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# Designing power mosfet circuit

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#### Fontaine

##### New Member
Hi, I need help to design a circuit

I'm buliding a scuba torch and need a small circuit to switch my ligth on and off.
I'm going to use a halleffect relay as switch and use a magnet to activate it.Then i need some kind of relay that can handle currents above 10A.

I've seen a small circuit with a power mosfet to act as relay and want to build something similar.
My knowlegde in electronics is very basic.

Torch spec:

Battery: 12V 9Ah
Bulb: 50W or 100W

Any help would be appreciated.

Fontaine said:
Hi, I need help to design a circuit

I'm buliding a scuba torch and need a small circuit to switch my ligth on and off.
I'm going to use a halleffect relay as switch and use a magnet to activate it.Then i need some kind of relay that can handle currents above 10A.

I've seen a small circuit with a power mosfet to act as relay and want to build something similar.
My knowlegde in electronics is very basic.

Torch spec:

Battery: 12V 9Ah
Bulb: 50W or 100W

If you have a halleffect switch design in mind, this power mosfet will be suitable to use:

https://www.electro-tech-online.com/custompdfs/2003/12/FDP038AN06A0.pdf

Or find something similar. Design a circuit using your hallefect sensor to drive the gate of the power fet. Then have the power fet switch on your 50W, or 100W bulb. Use a heat sink for the fet.

Any help would be appreciated.

You don't need a heat sink for the MOSFET that Optikon recommended.

Power=I^2*Rds;
Power=8^2*.0035;
Power<0.25w

80 amps thru a TO-220? I should think the source lead would melt!

Russlk said:
80 amps thru a TO-220? I should think the source lead would melt!
I agree it's pretty hard to believe, but I'm assuming that Rds(ON)=3.5 milliohms includes the package, so 80 amps will dissipate 22 watts - a lot of heat, but not impossible to handle with a good heat sink.

If the mosfet heats up dosen't that mean that it's draining power from the battery?

I want to get the longest possible burn-time of my battery so the power consumption should be lowest possible.

Sorry if my english is bad

I have some pictures of a circuit that works, but i can't figure out all the components.

Complete circuit:

1: A transistor,possibly an Op-Amp??

2: Unknown..maybe a switch?

3: Diode??

You will have to keep water out of the circuit, the same as you would for a switch. A mechanical switch can't be beat for efficiency, the circuit will have to be "on" all the time in order to turn the torch on. That is a continuous drain on the battery, so you should take the battery out or disconnect it when not in use. Therefore, why not have no switch, just put the battery in when you want to use the torch? If you HAVE to have a magnetic switch, this should work:

The hall effect is by Infineon Technology, available from Digikey.

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• magsw.jpg
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Thanks for the circuit, is it based on the mosfet recommended by Optikon?
How much power will it drain?

Have you looked at the pic's i've posted, the circut there is rated to use 3.6mA.

The problem with not using a switch is that i would drain the battery too much if i used to much time to get in the water or had a long surfaceswim before the dive started.

And i would drain the battery completely before i had a chance to disconnect it.

The reason for using a halleffect switch is to be able to operate the switch from outside the batterycannister with a magnet.

The mosfet recommended by Optikon is a good one. Yes, I looked at the pictures, the TLE4935g is an IC with the diode, opamp, etc inside. The current drain is between 1.4mA and 5 mA.

Ok, thanks
I will try your circuit as soon as i get the parts.

if you are unhappy with the heat dissipated by the MOSFET, try connecting 2 or more MOSFETS in parallel. MOSFETS, unlike bipolar transistors, increase their voltage drop with temperature, so they will share the current equally if simply connected in parallel. With 2 MOSFETS, each will have half the current, and one quarter of the heating.

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