Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Really basic question on voltage clamp

Status
Not open for further replies.

Scarr

Member
Hi all,

Right, I have a I/O that needs to source 1.5A but be clamped at 5v as the external device it's conected to can have between 5-24v and itself can source 15A


Please help!!!

Steve

P.S. just re-looked at resistor and know I think even the 15ohm is too large!! should be 3ohm if it's 5v to be able to supply 1.5A, I'm getting confused :(
 
Last edited:
hi,
Not having seen the circuit.?

Its possible to use a standard zener to drive the base of a shunt power transistor.

Post the circuit.
 
Last edited:
Hi ericgibbs,

This is the voltage limiting bit I have cobbled to the circuit board. Am I on right lines thinking it's the low value resistor causing the diode to pop and that if I want to source 1.5A I need to either up the wattage of the diode or create the shunt power transistor you suggest?

P.S. Ignore the part number on the circuit it is a BZX84-C5V1 I'm using

Thanks

hi,
This type of circuit, ref image.

Is it possible to explain what exactly you are wanting to do with this circuit.
Its a bit like a sledge hammer.!:)
 

Attachments

  • esp10 Nov. 27.gif
    esp10 Nov. 27.gif
    4.5 KB · Views: 155
I can only think it's the low value resistor allowing too much current to flow but I need this to be low to allow me to source 1.5A. the only other way (if I'm on right track) is to have a > 33.75w diode!
Why not look at the datasheet for the tiny BZX84 zener diode to see its max allowed dissipation and use arithmatic to calculate if it will survive?

Its max allowed dissipation is 300mW (0.3W).
Its test current is only 5mA.

You had a current of (12V - 5.1V)/15 ohms= 460mA. The tiny zener diode was trying to dissipate 5.1V x 460mA= 2.4W!

If you want 1.5A then the resistor would be (12V - 5.1V)/1.5A= 4.6 ohms/10.4W. The zener diode would dissipate 5.1V x 1.5A= 7.7W.
 
Explanation

I have made a test probe for serial communications this test probe can monitor AND inject data to a USART line at 5v, it injects commands by forcing the line high (max 1.5A) and pulling it low as it requires to send the command, this ensures the command issued is not overridden by another device talking over it. However normally it sits and listens. It is connected using bipolar transistors so as not to intefere until we need to.

Whilst roaming around a board testing it, I sometimes hit a higher voltage line (24v max) by accident these lines and can supply many amps.

I know, I know, "be more carfull" but it happens and it blows a component, whilst this does not happen every day when it does it's a pain so I want to clamp this line to +5v somehow.

Hope this explains it a bit better :)
 
I made calculations with a 12V input. If the input is 24V then the power rating of the resistor must be almost 3 times higher.
 
i would use a dc dc converter for that they are very efficient and simple to use
they are converter that accept 9-32v input and output 5V
 
OK whats the next step?

I cannot use a DC-DC regulator as this would intefere with th comms (wouldnt' it?)

Any other suggestions on how to keep this line at 5v whilst allowing comms on it?

Thx
 
ericgibbs

Hi Ericgibbs (and anyone else that may be able to answer this)

Eric, I didn't understand what you meant by "Its possible to use a standard zener to drive the base of a shunt power transistor" but I went looking and I found out what you meant it's called a 'amplified zener shunt stabilizer'

Shunt Voltage Stabilizer Circuits

I think this will work great but I have one question why do we still need the resistor if the transistor now regulates the current stopping it blowing the zener?

Thx
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top