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34063 voltage drop

sid2286

Aug 24, 2011
102
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Aug 24, 2011
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102
hi,

I'm using 34063 to step down voltage from 24vdc to 5vdc.
I'm using the same components as mentioned in the datasheet.
I'm driving 3 relays using uln2803. the current consumption when all the 3 relays are operating is 70mA.
but as soon as my relays are operated the voltage starts falling.
when all the 3 relays are operated voltage falls to 2.1v.

this shouldn't happen since max current in datasheet is 500ma,.

please help.

Sid
 

davenn

Moderator
Sep 5, 2009
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what's a 34063 ??

give a link to a datasheet :)

Dave
 

KJ6EAD

Aug 13, 2011
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Aug 13, 2011
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1,114
Make sure your inductor is big enough (low resistance & high inductance) to support the current draw and that you have enough output capacitance to handle the largish inrush to the relay coils.
 
Last edited:

shrtrnd

Jan 15, 2010
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Jan 15, 2010
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3,876
I think KJ6EAD pegged the problem. The inductance of the relay coils is probably
the issue (although daveen's point is well recognized).
It may be as simple as a more robust (higher current output) power supply.
 

OLIVE2222

Oct 2, 2011
690
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Oct 2, 2011
Messages
690
Hi,

KJ6EAD point out the MC34063 circuit inductance problem, not the inductance of the relays coils and indeed the MC34063 inductance must be sized according to the needed output current!

Olivier
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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8,393
The MC34063 is an ancient device with poor performance. I don't understand why people are still using it. There are MUCH better alternatives available.
 
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