# Confirmation of Base resistor for coil driver circuit

#### JulesP

Joined Dec 7, 2018
374
Hi all,

I am seeking advice on the correct configuration to power a transistor used to energise a coil inductor.

In the circuit shown the coil driver is either a FET or a BJT. With a FET the PWM signal driver is sufficient on its own to feed the coil driver Gate as hardly any current is needed to activate it and hence energise the coil. However, when using a BJT (MJE13009) to run the coil, the base current is too high for the PWM module to supply and it gets damaged. What I am proposing then is to put a 2N2222A in between the signal generator and the coil driver to boost the base current to the MJE13009 and wanted to check if I have the value of the Base resistor right. I have included all the relevant values.

This is a simple issue for someone well versed in electronics but Physics is my main subject. Any confirmation or feedback would be appreciated.

Thanks

Jules

#### ericgibbs

Joined Jan 29, 2010
13,277
hi Jules,
You require a current limiting resistor on the Base input of the MJE. ie: from the 2N2222 Emitter to the MJE Base say a 470R or 1K
Also a suppression diode across the inductor.
E

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#### JulesP

Joined Dec 7, 2018
374
Thank you!

#### Alec_t

Joined Sep 17, 2013
12,162
Since you are using the transistors as simple switches you need them to be driven hard so that they switch on fully (saturate), otherwise they dissipate a lot of power and can overheat. Accordingly, the usual design strategy is to assign each transistor a gain of only 10. For the MJE13009 that is already marginal (the rated gain range is only 8-40 @5A), making it a poor choice here in my opinion.
With this strategy, 2A coil current implies 200mA base current in the MJE13009, hence 20mA base current in the 2N2222A.
Using a base resistor value as high as Eric suggests will not allow enough base current for the MJE13009.
You could reconfigure the two transistors as a Darlington pair.

#### ericgibbs

Joined Jan 29, 2010
13,277
hi,
Without the 470R the current thru the 2N2222 into the MJE Base would not be limited and it would destroy the MJE, the 10K helps the MJE to turn Off a little faster.
E

#### ericgibbs

Joined Jan 29, 2010
13,277
hi Jules
I misread the coil current as 200mA, not 2A, so a 47R resistor NOT 470R.

As alec points a Darlington pair would be a better option.
E

#### JulesP

Joined Dec 7, 2018
374
hi Jules
I misread the coil current as 200mA, not 2A, so a 47R resistor NOT 470R.

As alec points a Darlington pair would be a better option.
E
Yes it may be better overall but I have limited space left on the board and its more components so if I can make it work with the simplest I would prefer it. Assuming the gain on both BJTs is 10 then I think I need a Base resistor of 600R for the 2N2222A and 47R for the MJE.

Jules

#### ericgibbs

Joined Jan 29, 2010
13,277
hi,
This simulation is for 47R, using an equivalent model for the MJE .
It saturates OK, a 39R will give more Base current, but the 2N2222 dissipation is high.

E

BTW: the 47R or 39R should be at least 1.5W rating

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#### JulesP

Joined Dec 7, 2018
374
hi,
This simulation is for 47R, using an equivalent model for the MJE .
It saturates OK, a 39R will give more Base current, but the 2N2222 dissipation is high.

E

BTW: the 47R or 39R should be at least 1.5W rating
Thanks for that. Do I need the 10k R4 between Base and Ground? Also the small blue rectangular pots that I have quite a few of, with a small brass screw on top, are they about 1.5W rating?

Julian

#### ericgibbs

Joined Jan 29, 2010
13,277
hi,
The purpose of the 10K is shunt to 0V any leakage current that may occur thru the 2N2222, rather than the leakage current going into the MJE Base.
It is not essential but preferable, I would certainly add it to a circuit I was building.

The small trim pots are not rated for 1.5W.
You can realise a close to 39R by using a parallel pair of say 68R, what low value resistors do you have in your bits box.?
E

#### JulesP

Joined Dec 7, 2018
374
hi,
The purpose of the 10K is shunt to 0V any leakage current that may occur thru the 2N2222, rather than the leakage current going into the MJE Base.
It is not essential but preferable, I would certainly add it to a circuit I was building.

The small trim pots are not rated for 1.5W.
You can realise a close to 39R by using a parallel pair of say 68R, what low value resistors do you have in your bits box.?
E
I have 0.25W ones for 47R and the next up is 100R but I'm happy to get a few specific values at a higher rating.

#### ericgibbs

Joined Jan 29, 2010
13,277
If you have the space/clearance, then 3 off 100R in parallel would give 33R [ the 100R's could be 0.5W each]

#### JulesP

Joined Dec 7, 2018
374
If you have the space/clearance, then 3 off 100R in parallel would give 33R [ the 100R's could be 0.5W each]
Ok thanks. I will find some option that meets the brief.

#### MisterBill2

Joined Jan 23, 2018
9,036
The other purpose of that resistor from the base to the emitter is to assist in a more rapid switching off of the power transistor.so it is rather important. The exact value is less critical except that it must be large enough to allow adequate base drive to switch the transistor on completely.

#### JulesP

Joined Dec 7, 2018
374
The other purpose of that resistor from the base to the emitter is to assist in a more rapid switching off of the power transistor.so it is rather important. The exact value is less critical except that it must be large enough to allow adequate base drive to switch the transistor on completely.
Fair enough. Does the 600R between the square wave generator and the base of the 2N2222A also need to be 1.5W rating?

Jules

#### ericgibbs

Joined Jan 29, 2010
13,277
hi
No Jules,
E

Query, Do you mean 600R, thats low for that 2N2222

#### Alec_t

Joined Sep 17, 2013
12,162
Bear in mind that the stated maximum PWM source current is 15mA. That won't cope if you actually need 20mA base current in the 2N2222A.
If you're going to have to buy parts anyway, why not simply invest in a FET to switch the coil?

#### ericgibbs

Joined Jan 29, 2010
13,277
hi alec,
Unless I am missing something, why do you consider he needs 20mA Base current into the 2N2222.
Eric

#### Alec_t

Joined Sep 17, 2013
12,162
why do you consider he needs 20mA Base current into the 2N2222
Merely the result of applying the 'forced gain of 10' strategy. A higher forced gain should work though.