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  #1  
Old 11-02-2008, 09:56 PM
relicmarks relicmarks is offline
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Default power supply short circuit protection, overcurrent draw

I have a 12 volt power supply , but what can i use "inbetween" the power supply and the circuit under test that has indicator lights to tell me if there is a short on the circuit under test?

I know they make variacs with a built in amp meter

But is there something cheap to put inbetween a external power supply or wallwart that has indicator lights or something to tell me there is a short on the circuit being test

Cause i don't want to turn the circuit under test on for the first time and it burns the PC board or burn some IC chips
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  #2  
Old 11-02-2008, 10:18 PM
relicmarks relicmarks is offline
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I tried using a Variac with the current meter, but if the board/circuit is drawing alot of current at a low voltage than its hard to troubleshoot where the short is at

I start the Variac at O zero volts and increase it up very slowly, but mostly around 3 or 4 volts if there shorts on the board i will see the current meter jump very high trying to draw alot of current

Than i try to use my DVM meter set to voltage , checking various grounds to see i read any voltages on grounds.

If there is voltages on the grounds then there is a SHORT

A short circuit protection circuit i need will still supply the 12 volts and "normal rating current draw" even tho its trying to draw alot of current since there is shorts on the circuit under test

It like it will Clamp or regulate the current/voltage from increase under test
but will have a indication lights tell me its trying to draw alot of current cause there is shorts on the board

example: circuit board under test 12volts at 1mA rating

Shorts on board under test:

1.) If i use a Variac with current meter is will be at 3 volts with 1 Amp
2.) Than i have to tests the grounds with a DVM set to volts
3.) Try to find which grounds have 3 volts on the ground nodes

Example: circuit board under test 12 volts at 1mA rating

1.) If i use a short circuit protection circuit
2.) the circuit board is trying to draw 1 amps
3.) The short circuit protection circuit lets me test the circuit under test still at 12 volts at 1mA rating, Clamps regulated the current so i can try to find the shorts

It will be easy to troubleshoot having the correct normal rating voltage and current while trying to find shorts
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  #3  
Old 11-03-2008, 12:29 AM
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mik3 mik3 is offline
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If the power supply 'sees' a short circuit directly on its terminals caused by the circuit then you can detect it simply with a continuity meter. Apply your multimeter probes on the circuit power lines and see if the buzzer sounds. However, if your circuit contains active components you cant do it because they may cause a short when a voltage is applied. In this case use a resistor in series with the circuit, which in case of a short cant cause a current of more than 10 amps to flow (calculate its value depending on the supply voltage), and use your multimeter set on 10 amps range to measure the current flowing to the circuit. If it much higher than expected then a problem occurs. Be careful, because some power supplies, when a short circuits occurs, they limit the current to a safe value so you wont be able to detect if there is a short circuit.
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Old 11-03-2008, 01:34 AM
relicmarks relicmarks is offline
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1.) If the power supply 'sees' a short circuit directly on its terminals caused by the circuit then you can detect it simply with a continuity meter. Apply your multimeter probes on the circuit power lines and see if the buzzer sounds.

Yes i'm not talking about this


2.) However, if your circuit contains active components you cant do it because they may cause a short when a voltage is applied.

Yes this is what i'm talking about only when voltage is applied

3.) In this case use a resistor in series with the circuit, which in case of a short cant cause a current of more than 10 amps to flow (calculate its value depending on the supply voltage), and use your multimeter set on 10 amps range to measure the current flowing to the circuit. If it much higher than expected then a problem occurs. Be careful, because some power supplies, when a short circuits occurs, they limit the current to a safe value so you wont be able to detect if there is a short circuit.

Is there another way than using a resistor in series because if i get the resistors value wrong than it will smoke the board or components

I need a circuit board protection network , so if any voltage or current increases it will just CLAMP and regulate it back to the normal rating

4.) How do you guys find SHORT when the circuit board is Active , having power ON ?

Continuity checks RULES::

a.) Can't use a DVM meter on continuity check mode when the applied voltage is ON only when the circuit board is off under test

b.) Continuity check with a Analog volt meter set to OHM's doesn't help either ON or off when the applied voltage is ON because there is series/parallel circuit resistance

c.) So its hard to Continity check with a Analog or DVM set to OHM's when trying to hind a short

d.) Maybe using a 12 volt LAMP with low current to do continuity check when power is applied to the circuit board under test

e.) Or Set your DVM meter to volts, apply power to the circuit under test and than probe around grounds,inputs,outputs,VCC , if you don't get any voltage drops than u might find a short but its still confussing to try to find no voltage drops around IC chips because if there is ONE 12 volt or 6 volt short if makes all the IC chips look like they all have 12 volt shorts because there is no voltage drop
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Old 11-03-2008, 02:09 AM
scubasteve_911 scubasteve_911 is offline
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Why not have the return path go through a small sense resistor, then use a differential amplifier off of the resistor, then run it into a comparator (with hysteresis)? The comparator is set to disconnect through a SSR or MOSFET. You can also run a circuit(LED?) to indicate when the SSR/MOSFET has switched.

Steve
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Old 11-03-2008, 05:40 PM
Bailey45 Bailey45 is offline
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The attached circuit limits when there is .7 volts acroos the emitter resistor independent of supply voltage.

The transistor will require a heat sink.
Attached Files
File Type: pdf limiter2.pdf (13.3 KB, 139 views)
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  #7  
Old 11-03-2008, 05:53 PM
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You might consider simply using an automotive light bulb. Light bulbs are unusual in that they have a relatively low resistance when they are cold, but increase in resistance considerably as the filament grows hot.

If your circuit under test was not shorted, your lightbulb would glow dimly. If it were shorted, it would glow brightly.

You would need to select the appropriate bulb wattage rating for the unit under test.
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  #8  
Old 11-04-2008, 12:38 AM
relicmarks relicmarks is offline
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If i use a automotive light bulb on the output of a 12 volt power supply in "series" with the + red positive lead going to the DUI circuit under test
and the circuit under test had a bunch of shorts or just one short its going to draw alot of current like 1Amp or so its going to still burn out the IC chips , PCB and other components

Yes the automotive light bulb will give me a indication that its drawing alot of current

But i still need a Current Limiter or something to STOP the circuit under test from or trying to draw alot of current since it has shorts on the board

I don't want to put a fuse , because a fuse would just blow out and i can't test voltages or find the shorts when power is applied if it keeps blowing fuses

example

DUI circuit under test normal rating is 12volts at 40mA

if the board has shorts its going to want to draw 1A, how can i current limit even if the board has shorts on it?
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  #9  
Old 11-04-2008, 12:56 AM
scubasteve_911 scubasteve_911 is offline
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I still fail to see why my current sense solution isn't feasible. It will not limit current, but it certainly will be able to disconnect the load.

Steve
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  #10  
Old 11-04-2008, 01:29 AM
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If i use a current sense resistor i would need a comparitor circuit or detecting
circuit plus it disconnects the load or circuit under test if any voltage or current goes "above" the threshold

I don't want to Turn OFF the power if the current raises really high or draws alot of current cause there is shorts on the board

Current sense resistor will turn the circuit under test OFF not keep it on

Some type of "Current Divider circuit" is all i can think of to keep the current constant or a current regulator?

A current regulator circuit that will always keep the source current out of a power supply at 100mA even if its trying to draw 1A or 800mA the current regulator circuit will not change
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