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| Increased current problem. |
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| stefan_trekkie:
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. |
| StillTrying:
"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. |
| Kleinstein:
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. |
| stefan_trekkie:
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. --- End quote --- 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. --- End quote --- Nope, a plastic .. I had somewhere in the parts a nice npn's for the TV vertical deflection with isolated collector |
| StillTrying:
"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. |
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