• 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.

Transistor as switch

Status
Not open for further replies.
Hi Experts,

I just have a query about the circuit attached. Well the problem with that circuit is the Q1, it always break. When the signal at TP10 is below 10V it means the Q1 B-E is open when you check it with the multimeter. This circuit provides an excitation voltage to wheatstone bridge, when you apply force (compression and tension) on the other side of bridge the voltage accross to it will be unequal. The transistor here works continoulsy(12-36Hrs) by the time you start to send power on it. I assumed that the transistor is over used thus it reaches its life span very easy. I want to improve my circuit by replacing it with the analog switch (dg304/309/adg419) since the transistor also works as a switch. Am i right with my assumption? Is it really better to use an analog switch instead of transistor?

Thanks in advance,
 

Attachments

ericgibbs

Well-Known Member
Most Helpful Member
Hi Experts,

I just have a query about the circuit attached. Well the problem with that circuit is the Q1, it always break. When the signal at TP10 is below 10V it means the Q1 B-E is open when you check it with the multimeter. This circuit provides an excitation voltage to wheatstone bridge, when you apply force (compression and tension) on the other side of bridge the voltage accross to it will be unequal. The transistor here works continoulsy(12-36Hrs) by the time you start to send power on it. I assumed that the transistor is over used thus it reaches its life span very easy. I want to improve my circuit by replacing it with the analog switch (dg304/309/adg419) since the transistor also works as a switch. Am i right with my assumption? Is it really better to use an analog switch instead of transistor?

Thanks in advance,
hi,
The 2N2222 is rated a 200ma/600mA continuous, replace it with a ZTZ451 1amp continuous.

Whats on the missing leftside of the cct.?
 

ericgibbs

Well-Known Member
Most Helpful Member
The missing parts are isolated and not important. Just focus on Q1, it switches the +12V continuosly (square wave).
The fact that the 2N2222 is blowing suggests the transistor is being over run, the rightside of the circuit would not do that.!
Assuming that the collector suppy to the 2N2222 is as shown.

Most likely the left side cct is drawing a high current or possibly being shorted.

Do you have a datasheet for the analog switches.?
 
Last edited:
Is it really the current that makes my transistor to breakdown? I dont think the circuit will takes more than 600mA. Well I found a cirucit like this but it use analog switch (HI9P5051) ad it never failed.
 

ericgibbs

Well-Known Member
Most Helpful Member
Is it really the current that makes my transistor to breakdown? I dont think the circuit will takes more than 600mA. Well I found a cirucit like this but it use analog switch (HI9P5051) ad it never failed.
Look at this clip from the data, its 625mW at 25C, so assuming it maintains 25C [which it wont] then with 2Vce, I make that ~300mA max.

If it gets over 25C its derated at 12mW/C rise, so after a while it could fail

How much current is the leftside cct drawing.?
 

Attachments

Last edited:

Chaerl

New Member
what do you mean by isolated? not connected? I dont think so, i want to know how did you get 10V. I'm trying to compute the total power of your transistor with the application in order to see if it is overstressed of not.

anyhow, I normally put a biased resistor in parallel with the BE junction and not with BC. in this case, I see 90mA of current going to base. though, the limit is 200mA, but note, its peak limit. therefore, it means transient, not continous. I dont know whats the continous limit.
 
Yes, the right side of the circuit would not do anything for the transistor to nreak down often. AS for the currentn on the left side, you can review the datasheet of AD708 together with the ADG419, I would prefer this switch. What do you think?
 

Attachments

ericgibbs

Well-Known Member
Most Helpful Member
Yes, the right side of the circuit would not do anything for the transistor to nreak down often. AS for the currentn on the left side, you can review the datasheet of AD708 together with the ADG419, I would prefer this switch. What do you think?
The continuous current of the AG is only 30mA.!!!

I would recommend that you measure the 2N2222 current.
 

Chaerl

New Member
if you just want a switch then this is fine. just try to consider the input parameters such as current limit, etc.
 
Status
Not open for further replies.

EE World Online Articles

Loading
Top