Logic level switching 12V high side

Discussion in 'General Electronics Chat' started by jnye6625, Dec 29, 2012.

1. jnye6625 Thread Starter New Member

Dec 27, 2012
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Hey Guys,
I was hoping someone might be able to help me with a circuit to use a logic level controller to switch a 12V load of about 9 amps from the high side. It should be capable of PWM. I'm pretty new to electronics so any help would be greatly appreciated!

2. ScottWang Moderator

Aug 23, 2012
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You may use -- high side mosfet to search on this site.

3. jnye6625 Thread Starter New Member

Dec 27, 2012
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Scott,
I tried a search for what you suggested and was unable to find what I'm looking for, or at least was unable to piece together the information I need. Do you have any other suggestions on where to find information on how to drive the load I described above?

Thanks!

4. #12 Expert

Nov 30, 2010
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Pick a mosfet.

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5. #12 Expert

Nov 30, 2010
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Here's a starter circuit based on, "I don't know what the frequency is".

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6. jnye6625 Thread Starter New Member

Dec 27, 2012
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#12 : After looking at those three devices, with my minimal knowledge, it looks like any of the three would switch a 9 amp load just fine. I'm interested in the calculations used to make a decision and also how to calculate what transistor to use with the mosfet. This circuit will normally be on and based on the programming will turn off momentarily. Probably for .1 seconds or less, and then back on for .1 seconds and then off. The actual frequency will have to be developed through testing. Any input on how to proceed from here?

7. #12 Expert

Nov 30, 2010
16,665
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The IRFH...looks like 4.6 milliohms at full on. 9 amps times .0046 ohms = .04 watts. Cool as a cucumber. The circuit I posted will work, as far as I can tell. Look up the driver and see how much current it can supply, then reduce the base driver resistor to allow most of the available current to get into the base of the transistor, just to insure fast switching. Then allow the same amount of current from the 12 volt supply to the collector of the transistor. Ohm's Law, you know...

Suppose your driver can supply 10ma. 5 volts, 10 ma, 560 ohms
12 volts, 10 ma, 1200 ohms

The driver transistor is any old POS like a 2N3904, MPSA18, 2N4401.

8. crutschow Expert

Mar 14, 2008
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Don't really understand your calculations. The base current needs to be no more than 1/10 of the collector current for good switching. It's not related to Ohm's law. It sounds like you are saying to have the same base current as collector current which is way overkill.

9. #12 Expert

Nov 30, 2010
16,665
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Let me think...Hard (saturation) switching requires a collector current capable of 10x the actual current. If I put 10 ma into the base, the collector can do 100 ma at a gain of 10 or a thousand ma at a gain of 100. Both gains are easily achievable with any common TO-92 transistor even though the 1000ma figure is generally not achievable in that small of a package.

So, if the design is 10 ma on the collector, the base can use 1 ma to get that done.
If the design is 100 ma on the collector, the base can use 10 ma to get that done.
I think the 100 ma design is overkill, so better to go with 10 ma on the collector and 1 ma on the base.

At these frequencies, even the 10 ma collector current might be called overkill.
Bottom line: set the base at one tenth the current of the collector, not equal currents as I suggested last night.

Are you OK with that crutschow?

10. crutschow Expert

Mar 14, 2008
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Sounds good to me.

11. jnye6625 Thread Starter New Member

Dec 27, 2012
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#12 : Can I use the IRFH for a prototype circuit where the components will be through-whole mounted?

12. #12 Expert

Nov 30, 2010
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Every mosfet I put up were surface mount chips. I could mount that chip with pad-per-hole perfboard with .1 inch centers. How skillful are you?

or you can search a vendor site for through hole mosfets. You have 12 volts to work with so the gate voltage limitations are not a problem in this design.

do you know how to search vendor sites?
which country you live in?

13. jnye6625 Thread Starter New Member

Dec 27, 2012
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Thanks again for the help! Can the IRFH be used in protoyping where I would typically use through hole mounting? Also, would there be a need for a diode for flyback purposes?

14. #12 Expert

Nov 30, 2010
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You have not said what the load is. I can not guess about protection from an unknown device. And yes, the IRFH transistor can be used for prototyping or final assembly. It simply works right for the job.

15. timescope Member

Dec 14, 2011
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Hi Everyone,

The load is not specified so a diode across the load might be a good idea if it is inductive.

Timescope

16. jnye6625 Thread Starter New Member

Dec 27, 2012
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#12 : To answer your questions... I am not very experienced searching vendor sites but could probably muddle through it. I live in Michigan. And as far as soldering goes, I have built a couple MegaSquirt EFI units and various other basic electrical circuits. I've dealt a lot with precision components and machining.

The load I am switching is an ignition coil so that would probably warrant the use of a diode for protection, correct?

17. Ron H AAC Fanatic!

Apr 14, 2005
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Powe r= I^2*R = 9*9*.0046=370mW.
I made the same mistake in a recent post.

18. jnye6625 Thread Starter New Member

Dec 27, 2012
8
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#12 When mounting the chip to perfboard, is there any special technique I should use or do you simply solder from the bottom side through the hole?

Thanks

19. crutschow Expert

Mar 14, 2008
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If you are switching the primary of the coil to generate a high voltage then you don't want a diode across the coil since that will kill the voltage. To protect the transistor, add a zener from the drain to source with a voltage rating about 75% of the transistors maximum.