections to carry 50A. Not trivial.

 

Since you'd only need 3 or 4 of them to have a decent safety factor, paralleling them is reasonable. (I assume you meant IRZF44?)

The transistor should be ok. You'll want an opto that can drive at least 5mA in order to take full advantage of the 5A and 1000x gain of the transistor, but that shouldn't be a problem.

I don't see any reason to pulse the plug unless you need to regulate down the (average) current.

Robustness will depend in part on how well your circuit is protected from the elements and from under-hood conditions. You'll need to consider the cables and connections to carry 50A. Not trivial.

Yea sorry irzf44.
The vehicle is being rewired with a second hand loom which looks like cat sick, I was welding the trans tunnel up and set fire to the new loom made a mess of some of the relays so going to improve it, by removing it.
I was looking at IRFP4368 in a bigger package?
Have you got any ideas for conectors?

 

The 4368 is a better choice than the 44, since it has a much lower RDS(on). However, both types are NFETs. Since a glowplug normally has one terminal grounded, driving it wth an NFET is problematic (the gate voltage would need to be ~22V if the plug is in a 12V system).

 

The 4368 is a better choice than the 44, since it has a much lower RDS(on). However, both types are NFETs. Since a glowplug normally has one terminal grounded, driving it wth an NFET is problematic (the gate voltage would need to be ~22V if the plug is in a 12V system).

Hi, thanks for your reply.

Could you explain the 22v gate voltage for a 12v system, there is a few 12v components I was thinking about controllinger ie such as wipers etc.
I was thinking fets as there pritty simple to control. But I'm open to any idea and I'm eager to learn about new things so I would be keen to hear any recommendations you have?
Best regards

 

Could you explain the 22v gate voltage for a 12v system

To switch the average N-FET on requires a gate-source voltage of about 10V. If the FET is switched fully on the source voltage will be almost the same as the drain voltage, i.e. 12V. Hence the gate voltage must be 10V+12V=22V.

 

To switch the average N-FET on requires a gate-source voltage of about 10V. If the FET is switched fully on the source voltage will be almost the same as the drain voltage, i.e. 12V. Hence the gate voltage must be 10V+12V=22V.

Thanks, that's great. would you use a NFET for this application?

Cheers

 

Is the vehicle 12V or 24V?
Which voltage would supply the glowplug?
Is one terminal of the glowplug grounded?

 

Is the vehicle 12V or 24V?
Which voltage would supply the glowplug?
Is one terminal of the glowplug grounded?

Arhhh the load is on the wrong side!
The vehical is 24v with some 12v stuff radio and instruments etc.
The glow plugs are earthed through the block and are 24v.
Cheers

 

I'm not to sure I would trust a 60 volt rated mosfet in a 24 volt system application to do the primary power switching of a glow plug system.

There's a reason that even the modern solid state glow plug control systems still use high current solenoids and relays instead of actual semiconductors to switch the glow plugs power on and off.

It's in the same concept range of solid state starter control. There's a reason it's still not used.

 

Ok, I thought it was a cost reason, semiconductors are used throughout industry for filters, vsd, inverters, relays etc for switching high current requirements. Why not some glow plugs?
What kind of rating would you choose? if you wanted to accomplish this task how would you tackle it? this is a learning experience for me mostly.
Best regards

 

Relays are used in part because of inertia: Widely used, cheap-ish, long history of successful use, and so on. But MOSFETs can certainly switch a 50A load. I believe people also start looking to IGBTs at that amperage.

I've only seen one solid state switch of any size on a vehicle. My Chrysler van uses a solid state relay to control the radiator fans. I have no idea why (or why not just another relay). It's bolted to the bumper or the frame for cooling.

 

Ok, I thought it was a cost reason, semiconductors are used throughout industry for filters, vsd, inverters, relays etc for switching high current requirements. Why not some glow plugs?
What kind of rating would you choose? if you wanted to accomplish this task how would you tackle it? this is a learning experience for me mostly.
Best regards

The issue with using them in those types of applications the inherent 'fail unsafe' mode of breakdown. That is the chance of them failing to full on without any practical way to shut the circuit off is their inherent problem and to make them have such an event takes little more than one voltage spike that goes beyond their breakdown voltage limit.

So given that if you glow plugs went to full on without being able to be shut down what sort of potential dame could it create? At minimal you just burn out your glow plugs and at worse you burn up your wiring or alternator or battery or entire machine or any combination of that.

That's why the old fashioned mechanical relay or contact disconnect device is still used in certain applications.

 

The issue with using them in those types of applications the inherent 'fail unsafe' mode of breakdown. That is the chance of them failing to full on without any practical way to shut the circuit off is their inherent problem and to make them have such an event takes little more than one voltage spike that goes beyond their breakdown voltage limit.

So given that if you glow plugs went to full on without being able to be shut down what sort of potential dame could it create? At minimal you just burn out your glow plugs and at worse you burn up your wiring or alternator or battery or entire machine or any combination of that.

That's why the old fashioned mechanical relay or contact disconnect device is still used in certain applications.

Fair enough, you reason is logical, I've melted wiring and fused relays on vehicals before. With out semiconductors. The risk is minimal really just a set of glowplugs there captivated in precombustion chamber they are only going to melt.

As for fail safe it dosent have one now, how could I make one with a igbt? what kind of curcuit porn do you know?

 

You use a IGBT the same as a mosfet. Given their much higher working voltage HV spike damage is less likely but I just don't see the point of making a solid state switch for glow plugs.
They are such a limited duty cycle application it's pointless given the overall components needed to make it work and work reliably will cost far more than just using a standard high current relay or contactor for the main power switching.

As for circuit porn I know much but I have to see justification in the application to share it and this is not something I see as being anywhere near justified. Doable but not the least bit justified cost wise or other.

The way I see things is solid state switching devices are best justified for applications where an electrical circuit needs to be varied or switched on and off several to billions of times a second. Glow plugs are on for a few to several seconds when the engine starts and that's it.

 

You use a IGBT the same as a mosfet. Given their much higher working voltage HV spike damage is less likely but I just don't see the point of making a solid state switch for glow plugs.
They are such a limited duty cycle application it's pointless given the overall components needed to make it work and work reliably will cost far more than just using a standard high current relay or contactor for the main power switching.

As for circuit porn I know much but I have to see justification in the application to share it and this is not something I see as being anywhere near justified. Doable but not the least bit justified cost wise or other.

The way I see things is solid state switching devices are best justified for applications where an electrical circuit needs to be varied or switched on and off several to billions of times a second. Glow plugs are on for a few to several seconds when the engine starts and that's it.

Quet an inspirational message there. The justification is irrelevant, cost or relivance. It's the learning process that has the only value I'm seeking here. thank you for your time anyway.

 

Well I am a professional Demotivational speaker in my free time.

 

What truck are you working on?

 

What truck are you working on?

It's a landrover with a nissan motor and drive chain, it's custom.