SMD Components for replacing motorcycle relay?

Thread Starter

evengravy

Joined Oct 6, 2012
11
Hi All,

I am looking for some advice on the best route to explore in a DC switching application. I am rewiring my vintage motorcycle and would like to use a microcontroller to switch the power to the DC lighting system so that I can eliminate a lot of complexity (and save space). I can not use a traditional mechanical relay in this situation so would like to have a solid state device to allow me to switch 200W of 12VDC to the headlight via a 3.3V or 5V microcontroller.

At the moment, I am not sure the best and most compact way to achieve this. I expect that MOSFETs would be the way to go and I have been searching around to this end, but would also very much appreciate any advice on components that would be suitable for logic level control and pro's/con's of N Channel or P Channel devices in this application. I am not entirely clear on the benefits/negatives of high vs low side switching but would prefer high side from a wiring point of view. I am also very aware that there may be a more refined component that I am not aware of.

Could anyone offer some suggestions? I am happy to implement gate drivers if necessary but space is at a premium so compact wins the day here.

Thank you in advance, J
 

Ya’akov

Joined Jan 27, 2019
9,230
Welcome to AAC.

First, a logic level MOSFET is the best route. Other options don’t really make sense given the job.

Second, concerning N- vs. P-channel—while I understand the convenience a PMOS device appears to give you it’s a trap.

There is a big problem that makes the wiring problem a better challenge to take on: turning the PMOS device on. There are two types of MOSFETs: enhancement mode and depletion mode. An enhancement mode device turns on when a positive voltage is applied to the gate, while a depletion mode part turns on when driven to 0V or less.

N-MOS comes in both types while P-MOS, for practical reasons (cost) are only enhancement mode devices. To turn on an enhancement mode P-Channel transistor you will have to apply n volts more than what you connect to its Drain*.
*the power for the switched thing goes into the Drain and the thing is connected to the Source.

This means that is the device needs 5V to turn on, you need 5V more than the 12V you are hoping to send to the headlight—totaling 17V (something you don’t have).

This immediately makes the N-Channel part much more attractive. It will also probably be less expensive, possibly quite a bit so. The part that @DickCappels suggested, is, I am sure, a fine choice. I would recommend the IRZ44N as another candidate. Note that the IRZ44N in in a TO-220 case, not an SMD package—i think that this will probably be a better choice in the end.

I also think you should consider putting the device in the headlight housing so the high current run can be kept shorter, allowing you to just run the control voltage to the headlight.
 

Ya’akov

Joined Jan 27, 2019
9,230
Might it be rather warm in there?
EDIT: by the way, I’ve seen motorcycle headlights that take a single supply and expose low current control inputs so it seems doable.

It might be, but wherever the device is mounted it will require effective heat dissipation, and the headlight is already rather toasty so I am making the (admitted) assumption it will be easier to dissipate the heat in something that rides in the airstream than something more internal.

In point of fact, the size benefit the TS wants may be nullified by the heatsink he will need. A relay may turn out to be the best option when all is said and done.
 

geekoftheweek

Joined Oct 6, 2013
1,240
To turn on an enhancement mode P-Channel transistor you will have to apply n volts more than what you connect to its Drain*.
Maybe I'm missing something here, but I have always seen this expressed as a voltage less than the source (Vgs) for PFETs. For instance a BSS84PW lists and Rds(on) of 10.5Ω with a Vgs of -2.7V and 4.6Ω with a Vgs of -10V.

Is there something more I need to learn?
 

Thread Starter

evengravy

Joined Oct 6, 2012
11
I believe that is blinding ilegal. Check the Deprtment of Transportation rules for your location on something like 55-65Watts.
Yes, you're right, the same circuit will be used to drive other aspects on the bike also (starter for example) and I want to have a lot of overhead for cooling also (like SMD) so overhead is a good thing.
 

Thread Starter

evengravy

Joined Oct 6, 2012
11
Welcome to AAC.

First, a logic level MOSFET is the best route. Other options don’t really make sense given the job.

Second, concerning N- vs. P-channel—while I understand the convenience a PMOS device appears to give you it’s a trap.

There is a big problem that makes the wiring problem a better challenge to take on: turning the PMOS device on. There are two types of MOSFETs: enhancement mode and depletion mode. An enhancement mode device turns on when a positive voltage is applied to the gate, while a depletion mode part turns on when driven to 0V or less.

N-MOS comes in both types while P-MOS, for practical reasons (cost) are only enhancement mode devices. To turn on an enhancement mode P-Channel transistor you will have to apply n volts more than what you connect to its Drain*.
*the power for the switched thing goes into the Drain and the thing is connected to the Source.

This means that is the device needs 5V to turn on, you need 5V more than the 12V you are hoping to send to the headlight—totaling 17V (something you don’t have).

This immediately makes the N-Channel part much more attractive. It will also probably be less expensive, possibly quite a bit so. The part that @DickCappels suggested, is, I am sure, a fine choice. I would recommend the IRZ44N as another candidate. Note that the IRZ44N in in a TO-220 case, not an SMD package—i think that this will probably be a better choice in the end.

I also think you should consider putting the device in the headlight housing so the high current run can be kept shorter, allowing you to just run the control voltage to the headlight.
Ah, thank you very much for that advice, I will try to digest the details. I likely have some logic level N-Channel Mosfets that would work for immediate testing, I'll be honest, after years of fiddling with electronics I haven't yet grasped the intricacies of P Channel devices.

These are the devices that I have to hand I believe, maybe they are suitable for testing at least: https://www.rapidonline.com/stp55nf06l-mosfet-logic-n-60v-55a-47-0550

I understand it is only rated at 95W but maybe suitable in reality. I need to check/measure the initial/actual current requirements I suppose.

Appreciate all the input from everyone also. So effectively, in that application I would be switching the GND connection and wiring 12VDC direct to the light it self. I can maybe get away with a relay to the starter solenoid if the current requirements are too high (I can review).
 
Last edited:

Alec_t

Joined Sep 17, 2013
14,370
The N-Channel Mosfet is likely more suitable.
It depends whether your light has its negative terminal permanently connected to chassis or not. If it is, then you would be using 'high-side' switching and a N-Fet would need its gate voltage several volts above the 12V positive supply to turn the Fet on.
 

crutschow

Joined Mar 14, 2008
34,679
To control a high-side P-MOSFET switch, its Vgs must be near 0V to turn on, and must be negative (positive source-to-gate) to turn on.
To generate that from a microcontroller 3.3-5V signal, a transistor driver can be used (below):
When the μC signal is low, Q1 is off, thus the PMOS Vgs is zero, and it is also off.
When the μC signal is high, Q1 is on, and the PMOS gate is at ground potential, giving a negative Vgs of 12V, turning it on.
1711290692333.png
 
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