# Simple Series/Parallel Equivalent Resistor Question

#### werd2501

Joined Aug 28, 2014
4
This one-port network is given and I have to find the equivalent resistance in it. My first attempt at this was:

300//200 + 400//100 = 120 + 80 = 200Ω

It seemed to me that the 300 was in parallel with the 200 resistor, and that was in series with a 400//100 resistor. But I realise why this is not the case.

I don't know anymore. Someone enlightened me today it was 100//400 in series with 200, in parallel with 300: (100//400 + 200)//300, which got the right answer.

I guess, can someone just explain to me how I can interpret stuff like this and what I should know/understand to avoid getting it wrong in the future?

Thanks.

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Offtopic, but is it me, or is the forums loading really slowly?

#### ericgibbs

Joined Jan 29, 2010
9,537
hi,
Why not calc' the 100R || 400R first.?

then that || combination value in series with the 200R

then the final || parallel value with the 300R.?

What do you make the final Requivalent value.?
E

EDIT:
I always look for the obvious 'parallel' resistor pairs in the circuit, then if necessary re sketch the circuit using a single resistor for the || pairs.

So its a simple method of reducing each parallel or series resistor path to its equivalent value.
If you find it difficult, do a simple circuit redraw for each reduction.

BTW: the site access appears normal to me.

Last edited:

#### MikeML

Joined Oct 2, 2009
5,444
Stand on the top right node in the circuit. Look left. Where does the current flow.

There are only two parallel branches (upper and lower) starting at that node. The "upper" branch is 100//400 + 200. The "lower" branch is just 300Ω.

This is just a consequence of KCL

#### Lestraveled

Joined May 19, 2014
1,946
When I solve networks like this my first step is to simplify the circuit and components.

1. The 100 and the 400 ohm resistors are in parallel. They resolve to a single 80 ohm resistor.
2. The 200 ohm resistor is in series with the 80 ohms which equals 280 ohms.
3. This leaves you with 300 ohm resistor in parallel with 280 ohms which equals 144.827 ohms.

Turn many components into fewer components.