Circuit problem

Thread Starter

GiannisMandelos

Joined Jan 19, 2019
20
Hi community. I have a question on something i built and i would like to listen to your thoughts about it. I have made a very simple circuit that consists of a power unit connected directly to the input of a boost dc-dc voltage converter that has both CV and CC modes. The output voltage of the converter falls on a resistor that consists of a precision 20 kOhms ten turn potentiometer in series with a 330 Ohm resistor. The output voltage of the converter has been regulated to 59 Volts , while the output current of it has been limited down to 80 mA max through the CC mode. When i turn on the power unit , i see exactly what i expected to , a 80 mA current running through the resistance , in the case I have turned the pot down to 0 Ohms.

But , when i connect a DPDT switch to the output of the converter , while the rest of the circuit remains the same , things change and i cannot understand why. In specific , the CC mode of the converter seems not to work , as when i turn on the power unit and turn the pot down to zero , the current is almost equal to 59(V)/330(Ohms)=178mA , and not 80 mA as it would suppose to be , with my CC regulation. What changes with the DPDT?
 

djsfantasi

Joined Apr 11, 2010
6,718
Can you show how the DPDT is wired with a picture? A DPDT switch isn’t just used for reversing polarity. Depending on how it’s wired, it could be an on/off switch. Or a two channel selector. Or nothing at all!
 

jpanhalt

Joined Jan 18, 2008
10,096
What is it not doing? Common DC ammeters I have seen are sensitive to polarity. Are you using a multimeter that can show negative current?
 

AlbertHall

Joined Jun 4, 2014
10,243
the current is almost equal to 59(V)/330(Ohms)=178mA
Did you measure the actual current. or measure the voltage and calculate the current?
80mA flowing in a 330Ω resistor is over 2W dissipation in that resistor. What power rating is the resistor?
Measure the resistance of the resistor plus the pot.
 

Thread Starter

GiannisMandelos

Joined Jan 19, 2019
20
Did you measure the actual current. or measure the voltage and calculate the current?
80mA flowing in a 330Ω resistor is over 2W dissipation in that resistor. What power rating is the resistor?
Measure the resistance of the resistor plus the pot.
I measured the actual current through an amperemoter in series with the resistors. I did the same measurement without the DPDT and the CC worked fine , the current was 80mA. It doesn't make any sense why with the dpdt the CC does not correspond. The resistor is 330Ohm 50Watt , the Pot is a precision ten turn Hybritron 20k Ohm.
 

Thread Starter

GiannisMandelos

Joined Jan 19, 2019
20
What is it not doing? Common DC ammeters I have seen are sensitive to polarity. Are you using a multimeter that can show negative current?
When the dpdt is on the circuit the current is not 80mA as it should be , due to converter's CC limitation down to 80mA , but the exact same circuit without the dpdt works fine and the current cannot surpass 80mA.
 

djsfantasi

Joined Apr 11, 2010
6,718
Makes no sense. If the only difference is the presence of the switch and it doesn’t matter what position the switch is in... there likely is a wiring error.

Your diagram shows how you think its wired. Can you post pictures showing how it’s actually wired? I haven’t seen a link to the DPDT switch you're using. Perhaps it has a different pinout.
 
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