Author Topic: Increased current problem.  (Read 884 times)

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Offline stefan_trekkieTopic starter

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Increased current problem.
« on: December 27, 2019, 11:13:07 am »
Hello. I have a design issue with finished project. I have to drive 4 separate coils at 60 to about 300 Hz and have been told that the current must be 1A each and i have done it with 4 PNP BJTs "КТ818Г" (Russian made ~ 10 amps, TO-220 package) FOr 1 amp i had them naked but not they tell me that has been mistake and the current must be 4 amps and it is not working in the moment. Have to increase it .. That is ok but i don't have space for radiators inside the tiny box ... Should i go for some high power mosfets and again - naked ? (expensive stuff) This device will work more then 90 min at time.
 

Offline StillTrying

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Re: Increased current problem.
« Reply #1 on: December 27, 2019, 12:45:34 pm »
"but i don't have space for radiators inside the tiny box"

If the box is metal could you use the box itself as the radiator, probably needing the TO-220's tabs to be isolated from the box.
...Which probably still won't be enough heatsinking.
.  That took much longer than I thought it would.
 

Online Kleinstein

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Re: Increased current problem.
« Reply #2 on: December 27, 2019, 05:44:21 pm »
It depends on the control signal that goes to the base. If the voltage level is right, it may work with P channel mosfets.  One may need a logic level type and a relatively low voltage one to get a R_on that is low enough.
 

Offline stefan_trekkieTopic starter

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Re: Increased current problem.
« Reply #3 on: December 27, 2019, 06:42:09 pm »
I was scrolling in the local electronics catalogs and found nice mosfets with high current ratings .. With 0.008 Ω D-S resistance.. with 4 amps .. That is below 0.2W .. 2€ per piece .. I was with wrong impression that will be at least 10€ peace

Quote
It depends on the control signal that goes to the base. If the voltage level is right, it may work with P channel mosfets.  One may need a logic level type and a relatively low voltage one to get a R_on that is low enough.
It is analog 12 volt with op amps and driver transistor (it was necessary for the power BJTs) .. Not a problem at all i will change it.

Quote
If the box is metal could you use the box itself as the radiator, probably needing the TO-220's tabs to be isolated from the box.
...Which probably still won't be enough heatsinking.
Nope, a plastic .. I had somewhere in the parts a nice npn's for the TV vertical deflection with isolated collector
« Last Edit: December 27, 2019, 06:43:58 pm by stefan_trekkie »
 

Offline StillTrying

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Re: Increased current problem.
« Reply #4 on: December 28, 2019, 01:02:51 pm »
"I have to drive 4 separate coils at 60 to about 300 Hz and have been told that the current must be 1A each and i have done it with 4 PNP BJTs "КТ818Г"
.... It is analog 12 volt with op amps and driver transistor (it was necessary for the power BJTs)"



I can't tell whether the coils are driven with square waves or sine waves, which makes quite a difference to the dissipation in the drivers.
.  That took much longer than I thought it would.
 

Offline stefan_trekkieTopic starter

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Re: Increased current problem.
« Reply #5 on: December 28, 2019, 08:42:41 pm »
Square waves on motorcycle fuel injectors. My friend is running experiments on that stuff.. It is ready and running
 

Offline daqq

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Re: Increased current problem.
« Reply #6 on: December 28, 2019, 09:09:37 pm »
You can get really cool MOSFETs these days for peanuts. Depending on how you are driving your transistors now, you may or may not need a rework of the driver. MOSFETs are awesome, provided they are used properly  ^-^
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Online langwadt

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Re: Increased current problem.
« Reply #7 on: December 28, 2019, 11:28:43 pm »
schematic?
 


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