Missing Voltage

[Dragon Tamer]

I was working on a circuit of min yesterday, it uses a lot of components and a couple of fans. the problem is that my 12v dc relay won't trigger because either the voltage or the current is too low. The relay is triggered by a 3904 transistor that is in turn driven by the out put of a LM2902 quad comparator. the problem that I am really concerned about is the fact that when the fans are triggered (by a different part of the circuit, but the same power supply) there is a certain amount of voltage that disapers. IE: the 12 volts may drop to 11.1 volts depending on the fan. This completely contradicts the law that voltage rise = voltage drop (the power supply is rated for 450mA, the small fan consumes 150mA and the large fan was rated at 430mA although it really consumes something like 100mA). One more question, not to be anoying, I was doing a supper position lab for my electronics class, my calculations and my measurements didn't match for the current through one of the resistors. If you want the circuit diagram then you will have to wait till I can find it.

 

[MikeMl]

Post the schematic. I'm guessing that your power supply is not regulated, or that the filter capacitor (upstream of the regulator) is too small.

 

[Dragon Tamer]

The power supply for the missing voltage? I don't have that circuit diagram, I bought it at radio shack.

here is the superposition schematic:

 

[Sceadwian]

There's no missing voltage, when loaded the supply voltage is sagging, this is perfectly normal especially with inductive loads like fans when starting. You're not taking everything into account.

 

[Dragon Tamer]

i was starting to think that it was related to current, i thought that it was similar to when you short out a power supply. and what about the superposition diagram? nobody that i talk to can figure that out. (the problem lies with R1 on the full circuit)

 

[audioguru]

An electric motor draws much more current when it starts than its running current.
The high current causes the voltage to drop if the power supply is a cheap Chinese one from Radio Crap.

 

[Sceadwian]

It's a little hard to guess on the diagram you posted because you didn't provide the values you got.

 

[Dragon Tamer]

there seems to be some confusion, the missing voltage and the superposition are 2 different isues, sorry i should have been more clear.

 

[Sceadwian]

There's no confusion on my part Dragon. You said you were having a mismatch from the observed and calculated valueson the superposition circuit... You gave no expected or measured values or how you came up with them.... Just a circuit with two batteries and three resistors. Seriously, are we supposed to guess? =)

 

[Dragon Tamer]

off the top of my head the current value thru R1 should be something like 400mA, yet when i measure it i get something like 136uA. everybody i ask is stumped by it.

 

[Sceadwian]

The current through R1 should be approximatly 464µa so neither of the values you mentioned make sense.

 

[Boncuk]

Here is the simulation result.

It says exactly the same as Sceadwian: Current flow across R1 is 464µA.

Using a pocket calculator it only says the truth if results are roughly estimated prior to calculating. (error of 1:1000)

Boncuk