# What are resistors in power supplies for?

1. Mar 23, 2016

### Guidestone

Hey guys. I just opened up a case from an AC to DC adapter and I noticed a 1kohm resistor in the circuit. I remembered a class I had about voltage supplies and I remember there were diagrams with a transformer, full bridge rectifiers, capacitors and at the end of them there was a resistor. The resistor was placed in such a way that we could measure the output rectified voltage across its terminals. However, if I'm not mistaken, we could obtain the same voltage measures whether that resistor was there or not, so, my question is, what is that resistor for? Is it to limit the current whenever I connect a load to it? The back of the case has 300mA maximum on it.
Thanks a lot!

2. Mar 23, 2016

### davenn

your question is too vague to give a decent answer.
Resistors have lots of uses depending on where in the PSU circuit they are

1) what type of power supply ?
2) where in the power supply is the resistor(s) ?

show us some sharp and well lit pics of the power supply both the component and track side of the circuit board

Dave

3. Mar 23, 2016

### nsaspook

Normally a simple unregulated full bridge rectifier, capacitor circuit unloaded output voltage would be the peak voltage of the AC input voltage to the full bridge rectifier. So usually a 'bleeder' resistor is used to give a minimum load on the circuit to improve DC voltage regulation from no to full load and to always discharge the output voltage to zero when the AC power is disconnected.

The filter 'Ripple' and the DC voltage varies with the load.

http://williamson-labs.com/powersupply.htm

4. Mar 23, 2016

### Guidestone

You are right Davenn, I should have uploaded some pics. In fact, the picture down below nsaspook posted is what I'm talking about. I never heard about those bleeding resistors before. However, if it's possible to be explained, how can a resistor help maintain voltage constant?
Thank you guys!

5. Mar 23, 2016

### davenn

the bleeder resistor isn't for maintaining a constant voltage ( not that I have been ever aware of)
it is, as its name suggests, used to slowly discharge the capacitor ( bleed off the voltage across the capacitor)
when the PSU is turned off. There are a couple of main purposes for this

1) for a high voltage DC PSU, a dual purpose of... a) discharging the cap so that the user doesn't accidentally zap themselves
b) discharging the cap so that when another piece of gear is connected to the PSU, it isn't damaged by a large voltage spike from the capacitor

2) for a low voltage PSU, purely for the same purpose as b) above ...
discharging the cap so that when another piece of gear is connected to the PSU, it isn't damaged by a large voltage spike from the capacitor

Dave

6. Mar 23, 2016

### nsaspook

It doesn't make it constant, it just makes the initial voltage drop from completely unloaded (Vripple ~0) to to the loaded value of Vripple less in a very cheaply made wall-wart.

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/rectct.html

Last edited: Mar 23, 2016
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